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Michelle Hechenbichler, Albert Prause, Michael Gradzielski, André

Laschewsky

Thermoresponsive self-assembly of twofold

fluorescently labeled block copolymers in aqueous

solution and microemulsions

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Citation details

Hechenbichler, M., Prause, A., Gradzielski, M., & Laschewsky, A. (2021). Thermoresponsive Self-Assembly of

Twofold Fluorescently Labeled Block Copolymers in Aqueous Solution and Microemulsions. In Langmuir (Vol.

38, Issue 17, pp. 5166–5182). American Chemical Society (ACS).

https://doi.org/10.1021/acs.langmuir.1c02318.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir,

edited and published work see https://doi.org/10.1021/acs.langmuir.1c02318.

This work is protected by copyright and/or related rights. You are free to use this work in any way permitted by

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Thermo-responsive Self-assembly of Two-fold

Fluorescently Labelled Block Copolymers in

Aqueous Solution and Microemulsions.

Michelle Hechenbichler a, Albert Prause b, Michael Gradzielski b,* and André Laschewsky a,c *

a Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476, Potsdam-

Golm/Germany

b Stranski-Laboratorium für Physikalische und Theoretische Chemie, FG Physical Chemistry/

Molecular Material Science Institute of Chemistry, Technische Universität Berlin, Straße des 17.

Juni 124, 10623, Berlin/Germany

c Fraunhofer Institute of Applied Polymer Research IAP, Fraunhofer Institute, Geiselbergstr. 69,

14476, Potsdam-Golm/Germany

KEYWORDS: watersoluble polymer, associative polymer, amphiphile, thermo-responsive, self-

assembly, fluorescence label, FRET.

ABSTRACT

A nonionic double hydrophilic block copolymer with a long permanently hydrophilic and a small

thermo-responsive block is synthesized by reversible addition fragmentation chain transfer

polymerization (RAFT). Employing a specifically designed chain transfer agent, the polymer is

functionalized with complementary end groups which are suited for Förster resonance energy

transfer (FRET). The end group attached to the permanently hydrophilic block of

poly(N,Ndimethylacrylamide) pDMAm is designed as permanently hydrophobic segment

(‘sticker’) comprising a long alkyl chain and the 4-aminonaphthalimide fluorophore. The other

end attached to the thermo-responsive block of poly(Nisopropylacrylamide) pNiPAm

incorporates a coumarin fluorophore. The temperature-dependent self-assembly of the two-fold

fluorescently labelled copolymer is studied in pure aqueous solution as well as in an o/w

microemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), and

fluorescence spectroscopy. It is compared to the behaviors of the analogous two-fold labelled

pDMAm and pNiPAm homopolymer references. The findings indicate that the block copolymer

behaves as polymeric surfactant at low temperatures, with one relatively small hydrophobic end

block and an extended hydrophilic chain forming ‘hairy micelles’. At elevated temperatures above

the LCST phase transition of the pNiPAm block, however, the copolymer behaves as associative

telechelic polymer with two non-symmetrical hydrophobic end blocks, which do not mix. Thus,

instead of a network of bridged ‘flower micelles’, large dynamic aggregates are formed. These are

connected alternatingly by the original micellar cores as well as by clusters of the collapsed

pNiPAm blocks. This type of structure is even more favored in the w/o microemulsion than in pure

aqueous solution, as the microemulsion droplets constitute an attractive anchoring point for the

hydrophobic dodecyl sticker, but not for the collapsed pNiPAm chains.

INTRODUCTION

Because of the inherently reduced mixing entropy and the resulting strong tendency for

(micro)phase separation, as well as by virtue of the substantially increased number of molecular

variables in comparison to low molar mass surfactants, polymeric amphiphiles give rise to a

plethora of self-organized structures in selective solvents, in particular in aqueous media.1-7 Two

major polymer classes are conveniently distinguished according to the origin of the amphiphilic

character. On the one hand, the constitutional monomer units (CRU), or at least short monomer

sequences, are inherently amphiphilic, as e.g. in the so-called polysoaps.7-9 On the other hand,

amphiphilicity can result from the overall macromolecular architecture that combines individual

hydrophobic and hydrophilic blocks, as e.g. in amphiphilic graft and block copolymers ("macro

surfactants").1, 4, 7 While formerly, amphiphilic block copolymers were restricted to a small number

of practical systems, the advent of the Reversible Deactivation Radical Polymerization (RDRP)

methods has diversified the synthesis of block copolymers exceedingly, and concomitantly the

investigation and use of the latter copolymer class enormously during the past two decades.10-14 A

particular aspect of amphiphilic block copolymers is the relative ease of implementing responsive

(also called ‘smart’ or ‘intelligent’) amphiphilic systems, which react to a small change of a trigger

parameter with important changes of their self-assembly behavior, and thus of their properties.14-

17 Arguably, the most explored trigger is a temperature change, as it is non-invasive and the

induced changes are fully reversible in many cases, thus enabling repeated switching of such

systems. In aqueous systems, thermo-responsive polymers exploit typically the crossing of a lower

consolute boundary, and are characterized by a lower critical solution temperature (LCST). The

best-known and most intensely studied example is poly(N-isopropylacrylamide) pNiPAm.18-24

Importantly, it exhibits LCST-behavior of the non-Flory-Huggins type (‘Type II’ behavior),25

reducing substantially the sensitivity of its transition temperature to variations in physical (such as

concentration) as well as molecular parameters (such as molar mass). In consequence, pNiPAm

has been frequently employed as versatile component in thermo-responsive ‘smart’ amphiphiles,

acting likewise as switchable hydrophobic or switchable hydrophilic block, in dependence on the

overall molecular architecture.26, 27 Up to now, studies have been mostly focused on AB diblock

and symmetrical ABA and BAB triblock copolymer systems.16, 28 ‘A’ and ‘B’, respectively,

signify the operative hydrophilic and hydrophobic blocks, notwithstanding the use of pNiPAm in

more complex architectures such as non-symmetrical triblock,27, 29, 30 so-called ‘schizophrenic’,31,

32 or 'molecular bottlebrush' copolymers.33, 34 The majority of studies deal with polymers, in which

pNiPAm acts as switchable hydrophilic block (A*) in combination with permanently hydrophobic

B blocks, or in reverse, as switchable hydrophobic block (B*) in combination with permanently

hydrophilic A blocks. Amidst these architectures, symmetrical triblock copolymers of the type

B*AB* found special interest as ‘smart” associative thickeners,35-38 in which the amphiphilic

character and thus, the aggregation driven modification of rheology can be implemented by a

simple temperature stimulus.

With this background, we started recently to explore thermo-responsive amphiphilic block

polymers with a non-conventional structure. Specifically, we explore non-symmetrical

architectures BAB*, in which a long permanently hydrophilic inner A block is framed on one end

by a short permanently hydrophobic end group B (‘hydrophobic sticker’), and on the other end by

a responsive, conditionally hydrophobic end block B*, which exploits an LCST transition (Scheme

1). Instead of implementing simple ‘on-off’ systems by rendering a double hydrophilic A*AA*

system into an amphiphilic and associative B*AB* system, we aim via this design at ‘smart’

systems, that switch between a classical surfactant-like BAA* type state and a BAB* triblock

system that behaves as rheology modifier (Scheme 1a). In this way, we aim at promoting systems

which transform from isolated ‘hairy micelle’-like assemblies, which produce low viscosity

solutions, into 'flower micelle'-like micellar assemblies, which are capable of network formation,

thus acting as associative thickeners (Scheme 1b).39, 40 In such 'hairy micelles', a relatively small

hydrophobic core formed by the hydrophobic blocks is surrounded by an extended hydrophilic

corona formed by dangling hydrophilic blocks. In contrast in 'flower micelles', the hydrophilic

corona is more compact as most of the hydrophilic blocks assume a looped conformation, because

the hydrophobic blocks of the individual copolymer reside predominantly in the same micellar

core.2

Scheme 1. a) Idealized architecture of an amphiphilic BAB*-type block copolymer consisting of

a long hydrophilic central block (A, in blue), capped by one short permanently hydrophobic end

group (B, in red) and one thermo-responsive end block (B*, in green); b) model for the

temperature-triggered switching of such polymers between isolated hairy micelles (left) and

networks of either interconnected flower-like micelles (upper right) or interconnected hairy

micelles (lower right).

Due to their particular structure, we may assume that the permanently hydrophobic sticker end B

of the copolymers enables their hydrophobic association in the BAA* state into large spherical

hairy micelles with small cores that are dynamically equilibrated.41, 42 For the aggregates formed

in the BAB* state, the scenario is a priori less clear after switching the character of the thermo-

responsive block from hydrophilic to water-insoluble (Scheme 1b). If the B and B* blocks are

sufficiently compatible, one will expect the transformation of the hairy into standard flower

micelles (Scheme 1b, upper right). If, however, the B and B* blocks are sufficiently incompatible,

additional separate hydrophobic micro domains will be formed by the collapsed B* blocks which

coexist with the micellar cores made of the B ends (Scheme 1b, lower right). Both scenarios allow

for network formation, and consequently, for implementing enhanced viscosity or inducing

gelation. However, the network properties are expected to differ markedly in these two scenarios.

For instance, the latter scenario (Scheme 1b, lower right) should markedly increase the average

distance between the micellar cores in the network, thus enabling percolation of the aggregates at

a much lower polymer concentration. Also, one can speculate whether the formation of separate

microdomains of the collapsed B* block can be favored, when o/w microemulsions are used

instead of pure water as medium. In fact, rheology control of microemulsions is of great practical

importance.43 In this case, the sticker blocks B would insert into the oil droplets, while the

collapsed B* blocks stay out and form separate microdomains, which act as a second population

of crosslinking sites. This scenario shows structural similarities to multicompartment micellar

hydrogels.44

To shed more light onto the aggregation behavior of such BAA*/BAB* systems, we designed the

thermo-responsive diblock copolymer pDMAm−b−NiPAm* (Figure 1). This polymer is

α,unsymmetrically functionalized with complementary fluorophores suited for Förster

resonance energy transfer (FRET).45 The efficiency of the FRET process depends sensitively on

the average distance of the fluorophore donor-acceptor pair at the nm scale. Therefore,

appropriately FRET labelled amphiphiles enable the differentiation between aggregates of diverse

structures, if these are correlated with the distance of the specific positions of the donor and

acceptor groups within the polymers.46-48 Most of the previous FRET-based studies on the

aggregation of amphiphilic polymers have used pairs of donor- and acceptor-labelled polymers

(often without defined positions of the fluorophores within a given polymer).47-58 Yet, the

investigation of the scenarios in Scheme 1b requires the incorporation of both donor- and acceptor

group into well-defined positions, which preferentially are as distant as possible within the same

block copolymer. This has been done rarely up to now.59-64 Taking the optimal layer substructure

of surfactants into account,65 we thus incorporated a naphthalimide fluorophore as FRET acceptor

at the joint of the permanently hydrophobic sticker group and the permanently hydrophilic block,

while a coumarin fluorophore was used for capping the thermo-responsive block, thus placing the

FRET donor at the opposite polymer terminus (Figure 1). If the collapse of the A* block results in

the transformation of the hairy into flower micelles (Scheme 1b, upper part), the FRET efficiency

will strongly increase after the temperature-induced transition. If, however, separate microdomains

are formed (Scheme 1b, lower part), the collapse of the responsive block will hardly affect the

FRET efficiency.

Figure 1. Chemical structures of the monomers (DMAm and NiPAm) and chain transfer agent

(FRET−TTC) used, and of the polymers synthesized (pDMAm*, pNiPAm*, and

pDMAm−b−pNiPAm*).

For the thermo-responsive AA*/AB* diblock fragment within the target structure, we used the

well-established combination of permanently hydrophilic non-ionic poly(N,Ndimethyl-

acrylamide) pDMAm and thermo-responsive poly(N-isopropylacrylamide) pNiAm.29, 66-70 The

polymer was synthesized by two successive RAFT polymerizations. The precise positioning of the

FRET donor-acceptor pair in the macromolecules was achieved by employing a dually

fluorophore-tagged trithiocarbonate, namely FRET−TTC (Figure 1), as RAFT chain transfer

agent.12, 71 Within FRET−TTC, the acceptor dye is part of the re-initiating so-called 'R-group',

and the donor dye is part of the deactivating so-called 'Z-group'. The unusual FRET pair based on

a naphthalimide and a coumarin fluorophore72 was chosen to cope with some boundary conditions

which are to be respected in our specific case. These comprise, e.g., relatively small fluorophore

size, inertness in free radical polymerization and the exclusion of potential electrostatic

interactions (thus e.g. excluding the popular fluorescein/rhodamine pairs), as well as good

tolerance to quencher groups such as trithiocarbonates,61 and rather high hydrophobicity of the

acceptor but low hydrophobicity of the donor dye (thus e.g. excluding the popular pairs of

hydrocarbon fluorophores such as naphthalene/anthracene/phenantrene/pyrene). Moreover, the

particular 4-amino-substituted naphthalimide fluorophore shows a pronounced

solvatochromism,73, 74 which may possibly provide additional clues about the changes of the

polymer's aggregation behavior.75

The dually fluorophore-tagged copolymer pDMAm−b−pNiPAm* was studied with respect to its

thermo-responsive behavior by temperature-dependent turbidimetry, dynamic light scattering

(DLS), small angle X-ray scattering (SAXS), and fluorescence behavior. The analogously dually

fluorophore-tagged homopolymers, pDMAm* and pNiPAm* (Figure 1), were also synthesized

and studied as references.

EXPERIMENTAL ‘SECTION

Materials

Nmethyldodecylamine was prepared from methylamine hydrochloride (≥ 98 %, Fluka) and

1bromododecane (≥ 95 %, Fluka) according to the literature.76 Reagents allylbromide (99 %

stabilized with propylene oxide, Sigma Aldrich), 2bromopropionyl bromide (97 %, Sigma

Aldrich), carbon disulfide (≥ 99.9 %, Merck), 4chloro-1,8naphthalic anhydride (95.0 %, Fluka),

7hydroxy-4methylcoumarin (97 %, Acros Organics), thioacetic acid (≥ 98 %, Merck),

triethylamine (≥ 99.5 %, Roth) were used as received. Ethanolamine (≥ 99.0 %, Sigma Aldrich)

was distilled prior to use. TetradecyldimethylamineNoxide (TDMAO, Ammonyx MO, Stepan)

was freeze-dried, recrystallized in acetone and dried under reduced pressure.

Nisopropylacrylamide (NIPAm, 97 %, Merck) was crystallized from nheptane prior to use.

N,Ndimethylacrylamide (DMAm, ≥ 99.0 %, stabilized with MEHQ, TCI) was distilled prior to

use to remove the inhibitor. 2,2′azobis(2methylpropionitrile) (AIBN, 98 %, Merck) was

crystallized from n-hexane prior to use. 1,1′azobis(cyclohexanecarbonitrile) (V40, 98 %, Merck)

was crystallized from chloroform prior to use. Solvents benzene (99.5 %, Roth), chloroform

(≥ 99.5 % stabilized with amylene, Th.Geyer), chloroformd (99.8 atom% D, Armar Chemicals),

ndecane (≥ 98 %, Fluka), deuterium oxide (99.8 atom% D, Armar Chemicals), dichloromethane,

(≥ 99.8 % stabilized with amylene, Th.Geyer), dichloromethane (≥ 99.5 %, Roth), diethylether

(≥ 95.5 %, Th.Geyer), N,Ndimethylformamide (> 99 %, Acros Organics), ethanol (absolute,

Merck), ethyl acetate (99.9 %, VWR), nhexane (≥ 95.0 %, Chemsolute/Th.Geyer), 1pentanol

(≥ 99 %, Sigma Aldrich), petrol ether (boiling range 60-80 °C, analytical grade,

Chemsolute/Th.Geyer), and tetrahydrofuran (≥ 99.5 % stabilized with BHT, Acros) were used as

received. For polymerizations, tetrahydrofuran was additionally distilled prior to use to remove

inhibitor. For spectroscopic studies, tetrahydrofuran and dichloromethane were additionally

distilled and stored over MgSO4 prior to use.

1 M Aqueous hydrochloric acid (Th.Geyer), concentrated hydrochloric acid

(Chemsolute/Th.Geyer), magnesium sulfate, (anhydrous, Applichem, Darmstadt/Germany),

potassium carbonate (anhydrous, ≥ 99 %, Roth), potassium iodide (≥ 99.99 %, Sigma Aldrich),

sodium chloride (≥ 99.0 %, Th.Geyer), and sodium hydroxide (≥ 98.8 %, Chemsolute) were used

as received. Deionized (DI) water was used for synthesis. For turbidimetry, DLS and fluorescence

studies, ultapure water (resistivity 18 MΩ·cm-1) was used, obtained by post-treating DI water by a

Millipore Milli-Q Plus water purification system (Merck Millipore, Darmstadt/Germany). The

nonionic microemulsion was prepared by dissolving precise amounts of TDMAO (190.0

mmol·L-1) and of decane (57.3 mmol·L-1) in ultapure water, which are known to form spherical

microemulsion droplets of about 3.1 nm in radius.77 Silica gel 60 (0.063-0.200 mm, 230-400 mesh

ASTM, Merck) was used as stationary phase for column chromatography.

Synthesis of 4−(N'−dodecyl−N'−methyl)amino)-N−2−hydroxyethyl-1,8−naphthalimide (2)

Adapting a procedure from the literature,73 4chloro-1,8naphthalic anhydride (10.00 g,

42.99 mmol) and N-methyldodecylamine (17.14 g, 85.98 mmol, 2.00 eq.) in pentanol (100 mL)

were refluxed for 24 h with stirring under argon atmosphere. After cooling to room temperature,

the crystals formed were filtered off, washed with ethanol, and crystallized twice from ethanol

before purification by column chromatography (silica gel, eluent petrol ether/ethyl acetate (5:1

(v/v), Rf= 0.16). Final crystallization from petrol ether/ethyl acetate (5:1 (v/v)) yielded 4.76 g

(12.0 mmol, 28 %) of 4−(N'−dodecyl-N'−methyl)amino-1,8−naphthalic anhydride 1 as orange

powder.

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 0.87 (t, J = 6.9 Hz, 3H, CH2-CH3), 1.24-1.29 (m,

18H, (CH2)9-CH3), 1.76 (br. tt, J1= J2= 7.3 Hz, 2H, N-CH2-CH2), 3.11 (s, 3H, N-CH3), 3.39 (br. t,

J = 7.6 Hz, 2H, N-CH2), 7.14 (d, J = 8.4 Hz, 1H, ArH3), 7.67 (dd, J1= J2= 7.3 Hz, 1H, ArH6), 8.45

(dd, J = 1.1 Hz, J = 8.2 Hz, 1H, ArH2), 8.47 (d, J = 8.2 Hz, 1H, ArH7), 8.57 (dd, J = 1.1 Hz,

J = 7.4 Hz, 1H, ArH5).

In the next step, 4−(N'−dodecyl-N'−methyl)amino-1,8−naphthalic anhydride 1 (1.59 g,

4.02 mmol) and ethanolamine (0.84 mL, 0.86 g, 14.03 mmol, 1.3 eq) were refluxed in ethanol

(100 mL) for 26 h. After cooling to room temperature, the formed yellow crystals were filtered

off. Concentration of the filtrate in the heat and subsequent cooling produced a second crop of

crystals. Yield: 1.45 g (3.31 mmol, 82 %) of 4(N'−dodecyl−N'−methyl)amino)-

N−2−hydroxyethyl-1,8−naphthalimide 2.

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 0.87 (t, J = 6.9 Hz, 3H, CH2-CH3), 1.24-1.29 (m,

18H, (CH2)9-CH3), 1.74 (br. tt, J1= J2= 7.0 Hz, 2H, N-CH2-CH2), 3.07 (s, 3H, N-CH3), 3.33 (br. t,

J = 7.6 Hz, 2H, N-CH2), 3.97 (t, J = 5.1 Hz, 2H, -N-CH2-O), 4.46 (t, J = 5.2 Hz, 2H, O-CH2-),

7.15 (d, J = 8.4 Hz, 1H, ArH3), 7.66 (dd, J = 7.3 Hz, J = 8.4 Hz, 1H, ArH6), 8.42 (dd, J = 1.1 Hz,

J = 8.5 Hz, 1H, ArH2), 8.48 (d, J = 8.2 Hz, 1H, ArH7), 8.58 (dd, J = 1.1 Hz, J = 7.3 Hz, 1H, ArH5).

(see Figure S1)

Synthesis of 4−(N'−dodecyl−N'−methyl)amino)-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)yl)-

ethyl 2-bromopropanoate (3)

2Bromopropionyl bromide (0.40 mL, 0.82 g, 3.82 mmol, 1.15 eq.) was added dropwise to a

solution of 4(N'−dodecyl−N'−methyl)amino)-N−2−hydroxyethyl-1,8−naphthalimide 2 (1.45 g,

3.31 mmol) and triethylamine (0.52 mL, 0.38 g, 3.77 mmol 1.14 eq.) in dry CH2Cl2 (40 mL), while

cooling to 0 °C. After 1 d, more triethylamine (0.52 mL, 0.38 g, 3.77 mmol 1.14 eq.) and

2bromopropionyl bromide (0.40 mL, 0.82 g, 3.82 mmol, 1.15 eq.) were added, maintaining the

temperature at 0 °C. After 1.5 h, the reaction was complete according to thin layer chromatography

(TLC, eluent petrol ether/ethyl acetate 5v:1v). After cooling and dilution by more CH2Cl2 (35 mL),

the organic phase was separated, washed successively with water (100 mL), 1 M HCl (2 × 50 mL),

saturated NaHCO3-solution (50 mL) and brine (50 mL), dried over MgSO4, and the solvent

removed under reduced pressure. The orange residue is purified by column chromatography

(gradient eluent: petrol ether/ethyl acetate 10:1 (v/v) increasing to 5:1 (v/v)). The thus obtained

intermediate 4−(N'−dodecyl−N'−methyl)amino)-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-

yl)ethyl-2−bromopropanoate 3 was used without further purification. Yield: 1.50 g (2.62 mmol,

79 %). (see Figure S2)

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 0.87 (t, J = 6.9 Hz, 3H, CH2-CH3), 1.24-1.29 (m,

18H, (CH2)9-CH3), 1.74 (br. tt, J1= J2= 7.0 Hz, 2H, N-CH2-CH2), 1.80 (d, J = 7.0 Hz, 3H, CH-CH3),

3.06 (s, 3H, N-CH3), 3.32 (br. t, 2H, N-CH2), 4.33 (q, J = 6.9 Hz, 1H, CH-CH3) 4.45-4.60 (m, 4H,

-N-CH2-CH2-O), 7.15 (d, J = 8.3 Hz, 1H, ArH3), 7.66 (dd, J = 8.4 Hz, J = 7.3 Hz, 1H, ArH6), 8.42

(dd, J = 8.5 Hz, J = 1.1 Hz, 1H, ArH2), 8.47 (d, J = 8.2 Hz, 1H, ArH7), 8.57 (dd, J = 7.3 Hz, J =

1.1 Hz, 1H, ArH5).

Synthesis of 7(3'mercaptopropyloxy)-4methylcoumarin (6)

Adapting a procedure from the literature,56 K2CO3 (9.44 g, 68.31 mmol, 1.00 eq.) and KI (0.38 g,

2.29 mmol, 0.03 eq.) were added to the solution of 7hydroxy-4methylcoumarin (12.04 g,

68.34 mmol) and allylbromide (8.90 mL, 12.46 g, 102.99 mmol, 1.5 eq.) in DMF (100 mL). After

stirring at 100 °C for 24 h under argon atmosphere, the mixture was cooled, and water (300 mL)

was added. The precipitated raw product was filtered off, and crystallized repeatedly from ethanol

(200 mL) to remove all impurities. Yield of 7-allyloxy-4-methylcoumarin 4: 11.19 g (51.75 mmol,

76 %).

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 2.17 (d, J = 1.0 Hz, 3H, CH3), 4.38 (dt, 2H, J = 1.4

Hz, J = 5.3 Hz, O-CH2), 5.12 (dd, J = 1.2 Hz, J = 10.5 Hz, 1H, CH2cis=CH), 5.22 (dd, J = 1.4 Hz,

J = 17.3 Hz, 1H, CH2trans=CH), 5.81 (m, 1H, CH2=CH), 5.91 (br. q, J = 1.0 Hz, 1H, CoumH3), 6.60

(d, J = 2.5 Hz, 1H, ArH8), 6.66 (dd, J = 2.5 Hz, J = 8.8 Hz, 1H, ArH6), 7.27 (d, J = 8.8 Hz, 1H,

ArH5).

Subsequently, the mixture of intermediate 7allyloxy-4methylcoumarin 4 (2.00 g, 9.25 mmol),

thioacetic acid (1.30 mL, 1.41 g, 18.50 mmol, 2.0 eq.) and AIBN (0.76 g 4.62 mmol, 0.5 eq.) in

benzene (100 mL) was refluxed for 24 h. After cooling to room temperature, 10 wt% aqueous

NaOH (70 mL) was added slowly and stirred for 10 min. The organic layer was separated and

washed with more 10 wt% NaOH-solution (2  70 mL), water and brine (70 mL). The organic

layer was dried over MgSO4, and the solvent was evaporated. The residue was crystallized four

times from ethanol, to yield 2.02 g (75 %) of 7(3’(acetylthio)propyloxy)-4methylcoumarin 5.

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 2.11 (tt, J = 6.6 Hz, 2H, S-CH2-CH2-CH2-O), 2.35

(s, 3H, CH3-COS), 2.40 (d, J = 1.1 Hz, 3H, Ar-CH3), 3.07 (t, J = 7.1 Hz, 2H, S-CH2), 4.07 (t, J =

6.1 Hz, 3H, O-CH2), 6.14 (d, J = 1.1 Hz, 1H, CoumH3), 6.80 (d, J = 2.5 Hz, 1H, 1H, ArH8), 6.85

(dd, J = 8.8 Hz, J = 2.5 Hz, 1H, ArH6), 7.49 (d, J = 8.8 Hz, 1H, ArH5).

The intermediate thioacetate 5 (0.63 g, 2.15 mmol) in ethanol (15 mL) is cooled to 0 °C. NaOH

(0.50 g, 12.5 mmol, 5.8 eq.) was added, and the mixture was stirred overnight under argon

atmosphere. Within the initial 1-2 h of the reaction, the mixture became clear. After acidifying the

solution with conc. HCl(aq), a precipitate was formed, which was filtered off and washed with

water. The raw product was dissolved in CH2Cl2 (40 mL), the organic phase washed with brine

(20 and 50 mL), dried over MgSO4, and the solvent removed under reduced pressure. The

intermediate 7-(3'-mercaptopropyloxy)-4-methylcoumarin 6 was obtained as slightly yellow solid.

Yield: 0.52 g (2.08 mmol, 97 %).

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 1.40 (t, J = 8.1 Hz, 1H, SH), 2.12 (tt, J = 6.5 Hz,

J = 6.4 Hz, 2H, S-CH2-CH2-CH2-O), 2.40 (d, J = 1.1 Hz, 3H, -CH3), 2.75 (dt, J = 7.0 Hz, J = 7.5

Hz, 2H, S-CH2), 4.15, (t, J = 5.9 Hz, 2H, O-CH2), 6.14 (d, J = 1.0 Hz, 1H, CoumH3) 6.83 (d, J =

2.4 Hz, 1H, ArH8), 6.86 (dd, J = 8.8 Hz, J = 2.5 Hz, 1H, ArH6), 7.50 (d, J = 8.7 Hz, 1H, ArH5) (see

Figure S3).

Synthesis of 4−(N'−dodecyl−N'−methyl)amino)-1,3dioxo−1H−benzo[de]isoquinolin-2(3H)

−yl)ethyl 2methyl-3((((3((4methyl-2oxo-2Hchromen-7yl)oxy)propyl)thio)carbono-

thioyl)thio)propanoate (FRET-TTC).

Triethylamine (0.32 mL, 0.23 g, 2.28 mmol, 1.1 eq.) was slowly added to

7−(3'−mercaptopropyloxy)-4−methylcoumarin 6 (0.39 g, 1.55 mmol) in CH2Cl2 (7 mL) while

stirring. After 30 min, CS2 (0.11 ml, 0.14 g, 1.82 mmol, 1.2 eq.) was added, and the solution was

stirred for another 1 h. Then, a solution of naphthalimide derivative 3 (0.89 g, 1.55 mmol, 1.0 eq.)

in CH2Cl2 (7 mL) was slowly added. The solution was stirred overnight at room temperature. The

solvent was evaporated, and the residue purified by column chromatography (silicagel, gradient

eluent petrolether/ethyl acetate 5:1 (v/v) increasing to 1:1 (v/v)). The product FRETTTC was

obtained as orange oil. Yield: 0.24 g (0.29 mmol, 19 %)

1H-NMR (400 MHz in CDCl3, δ in ppm): δ = 0.87 (t, J = 6.9 Hz, 3H, CH2-CH3), 1.24-1.29 (m,

18H, (CH2)9-CH3), 1.57 (d, J = 7.4 Hz,1H, CH-CH3) 1.73 (br. tt, 2H, N-CH2-CH2), 2.16 (tt, J1=

J2= 6.5 Hz, 2H, S-CH2-CH2), 2.39 (d, J = 1.1 Hz, 3H, Coum-CH3), 3.07 (s, 3H, N-CH3), 3.33 (br.

t, J = 7.4 Hz, 2H, N-CH2), 3.46 (t, J = 7.1 Hz, 2H, S-CH2), 4.05 (t, J = 6.0 Hz, 2H, Aryl-O-CH2),

4.32-4.56 (m, 4H, N-(CH2)2-OOC), 4.79 (q, J = 7.4 Hz, 1H, OOC-CH-CH3), 6.13 (d, J = 1.1 Hz,

1H, CoumH3), 6.78 (d, J = 2.5 Hz, 1H, CoumH6), 6.84 (dd, J = 8.8 Hz, J = 2.5 Hz, 1H, CoumH8),

7.18 (d, J = 8.3 Hz, 1H, NaphH3), 7.49 (d, J = 8.8 Hz, 1H, CoumH5), 7.67 (dd, J = 7.9 Hz, J = 7.9

Hz, 1H, NaphH6), 8.46-8.49 (m, 2H, NaphH2 + NaphH7), 8.57 (dd, J = 7.2 Hz, J = 1.0 Hz, 1H,

NaphH5). Attribution of signals confirmed by 1H-1H correlation (COSY) experiments.

13C-NMR (75 MHz in CDCl3, δ in ppm): δ = 14.22 (CH3-CH2), 16.89 (CH3-CH-COO), 18.77

(CH3-Coum), 22.77 (CH3-CH2), 27.11 (N-(CH2)2-CH2), 27.53 (N-CH2-CH2-CH2), 27.82 (S-CH2-

CH2), 29.43-29.71 (N-(CH2)3-(CH2)6), 32.00 (CH3-CH2-CH2), 33.34 (S-CH2-CH2), 38.65 (

(C=O)2N-CH2), 41.77 (N-CH3), 48.26 (CH3-CH-COO), 57.34 (N-CH2-(CH2)2), 63.29 (COO-

CH2), 66.73 (aryl-O-CH2), 101.64 (CoumC8), 112.21 (CoumC3), 112.57 (CoumC6), 113.88

(CoumC4a), 114.73 (NaphC1 + C3), 122.98 (NaphC8), 125.16 (NaphC6), 125.70 (CoumC5), 126.02

(NaphC4a), 130.51 (NaphC8a), 131.29 (NaphC7), 131.39 (NaphC5), 132.82 (NaphC2), 152.60

(CoumC4), 155.35 (NaphC8a), 161.36 (CoumC7), 161.78 (CoumC2=O), 164.10 (CON), 164.73

(NaphC4), 170.97 (COO), 221.48 (C=S). Attribution of signals confirmed by 1H-13C heteronuclear

single quantum correlation (HSQC) and Heteronuclear Multiple Bond Correlation (HMBC)

experiments.

Elemental analysis (C44H54N2O7S3) calculated: C 64.52 %, H 6.65%, N 3.42 %, S 11.74% found:

C 64.19 %, H 6.74 %, N 3.35 %, S 10.80 %

ESI: calculated mass Mr = 819.10 g/mol, found: 841.64 g/mol [M+Na]+:

FTIR (selected bands cm−1): 2924, 2852, 1730, 1693, 1653, 1612, 1585, 1514, 1466, 1452, 1387,

1369, 1354, 1292, 1281, 1263, 1240, 1201, 1147, 1068, 1024, 1016, 872, 849, 833, 816, 781, 760,

735, 706.

UV-vis absorbance: in CH2Cl2 (λmax=424 nm, ε = 8500 L∙mol-1∙cm-1), in THF (λmax=424 nm,

ε = 10700 L∙mol-1∙cm-1)

Synthesis of polymers

In the typical procedure for homopolymers pDMAm* and pNiPAm*, the monomer, initiator V-

40 and chain transfer agent FRETTTC were dissolved in benzene. The solution was purged with

argon for 45 min and immersed into a preheated oil bath with a temperature of 90 °C. After stirring

for a specific time, the reaction was stopped by opening the flask to the air and cooling the flask

with liquid nitrogen. For chain extension block copolymerization, NIPAm and macro chain

transfer agent pDMAm* were dissolved in benzene. A stock solution of V-40 in benzene

(2 mg/mL) was prepared and an appropriate volume of this solution was added. The solution was

purged with argon for 40 min and immersed into a preheated oil bath with a temperature of 90 °C.

After stirring for a specific time, the reaction was stopped by opening the flask to the air and

cooling the flask with dry ice/i-propanol. While homopolymer pNiPAm* was precipated

successively in diethylether and in pentane, all other polymers were precipitated twice into

diethylether for purification. The isolated polymers were dried in a vacuum oven, dissolved in

distilled water and lyophilized. The detailed amounts engaged are specified in Table 1.

Table 1. Reaction conditions for polymerization in benzene (ca. 33 wt% monomer and RAFT

chain transfer agent CTA) at 90 °C using initiator V-40

Sample

monomer

ount

of M

[g]

CTA

amount

of CTA

[mg]

initiator

I [mg]

molar

ratio

M:CTA:I

[h]

pDMAm

DMAm

2.44

FRET



TTC

2050:10:1

pDMAm2*

DMAm

3.47

FRET



TTC

150

4.5

1940

:10:1

pNiPAm*

NiPAm

2.78

FRET



TTC

2.9

2050:10:1

pDMAm



pNiPAm1*

NiPAm

0.12

pDMAm

500

0.57

450:10:1

pDMAm



pNiPAm2*

NiPAm

0.47

pDMAm

500

2.2

450:10:1

a) 50 ca. wt% solids monomer and CTA

Methods and instrumentation

Elemental analysis was done with a Vario ELIII microanalyzer (Elentar Analysensysteme, Hanau,

Germany). NMR spectra were recorded with a spectrometer Avance 300 (Bruker) operating at 300

MHz (1H), and 75 MHz (13C) respectively. Chemical shifts δ are reported in ppm vs. the respective

solvent peaks at δ (1H) 4.79 ppm for D2O, and δ (1H) 7.26 ppm and δ (13C) 77.16 ppm for CDCl3.

Fourier-transform infrared (FTIR) spectra were taken in the ATR mode with a spectrometer

Nicolet Avatar 370 FT-IR (Thermo Fisher Scientific) equipped with an AMTIR crystal and an

ATR Smart Performer element.

UV/Vis spectra were recorded on a Lambda 25 UV/Vis Spectrometer (Perkin Elmer) in quartz

sample cells (path length 1 cm). Assuming one naphthalimide chromophore in each

macromolecule due to the ‘R’-group of the FRET-TTC, number average molar masses MnUV were

calculated by end group analysis via UV vis spectroscopy from the absorbance at λmax = 424 nm

in CH2Cl2. Values were calculated according to MnUV = ε∙c∙d∙E-1 where ε [L∙mol-1∙cm-1] is the

extinction coefficient, c [g∙L-1] is the concentration of the polymer in solution and d [cm] is the

optical path length. The molar extinction coefficient ε of the naphthalimide chromophore in the

polymers was taken as 8500 L∙mol-1∙cm-1 at 424 nm, assuming to be identical to the value

determined for FRET-TTC in CH2Cl2.

Temperature-dependent static fluorescence experiments were performed with a thermostated

fluorimeter FluoroLog-3 (HORIBA Jobin Yvon, France). Optical silica cuvettes with an optical

path length d = 1 cm were used. The excitation wavelength was set to 318 nm, and emission

wavelength to 376 nm for the coumarin chromophore and to 520 nm for the naphthalimide

chromophore. Temperature was precise within 1 K. The polymer samples were dissolved in

Millipore water or in the tetradecyldimethylamineNoxide TDMAO/decane microemulsion.

Size exclusion chromatography (SEC) was performed by a self-made apparatus with simultaneous

UV and RI detection at room temperature (flow rate 0.5 mL∙min-1). The stationary phase was a

300 x 8 mm2 PSS GRAM linear M column (7 μm particle size), with eluent 0.1% LiBr in NMP

(injection volume 100 μL). Samples were filtered through 0.45 μm filters. The system was

calibrated with narrowly distributed polystyrene standards (PSS, Mainz, Germany).

Thermogravimetric analysis (TGA) was performed under N2 atmosphere using an apparatus

SDTA851e (Mettler-Toledo, Gießen/Germany, heating rate of 10 K min-1). Differential scanning

calorimetry (DSC) employed an apparatus DSC822e (Mettler-Toledo, Gießen/ Germany),

applying heating and cooling rates of 10 K min-1 for the first and second, and 30 K min-1 for the

third and fourth heating and cooling cycles. Glass transition temperatures Tg were taken from the

second heating cycle that used a heating rate of 10 K min-1 via the midpoint method.78

The temperature dependent transmittance was recorded with a Cary 5000 (Varian) spectrometer at

600 nm with heating and cooling rates of 0.5 K∙min-1. Temperatures are precise within 0.5 K. The

temperature at which the solution's transmittance starts to decrease (‘onset’) was taken as cloud

point (CP). Dynamic light scattering (DLS) was performed with a high performance particle Sizer

(HPPS-5001, Malvern Instrument, Malvern/UK) equipped with a He-Ne laser beam and a

thermoelectric Peltier element to control the temperature. The backscattering mode was used at a

scattering angle of Θ = 173 °. Samples were diluted with ultrapure water to the desired

concentration, and measured in heating runs by raising the temperature in steps of 1 °C

equilibrating the sample for 120 s prior to each measurement.

More comprehensive static (SLS) and dynamic (DLS) light scattering experiments were performed

with a 3DSpectrometer (LSinstruments, Switzerland). The instrument is equipped with a He-Ne

laser and operates at a wavelength 𝜆 =632.8 nm. All measurements were carried out with an

angle scan (2𝜃) between 30 and 135° in 5° steps and a temperature ramp from 20 to 60 °C in 5 °C

steps. At each angle three repetitions were performed with a duration of 60 s.

The static intensity was deduced according to 𝐼

 (𝑞)=,,

, ⋅𝑅, where

𝑞 =

sin

 is the magnitude of the scattering vector, 𝐶, =

 is the count rate 𝐶 divided by

the laser power 𝑃 of species 𝑖, and 𝑅 =1.37⋅10 cm is the Rayleigh ratio of toluene

for the given laser wavelength.79 For calculating the apparent molecular weight of aggregates, 𝐼(0)

is needed and was estimated via a Guinier fit, using eq. 1, in the 𝑞-range of 0.0066–0.0128 nm-1

(30–60°).

𝐼(𝑞)=𝐼(0)⋅exp−𝑅

𝑞/3 (1)

The effective aggregation number (𝑁

) from the obtained 𝐼(0) values, was estimated according

to: 𝑁

 =()

⋅⋅ (2)

with 𝐾 = 





, where K is the optical constant which corresponds to the value reported for

polyDMAm,80 𝑐 the mass concentration of polymer in solution, M the molar mass (Mntheo is used

for calculation), 𝑛 the refractive index of the solvent, 

 the refractive index increment for the

polymer in solution, and 𝑁 the Avogadro constant. The refractive index increment values were

determined experimentally with an Orange Analytics 19” dn/dc instrument. The DLS data were

analyzed based on the optimized regularization techniques (ORT). 81, 82 The analysis was

performed with SimplightQt (Python based software for analyzing light scattering data). Starting

with the measured intensity autocorrelation function (𝑔()(𝜏)−1), the field autocorrelation

function (𝑔()(𝜏)) was calculated via the Siegert relation: 𝑔()(𝜏)−1=𝛽⋅𝑔()(𝜏) where 𝛽

is the coherence factor and an instrument specific parameter below 1. The size distributions were

analyzed by using a set of log-normal distributions instead of bell-shaped B-splines as reported in

17,18. From the analysis a weight is obtained for each contribution.

𝑔()(𝜏)=∑𝑤𝑔()(𝜏)

 (3)

where 𝑔()(𝜏) is the field autocorrelation function, 𝑤 the weight and 𝑔()(𝜏) the Laplace

transform of the underlying correlation time distribution of the ith component.

𝑔()(𝜏)=∫𝐿(𝜏,𝜏,𝜎)⋅exp󰇡−

󰇢dτ



 (4)

The correlation time distribution 𝐿(𝜏,𝜏,𝜎) is defined as

𝐿(𝜏,𝜏,𝜎)=

√exp−(()())

⋅ (5)

where 𝜏 is the median and 𝜎 the standard deviation of the distribution. The apparent

hydrodynamic radius is obtained via the Stokes-Einstein-Smoluchowski equation (𝑅

 =

𝑘𝑇 (6𝜋𝜂𝐷)

⁄, where kB is the Boltzmann constant, T the absolute temperature and 𝜂 the solvent

viscosity) from the diffusion coefficient (𝐷=

, where q is the magnitude of the scattering

vector).

RESULTS AND DISCUSSION

The two-fold fluorescently labelled RAFT chain transfer agent FRET-TTC bears the coumarin

donor chromophore as part of the deactivating so-called 'Z-group' (forming the -end group of

polymers synthesized with the RAFT agent), and the naphthalimide acceptor dye as part of the re-

initiating 'R-group' (forming the -end group of polymers synthesized with the RAFT agent).

FRET-TTC was synthesized via seven steps starting from 7hydroxy-4−methylcoumarin

(‘4methylumbelliferone’) and 4chloro1,8naphthalic anhydride. Despite its complexity, the

1H NMR spectrum of FRET-TTC displays a number of well-resolved aromatic proton signals

(Figure 2), which can be unambiguously attributed to either the coumarin (e.g., signals 35, 40 and

43) or the naphthalimide chromophores (e.g., signals 16, 22 and 24). A priori, this enables not only

the end group analysis of the molar masses of polymers which are devoid of aromatic protons,

synthesized with this RAFT agent by 1H NMR spectroscopy using either end group, as it is typical

for most (meth)acrylic polymers including pDMAm and pNiPAm. Also, their direct comparison,

i.e., the molar ratio of incorporated R and Z groups, allows for estimating the preservation of

trithiocarbonate moiety (‘end group fidelity’), and thus of the ‘livingness’ of the RAFT

polymerization process.39, 83, 84 Additionally, the intense UV-vis absorbance at about 425 nm of

the naphthalimide chromophore attached to the R-group enables end group analysis of the molar

masses of polymers of virtually any chemical structure by an additional, independent method.84

Figure 2. NMR spectra of two-fold fluorescently labelled FRET-TTC in CDCl

: (a)

H-NMR,

(b)

C-NMR.

As illustrated in Figure 3, the emission maximum of the coumarin chromophore is at about 376 nm,

while the emission maximum of the naphthalimide chromophore is at about 520 nm. Importantly,

the emission maximum of the coumarin matches very well the absorbance band of the

naphthalimide between 350 and 480 nm. Furthermore, the coumarin has its excitation maximum

at about 324 nm, which coincides with the excitation minimum of the naphthalimide chromophore

in the UV-region (318 nm). Thus, the combined coumarin (as donor) and naphthalimide (as

acceptor) chromophores incorporated into the functional RAFT agent FRETTTC represent a

well-suited pair of fluorophores for FRET.

Figure 3. Fluorescence spectra in tetrahydrofuran for intermediates (a) 7-allyloxy-4-

methylcoumarin 4 (emission spectra excited at 318 nm, excitation spectra recorded by emission at

376 nm), and (b) naphthalimide 3 (emission spectra excited at 318 nm, excitation spectra recorded

by emission at 520 nm). Excitation spectra are shown in magenta (emission wavelength indicated

as dashed vertical line), and emission spectra in blue color (excitation wavelength indicated as

broken vertical line), the shades intensifying with increasing dye concentration.

200 300 400 500 600 700

10000

20000

30000

40000

50000

60000

70000

80000

Wavelength [nm]

Coumarin

excitation spectra

= 376 nm)

0.010 g/L

0.008 g/L

0.005 g/L

0.004 g/L

Emission spectra

exc

= 318 nm)

0.010 g/L

0.008 g/L

0.005 g/L

0.004 g/L

intensity

200 300 400 500 600 700

500000

1000000

1500000

2000000

2500000

Wavelength [nm]

Naphthalimide

excitation spectra l

= 520nm

0.050 g/L

0.030 g/L

0.020 g/L

0.010 g/L

0.001 g/L

emission spectra l

exc

= 318 nm

0.050 g/L

0.030 g/L

0.020 g/L

0.010 g/L

0.001 g/L

intensity

a) b)

Chain transfer agent FRETTTC was successfully employed in the RAFT polymerization of

acrylamides, for obtaining homopolymers pDMAm* and pNiPAm*, and block copolymer

pDMAm−b−pNiPAm* adapting an established procedure (cf. Table 1).39 The reactions

proceeded rather slowly, requiring 10  20 h and high monomer concentrations to achieve

conversions in the range of 60 – 90 %, which might be due to the bulkiness of the bi-functionalized

trithiocarbonate. Still, when the concentration of monomer and chain transfer agent was increased

from 33 to 50 wt%, the duration of the reaction could be reduced to 3 h. Table 2 summarizes the

results of the polymerization. The molar masses determined by different methods match

reasonably well with each other within the precision of the methods, and also with the theoretically

expected ones. Further, the dispersities Ɖ are rather low. Importantly, the molar mass distributions

of all polymers were monomodal, and the molar masses of the pDMAm* samples increased after

chain extension polymerization with NiPAm (see Figure S4). This indicates the successful

synthesis of block copolymers pDMAm−b−pNIPAm1* and pDMAm−b−pNiPAm2*.

Table 2. Molecular characterization of the polymers synthesized (cf. Figure 1). The applied

reaction conditions and chain transfer agents are specified in Table 1.

Polymer Yield

[%]

Monomer

conversion

[%] b)

Mntheo

[kg mol-1]

DPn

theo

MnNMR

[kg mol-1]

DPn

NMR

Z/R

MnUV

[kg mol-1]

Mnapp

[kg mol-1]

pDMAm1* 61 80 17 166 34 334 0.9 22 16 1.25

pDMAm2* 80 81 16 157 22 217 1.0 15 20 1.21

pNIPAm* 69 88 25 180 36 311 - 20 27 1.23

pDMAm-b-

pNiPAm1*

48 k) 58 25 26 39 34 0.8 23 24 1.43

pDMAm-b-

pNiPAm2*

84 l) 81 21 37 30 64 - 20 28 1.26

a) by gravimetry; b) by 1H-NMR analysis of the raw reaction mixture; c) theoretically expected number average molar

mass, calculated as the molar ratio of the monomer to the RAFT agent employed [M]/[CTA] corrected by the

monomer conversion as determined by 1H-NMR, assuming ideal (‘living’) conditions for the RAFT process;12, 39 d)

theoretically expected number average degree of polymerization of the newly added polymer block; e) number

average molar mass determined by end group analysis via 1H-NMR, using the integrals of the aromatic protons of

the naphthalimide moiety (from R-group), f) number average degree of polymerization of the newly added polymer

block via 1H-NMR analysis, g) relative ,-end group fidelity, by comparing the integrals of the 1H-NMR of the

aromatic protons of the naphthalimide (from R group) and coumarin (from Z-group) moieties, h) number average

molar mass determined by end group analysis via UV/Vis spectroscopy in dichloromethane using the band at

λmax = 424 nm (ε = 8500 L·mol-1·cm-1), i) apparent number average molar mass determined by SEC (calibration with

polystyrene standards), full elution diagams shown in Figure S4, j) dispersity Ð = Mw/Mn from SEC analysis, k)

pDMAm1* used as macroCTA, l) pDMAm2* used as macroCTA. Precision of all molar mass values is ± 20 %.

Moreover, the signals of the end groups were clearly visible in the 1H NMR spectra and could be

resolved for the naphthalimide and coumarin moieties despite the relatively high molar masses

(Figure 4 and Figure S5). The accordingly derived Z/R ratios are close to unity. Only when the

NiPAm content of the polymers becomes high, the signal of the NH-group interferes too strongly

with the end group signals (Figure 4b) to allow for a meaningful determination of the Z/R ratio.

All these findings point to a well-controlled polymerization process. Note that compared to the 1H

NMR spectra of the polymers in CDCl3, the characteristic signals of the end groups, in particular

of the R-group, were markedly broadened and attenuated when the spectra are recorded in D2O.

This points to a poor solvation of the end groups and possibly their aggregation into hydrophobic

nanodomains.

Figure 4.

H NMR spectra of two-fold fluorescently labelled polymers in CDCl

: (a) pDMAm1*,

(b) pNiPAm* , (c) pDMAmbpNiPAm1*.

According to thermogravimetric analysis TGA, all polymers were thermally stable up to at least

200°C. Significant mass losses were observed only when the temperature exceeded 300 °C.

Differential scanning calorimetry DSC revealed a glass transition for homopolymers pDMAm1*

at 118 °C and pNiPAm* at 132 °C, in good agreement with the literature.39, 85 Block copolymer

pDMAmbpNiPAm1* showed only one glass transition at 122 °C, i.e., a value between those

of the respective homopolymers. This is an indication that the two polyacrylamide blocks are

compatible in the bulk phase and do not (micro)phase separate.

Behavior in aqueous media

All the polymers synthesized were directly soluble in water at ambient temperature. However, in

contrast to the behavior of the solutions of homopolymers pDMAm1* and pDMAm2*, the

solutions of block copolymers pDMAm−b−pNiPAm1* and pDMAm−b−pNiPAm2* became

turbid upon heating beyond 35 °C (Figure 5). The solution of reference homopolymer pNiPAm*

showed a cloud point CP of 26 °C. Accordingly, the attachment of the two fluorophores at chain

ends reduced the phase transition temperature of about 31−32 °C which is typically reported for

atactic pNiPAm samples of comparable molar mass,18, 21, 22, 86 by a few degrees. As the -end

group with the dodecyl chain and the large naphthalimide fluorophore, i.e. the hydrophobic sticker

group B, is too hydrophobic to affect the CP of pNiPAm,83, 87-91 its lowering for pNiPAm* is

presumably due to the -end group with the coumarin fluorophore. The moderately reduced CP

in comparison with other end-group effects reported, suggests that the effective hydrophobicity of

the -end group is lower than the one of end groups bearing a naphthyl, azobenzene or dodecyl

moiety.92-94

15 20 25 30 35 40 45 50 55 60 65 70 75

100

Transmission [%]

Temperature [°C]

pDMAm-

pNiPAm1*

1 g/L

3 g/L

5 g/L

10 g/L

pDMAm-

pNiPAm2*

1 g/L

3 g/L

5 g/L

pNiPAm*

5g/L

Figure 5. Optical transmittance measurements for aqueous solutions of homopolymer pNiPAm*

(left curve, green), and block copolymers pDMAmbpNiPAm1* (right curves, magenta), and

pDMAmbpNiPAm2* (center curves, grey), for varying concentrations (heating runs). The

onset of decline in transmittance is taken as cloud point.

CP values of the block copolymers pDMAm−b−pNiPAm1* and pDMAm−b−pNiPAm2* were

40 °C and 36 °C, respectively, which are substantially higher than for homopolymer reference

pNiPAm*. The increase is attributed to the covalent attachment of the pNiPAm block to a large

hydrophilic polymer. The numerous reports of this general effect in the literature include also the

specific case of block copolymers made from DMAm and NiPAm.66-68, 94, 95 The effect is the more

pronounced the shorter the pNiPAm block is, as observed for samples pDMAm−b−pNiPAm1*

and pDMAm−b−pNiPAm2*. Also, the CP’s shift with increasing concentration slightly to lower

temperatures for both polymers, again in agreement with the literature for such dilute solutions.20

After the heating run, the polymers separated macroscopically in a polymer rich and polymer poor

phase, so that no cooling cycle was applied. However, under stirring, the polymer dispersions were

stable, and the clouding transitions were found fully reversible. A closer inspection of the evolution

of turbidity with increasing temperature reveals a striking difference between the two block

copolymers. Whereas the clouding transition is sharp and pronounced for

pDMAm−b−pNiPAm2*, indicating the rapid formation of rather large aggregates once the phase

transition temperature is crossed, the turbidity of the solutions of pDMAm−b−pNiPAm1* evolves

in two stages. In a first step, the drop of transmittance is small, before in a second step the solutions

become opaque at about 15 °C higher than CP. Qualitatively, this suggests the formation of small

aggregates initially, which transform into much larger ones only after further dehydration of the

polymer coils at more elevated temperatures.

An analogous behavior was observed in dynamic light scattering (DLS) experiments (Figure 6).

While the solutions of homopolymers pDMAm1* and pDMAm2* showed virtually temperature

independent scattering behavior, the solution of pNiPAm* exhibited a sudden and steep increase

of the polymers Z-average hydrodynamic diameter DH upon heating to 27°C. Also the solutions

of block copolymers pDMAm−b−pNiPAm1* and pDMAm−b−pNiPAm2* showed a marked

increase of the DH upon heating at 41 °C and 38 °C, respectively.

Figure 6. Temperature dependent evolution of the Z-average hydrodynamic radius Dh of polymers

pDMAm1* (), pNiPAm* (), pDMAmbpNiPAm1* (), and pDMAmbpNiPAm2*

() in aqueous solution (5 g∙L-1), followed by DLS (HPPS-5001, Malvern Instrument,

Malvern/UK) in (a) linear and (b) semi-logarimthmic presentation. Dashed lines are meant as

guide to the eye..

Moreover, Figure 6 reveals that independently whether they are permanently hydrophilic or

thermo-responsive, all of the two-fold fluorescently labelled polymers display Dh values of about

25 nm. This suggests the formation of small aggregates due to the surfactant-like structure of the

α-terminal hydrophobic end group.40, 96, 97 When heating beyond the cloud point, homopolymer

pNiPAm* as well as block copolymer pDMAmbpNiPAm2* immediately form large

aggregates (with Dh >1 µm). In contrast, block copolymer pDMAmbpNiPAm1* shows an

apparent two-step transition above the cloud point. Initially, aggregate size increases slowly to a

moderate value (Dh ≈ 125 nm), but rises markedly only above 55 °C. The two-step aggregation

matches the observations on the clouding transitions (Figure 5) discussed above. We attribute the

differing aggregation behavior of the two block copolymers to the considerably shorter thermo-

15 20 25 30 35 40 45 50 55 60 65 70 75

500

1000

1500

Hydrodynamic Diameter [nm]

Temperature [°C]

15 20 25 30 35 40 45 50 55 60 65 70 75

100

1000

lg(D

) [nm]

Temperature [°C]

a) b)

responsive pNiPAm block in copolymer pDMAmbpNiPAm1*. Notwithstanding the possible

differences between the specific aggregation processes, the DLS experiments demonstrate the

temperature-controlled transition of associating BAA* to BAB* systems for the block copolymers

studied, as sketched in Scheme 1.

Fluorescence Spectroscopy of Aqueous Polymer Solutions and Microemulsions

The temperature-dependent fluorescence of the two-fold labelled polymers in aqueous media was

studied with the fixed excitation wavelength of 318 nm (Figure 7). At this wavelength, the

excitation of the donor fluorophore is efficient but of the acceptor fluorophore at a minimum (cf.

Figure 3), and thus, the spectra are sensitive to the FRET process. Figure 7 reveals on a first view

that the spectra of all polymersolvent systems are subject to changes with increasing temperature.

Furthermore, these changes vary markedly not only between the different polymers studied, but

also when pure water as solvent (Figure 7a, c, e) is replaced by a TDMAOdecane microemulsion

(Figure 7b, d, f).

Figure 7: Temperature-dependent fluorescence of solutions (1 g∙L-1 ) of pDMAm1* (a-b),

pNiPAm* (c-d) and pDMAmbpNiPAm1* (e-f) in pure water (a, c, e) and microemulsion (b,

d, f; 190 mM TDMAO in decane).

350 400 450 500 550 600 650 700

Fluorescence Intensity [a.u.]

Wavelength [nm]

350 400 450 500 550 600 650 700

Fluorescence Intensity [a.u.]

Wavelength [nm]

350 400 450 500 550 600 650 700

Fluorescence Intensity [a.u.]

Wavelength [nm]

350 400 450 500 550 600 650 700

Wavelength [nm]

350 400 450 500 550 600 650 700

Wavelength [nm]

a) b)

c) d)

e) f)

350 400 450 500 550 600 650 700

10 20

The spectra were analyzed with respect to the position of the emission maximum of the acceptor

fluorophore, i.e., possible solvatochromism of the naphthalimide, and to the relative emission

intensities of the donor and acceptor chromophores, i.e., possible changes in the extent of FRET

occurring (Figure 8). In pure aqueous solution, we note that the emission maximum of the acceptor

chromophore is located at 561 nm below 25 °C for all polymers studied. This value corresponds

to a surrounding of the chromophore close to pure water.73 Whereas the peak position did not

change between 15 and 65 °C for permanently hydrophilic homopolymer pDMAm1*, a

hypsochromic shift of about 10 nm was observed for thermoresponsive homopolymer pNiPAm*

in this temperature range with increasing temperature (Figure 8a). The value indicates a less,

though still highly polar surrounding of the chromophore at elevated temperatures, which could,

for instance, be equivalent to a mixture of water and N-methylformamide.73 It is also evident that

the solvatochromic shift does not occur linearly, but seems to follow an S-shape with the maximum

slope between 25 and 35 °C, i.e., around the cloud point of pNiPAm*. The combined findings

suggest that the shift is a consequence of the coil-to-globule collapse of the pNiPAm chains and

their concomitant partial desolvation, changing the local environment of the fluorophore from

‘nearly aqueous’ to ‘water-swollen NiPAm groups’. In the case of block copolymer

pDMAmbNiPAm1*, a similar behavior is seemingly observed, though being much less

pronounced. The hypsochromic shift amounts only to about 2 nm, and the maximum slope of the

S-shaped curve seems shifted to higher temperatures, between 30 and 45 °C (Figure 8a). Again,

this behavior can be correlated with the cloud point transition of the block copolymer. The

weakness of the solvatochromic shift compared to the homopolymer may be easily explained by

the much shorter pNiPAm block in the copolymer, as well as by the separation of the collapsed

pNiPAm block and the naphthalimide moiety by the long pDMAm block in between.75

It is interesting to note that the form of the fluorescence spectra of pDMAmbNiPAm1* did not

alter upon dilution down to a concentration of 1 mg L-1 (lowest concentration studied). Neither the

positions of the emission maxima nor the intensity ratio between the donor and acceptor bands

changed. This suggests that the hairy micelles exist still at such low concentrations, and that any

critical aggregation concentration must be lower than 10-7 M, if existing at all.

0 10 20 30 40 50 60 70

550

552

554

556

558

560

562

564

LCST

(pDMAm-b-NiPAm1*)

LCST

(pNiPAm*)

Emission maximum [nm]

Temperature [°C]

0 10 20 30 40 50 60 70

Emission intensity ratio A/D

Temperature [°C]

0 10 20 30 40 50 60 70

0.2

0.3

0.4

0.5

0.6

0.7

Emission intensity ratio A/D

Temperature [°C]

)

Figure 8: Temperature-dependent fluorescence of solutions (1 g∙L-1 ) of pDMAm1* ( ,  ),

pNiPAm* ( , , ), and pDMAmbpNiPAm1* ( ,  ) in pure water ( , ,  ) and in 190

mM TDMAO in decane ( , , ): (a) shift of the acceptor emission maximum with temperature;

(b) intensity ratio of acceptor emission / donor emission ; (c) magnified section of (b).

When analyzing the temperature-dependent relative emission intensities of the donor and acceptor

chromophores, i.e., the extent of FRET of the polymers in pure water (Figure 8b-c), the structure-

dependent effects seem to follow an analogous pattern as discussed for the solvatochromism.

While the extent of FRET is virtually independent of the temperature for the homopolymer

reference pDMAm1*, a strong increase of FRET is seen for the reference pNiPAm* just above

its cloud point. The extent of FRET increases also for block copolymer pDMAmbpNiPAm1*

when crossing the cloud point (Figure 8c), but the effect is much weaker than for pNiPAm*

(Figure 8b).

The picture differs characteristically when the polymers are dissolved in the TMDAOdecane

microemulsion. The position of the emission maximum of the acceptor fluorophore is located at

551 nm for pDMAm1* and pDMAmbNiPAm1* at all temperatures, and also for pNiPAm* at

temperatures above 30 °C. This indicates a less, though still highly polar surrounding of the

chromophore in the presence of the microemulsion’s oil droplets compared to the pure aqueous

solution. Interestingly, below 30°C, the emission maximum of pNiPAm* is further

hypsochromically shifted to 548 nm, indicating an even less polar surrounding at low

temperatures. Concerning the temperature-dependent extent of FRET, the general behavior is

rather similar to the one observed in pure aqueous solution (Figure 8b). The FRET efficiency is

strongly enhanced for pNiPAm* when crossing the cloud point, whereas temperature effects are

small for the permanently hydrophilic reference pDMAm1* and the block copolymer. Figure 8c

shows that overall, at room temperature, the extent of FRET is slightly higher for pDMAm1* and

pDMAmbNiPAm1* in the microemulsion compared to the aqueous solution. Still, a closer

look reveals important differences between pDMAm1* and pDMAmbNiPAm1* in the

microemulsion when heated. On the one hand, the FRET effect increases continuously, albeit very

slightly, for pDMAm1* with increasing temperature, in contrast to its virtual independence in pure

water. On the other hand, the increase of FRET for pDMAmbNiPAm1* when passing the cloud

point is markedly weaker in micromemulsion than in water, and parallels the behavior of

pDMAm1* (Figure 8c).

Combining the findings of the fluorescence studies, the following picture emerges. (i) In water at

ambient temperature, the acceptor dye is not located in the hydrophobic domain of micellar

aggregates, but rather at their interface. The moderately hydrophobic termini with the donor

dye are neither located in the hydrophobic domain of micellar aggregates nor do they tend to

approach the micelles surface, either by backfolding of the polymer chains or by bridging different

micelles. (ii) The solvatochromic effects suggest that the hydrophobic sticker group B has a

marked affinity for inserting into the oil droplets of the microemulsion; (iii) The distance between

 and termini of the permanently hydrophilic polymer is virtually unaffected by temperature.

(iv) The coil-to-globule collapse transition of -two-fold fluorophore labeled polymers

separated by a thermo-sensitive polymer chain, here pNiPAm, increases FRET strongly. Still, it

cannot be distinguished whether the effect indicates an increased backfolding of the termini

with loop formation to form flower micelles, or whether it is mainly due to the inherent contraction

of the chain conformation above CP and the concomitant reduction of the average end-to-end

distance of the chains, thus approaching the donor and acceptor chromophores. (v) The block

copolymer shows little backfolding, if at all. It forms hairy micelle in the BAA* state (cf. Scheme

1b left). In the BAB* state, the extent of backfolding of the B* block might be slightly higher than

in the A* state, but if so, the increase is very limited. The same reasoning applies to the possibility

of bridging micellar cores. Importantly, backfolding is further reduced in the microemulsion

compared to in the pure aqueous solution. This implies that the collapsed pNiPAm segments of

the copolymer have little tendency to assemble onto oil droplet interface, let alone to enter the oil

droplets. Accordingly, the scenario sketched on the lower right side of Scheme 1b seems

predominant. This means, that the polymer micelles are interconnected by clusters of collapsed

pNiPAm chains, eventually forming a network with two different, alternating types of crosslinks

formed by micellar cores (or oil droplets, respectively) and by pNiPAm microdomains.

Static (SLS) and Dynamic Light Scattering (DLS) Results for Aqueous Polymer Solutions and

Microemulsions

More detailed light scattering experiments of the pure polymers in aqueous solution as a function

of temperature in the range from 20 to 60 °C showed distinctly different behavior for the three

types of polymer studied. As expected, the homopolymer reference pDMAm1* shows very little

effect of temperature (just some shift to shorter relaxation times with increasing temperature that

can largely be attributed to the reduced water viscosity) and very similar correlation functions

(Figure 9a), dominated by a rather monoexponential decay and some tailing for longer times. This

shows that small micellar aggregates are present with a hydrodynamic radius Rh of 10-15 nm

(Figure 9b). This is in good agreement with the simple picture of having aggregated dodecyl chains

surrounded by the permanently hydrophilic pDMAm shell. The unchanged state of aggregation is

confirmed by the static light scattering (SLS) intensity measured for concentrations of 1.0 to 10.0

g L-1 from 20 to 60 °C (Figure 9c). No change of aggregation is seen in this temperature range,

and aggregation numbers of 20 to 80 are observed, which generally decrease with increasing

concentration. The tailing seen in DLS might be attributed to overlapping polymer chains, as often

seen for polymers.

In the case of the thermo-responsive reference homopolymer pNiPAm*, the situation is clearly

different. A fast decay of the correlation function is still present, but slower than for pDMAm1*.

For lower temperature, a second, even slower decay is noted (Figure 9d). This can be interpreted

such that the 10-15 nm Rh sized micelles are still seen, but with a pNiPAm shell instead of a

pDMAm shell. They are partly aggregated, presumably in dynamic equilibrium, into clusters of

about 800 nm in radius (Figure 9e; note that the results were obtained with a different instrument

than those in Fig.6, focussing on correctly measuring longer correlation times. For the data

displayed in Figure 6, measurements simply run out of the experimental size observation window

of the instrument). This must be due to attractive forces arising from the presence of the pNiPAm

block.97 With increasing temperature, pNiPAm is expected to shrink in size and lead to stronger

attractive interactions. This is manifested by the decreasing value of Rh which reaches a value of

4050 nm for 55 °C (Figure 9e). SLS (Figure 9f) shows that single micelles are present still at 20

°C. However upon reaching 25 °C, the pNiPAm shall provides sufficiently attractive interactions

to induce clustering. Figure 9f also demonstrates that the clusters retain their size as a function of

temperature. Only for the highest concentration of 10 g L-1, some growth occurs above 45 °C. This

suggests that the shrinkage seen by DLS corresponds to a compaction of the formerly more loosely

connected micellar aggregates.

Strikingly, the diblock copolymer pDMAmbNiPAm1* shows a behavior somewhere in

between of the two homopolymers. In DLS (Figure 9g), monomodal decay is observed, which

becomes much slower for temperatures above 40 °C. At lower temperatures, we see in Figure 9h,

as for the homopolymer references, small individual micellar aggregates with Rh values of 1015 nm.

These agglomerate at higher temperature into clusters with Rh values of 150250 nm, increasing

in size with rising temperature. Apparently, the pDMAm block reduces the attractive interaction

exerted by the pNiPAm block, and clustering occurs only above the phase transition temperature

of the thermo-responsive block. This is mirrored by the SLS results (Figure 9i) which show a

constant aggregation number of about 20 until 35 °C, irrespective of the concentration. Above 35

°C, the aggregation increase number substantially, approaching a constant value for temperatures

above 45 °C. The total aggregation number of ~2000 macromolecules corresponds to a situation

where about 100 of the copolymer micelles are contained within bigger clusters of Rh = 150-250

nm. This means that these are rather weakly compacted aggregates.

Figure 9. Field correlation curves obtained from DLS measurements (LSinstruments) (a):

pDMAm1*, (d): pNiPAm*, (g): pDMAmbNiPAm1*) and corresponding intensity weighted

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.1

0.2

0.3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.1

0.2

0.3

-8

-6

-4

-2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.1

0.2

0.3

15 25 35 45 55 65

25°C

35°C

45°C

55°C

(a)

25°C

35°C

45°C

55°C

(b)

(1)

(90°)

(d)

intensity weights (90°)

(e)

t / s

(g)

/ nm

(h)

10 g L

-1

5.0 g L

-1

3.0 g L

-1

1.0 g L

-1

(c)

eff

agg

(f)

temperature / °C

(i)

size distribution of the pure polymer solutions ( (b): pDMAm1*, (e): pNiPAm*, (h):

pDMAmbNiPAm1*), obtained from the measurement at 90° and a concentration of 3.0 g L-1.

The corresponding fit curves of the correlation functions are shown as solid lines. In addition, we

show the effective aggregation number calculated based on the estimated forward scattering

intensity I(0) obtained via a Guinier fit, by using eq. 2, as a function of temperature ( (c):

pDMAm1*, (f): pNiPAm*, (i): pDMAmbNiPAm1*).

For the microemulsion based systems, the DLS measurements showed basically a negligible effect

of the presence of the different polymers, except for the case of pNiPAm* (Table 3, the

corresponding autocorrelation functions are shown in Figure S6). This can be interpreted such that

the lifetime of binding of an individual micromulsion droplet in the clusters is shorter than the

relevant measurement time, which is in the range of ~1 ms. Accordingly, their diffusion occurs

rather freely. Only for the polymers with a pNiPAM shell, the hydrophobic binding becomes

apparently strong enough at high temperature so that the clustering of the microemulsion droplets

becomes so extensive, that their diffusion is substantially slowed down and clusters of a mean

radius of ~ 100 nm are generated.

Table 3. Collective diffusion coefficient D and hydrodynamic radius Rh for the pure

TDMAO/decane o/w microemulsion (ME) and its mixtures with the different polymers. Data are

obtained from the corresponding simple exponential fit of the initial slope of the autocorrelation

function. Rh was calculated by the Stokes-Einstein relation. Errors of the last digit are given in

parentheses.

Sample ME ME + pDMAm1* ME + pNiPAm* ME + pDMAm-b-

NiPAm1*

25°C 55°C 25°C 55°C 25°C 55°C 25°C 55°C

D / µm2 s-1 76.8(4) 113(2) 65.2(6) 144(2) 64.4(6) 4.59(5) 66.2(5) 149.2(9)

Rh / nm 3.20(9) 4.24(13) 3.76(11) 3.31(9) 3.81(12) 103(4) 3.71(11) 3.20(9)

In summary, it can be stated that the tendency for clustering of these micellar aggregates depends

on temperature and can be largely controlled via the type of block attached. For the purely

hydrophilic pDMAm sample, no clusters are observed. In contrast, clustering takes place for the

pNiPAm sample already above 20 °C, i.e. below the cloud point, with a marked tendency for

shrinkage with increasing temperature. The diblock pDMAmbNiPAm* system requires

temperatures above 35 °C, i.e., to reach the coil-to-globule transition of the thermo-responsive

block, to undergo clustering of its micelles, and the cluster size increases somewhat with increasing

temperature.

CONCLUSIONS

Nonionic water-soluble polyacrylamides labelled with complementary fluorophores at the

opposite termini of the chains were synthesized, which are suited for intramolecular FRET. The

precise placement of the fluorophores is conveniently achieved by using a bi-functional

trithiocarbonate (FRET-TTC) as chain transfer agent in RAFT polymerizations. Employing a

coumarin of the methylumbelliferone family as donor and a 4-aminonaphthalimide as acceptor

groups, the polymers show effective FRET with a large spectral shift between excitation (at 318

nm) and emission (at 560 nm) wavelengths. Moreover, the marked hydrophobic character

supported by an n-dodecyl substituent renders the acceptor chromophore effective as hydrophobic

end group (‘sticker’) for designing polymeric surfactants. Two-fold labelled macroRAFT agents

pDMAm*, which were first synthesized from N,N-dimethylacrylamide (DMAm) using FRET-

TTC, could be successfully chain extended by N-isopropylacrylamide (NiPAm). The pNiPAm

block of the produced dual hydrophilic block copolymers is thermo-responsive, showing an LCST

transition in aqueous media. Due to the hydrophobic sticker group containing the acceptor

fluorophore, these block copolymers pDMAm-b-pNiPAm* represent ‘smart’ polymeric

surfactants. At low temperature, they behave as B-A-A* systems forming hairy micelles. At

elevated temperatures, above the LCST-type phase transition of pNiPAm switching its character

from water-soluble to insoluble, they behave as associative telechelics BAB* with unsymmetrical

hydrophobic end blocks (A, A*= hydrophilic block, B, B* = hydrophobic block). This allows for

switching the surfactant properties significantly by a thermal stimulus, inducing e.g. the formation

of large aggregates as required for associative thickeners. Interestingly, the findings from

fluorescence experiments suggest that the aggregation at elevated temperatures is not due to the

conventional bridging of flower-micelles by the BAB*-type polymers. In fact, FRET gives no

evidence for increased backfolding or bridging by the polymer chains above the coil-to-collapse

transition of the pNiPAm block. Instead of, it appears that hairy micelles persist, which are

connected to larger assemblies by separate microdomains of collapsed pNiPAm blocks at the end

of the ‘hairs’. This picture is confirmed by light scattering experiments, which show that the

presence of the pNiPAm block alone is sufficient to induce cluster formation of the micelles

present above 20 °C, where the clusters shrink substantially with increasing temperature. In

contrast, the BAB*-type polymer shows the more typical pNiPAm transition around 35 °C, also

leading to clusters of micelles. However in this case, the clusters increase somewhat in size with

increasing temperature. In general, the aggregation behavior of these polymers can be tuned largely

by temperature, and such a behavior can be expected to boost the thickening and gelling efficiency

of such block copolymers compared to conventional BAB associative thickeners. Accordingly,

such unsymmetrically end-capped thermo-responsive water-soluble polymers represent a

promising design for both ‘smart’ and particularly efficient rheology modifiers. The tendency for

the particular structure formation of these polymers is enhanced when exchanging pure water as

solvent by an o/w microemulsion.

ASSOCIATED CONTENT

Supporting Information.

The supporting information is available free of charge at

https://pubs.acs.org./doi/10.1021/acs.langmuir.xyz

SEC elugrams of the polymers studied, solution 1H NMR spectra of diblock copolymer pDMAm-

pNiPAm2* and key intermediates in the synthesis of chain transfer agent FRET-CTA, and field

correlation functions of the polymers in the microemulsion in dependence on the correlation time

τ together with decay rates as function of scattering vector q at 25 and 55 °C (PDF)

AUTHOR INFORMATION

Corresponding Authors

* Prof. Michael Gradzielski, E-mail: michael.gradzielski@tu-berlin.de, Address: Technische

Universität Berlin, Stranski-Laboratorium für Physikalische und Theoretische Chemie TC7, FG

Physical Chemistry/ Molecular Material Science Institute of Chemistry, Strasse des 17. Juni 124,

10623 Berlin, Germany

* Prof. André Laschewsky, E-mail: laschews@uni-potsdam.de, Address: Universität Potsdam,

Institut für Chemie, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam-Golm, Germany

AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors. All authors have given approval

to the final version of the manuscript.

FUNDING SOURCES

This work was supported by Deutsche Forschungsgemeinschaft (DFG), grants GR 1030/22-1 and

LA 611/17-1.

ACKNOWLEDGMENT

The authors gratefully acknowledge support for fluorescence spectroscopy by K. Brennenstuhl

and M. Kumke (Universität Potsdam) as well as by A. Gessner and in particular O. Sakhno

(Fraunhofer IAP). They also acknowledge the support for SEC analysis by S. Prenzel and H.

Schlaad, and for thermal analysis by D. Schanzenbach (all Universität Potsdam).

CONFLICT OF INTEREST

The authors declare no conflict of interest.

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ToC Table of Content graphics

FRET

no FRET