Emulsion copolymerization of 2-methylene-1,3-dioxepane (MDO) and acrylate monomers: Incorporation vs hydrolysis

Polymer 307 (2024) 127285 Available online 11 June 2024 0032-3861/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Emulsion copolymerization of 2-methylene-1,3-dioxepane (MDO) and acrylate monomers: Incorporation vs hydrolysis Fabian Wenzel 1 , Miren Aguirre ** , Jose R. Leiza * POLYMAT and Kimika Aplikatua Saila, Kimika Fakultatea, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Hiribidea 72, 20018, Donostia-San Sebastian, Spain ARTICLE INFO Keywords: 2-Methylene-1,3-dioxepane (MDO) Acrylate monomers Emulsion copolymerization Hydrolysis of MDO ABSTRACT The incorporation of polyester moieties to the backbone of waterborne polymer dispersions by the emulsion copolymerization of 2-methylene-1,3-dioxepane (MDO) and acrylate monomers (n-butyl acrylate and 2-octyl acrylate) is explored in this work. Due to the competition between the consumption of MDO by hydrolysis (MDO is very sensitive to water) and by copolymerization reactions, seeded semibatch emulsion copolymerizations were carried out varying the temperature of the reaction, the feeding rates of the comonomers, and maintaining a basic pH (~pH =8) that decreases notably the rate of hydrolysis of the MDO. Interestingly, we found that the lower the temperature of the reaction (T =20 ◦C) and the higher the monomer addition flow rate, allowed the highest incorporation of MDO in the copolymer (up to 86 mol %). Unfortunately, the MDO incorporated in the closed formed (i.e., in the acetal form) and therefore, the acrylate copolymers produced did not contain ester groups in the backbone. Furthermore, increasing the MDO composition in the copolymers under the same polymerization conditions favored more hydrolysis than copolymerization and hence, incorporation content decreased. On the other hand, the ester size (2-octyl vs n-butyl) of the acrylate did not have any noticeable effect in the MDO incorporation content. 1. Introduction The global environmental crisis, together with the depletion of fossil resources and the governmental policies towards carbon neutrality has given rise to an increasing attention towards degradable and/or biobased polymers. The emulsion copolymerization of cyclic ketene acetals (e.g., MDO) with other vinyl monomers is a very appealing route to produce degradable polymer dispersions that might find application in materials or liquid polymer formulations where the term of use is short and their recyclability is not viable [1]. Therefore, there is a strong interest in industry and academia to synthesize latices with backbone degradable polymer particles [2]. However, the polymerization of cyclic ketene acetals in waterborne systems is challenging due to their high water sensitivity, which leads to rapid hydrolysis [3–6]. Furthermore, MDO polymerizes following two different routes, either by ring opening or by ring retention (see Scheme 1). The MDO unit that undergoes ring opening is incorporated into the polymer backbone with an ester unit, providing a degradable unit to the polymeric chain. However, when the MDO unit is incorporated as ring retention, an acetal unit is added to the polymer chain without adding any degradable unit. The ratio of ring opening to ring retention is strongly influenced by the polymerization parameters, mainly by the temperature [7–9]. It is reported in literature that increasing the temperature increases the ratio of ring opening to ring retention [8]. For instance, at 70 ◦C 28 % of the MDO was incorporated into a nBA/MDO-copolymer (by solution polymerization in cyclohexane) as ring opened units, whereas at 90 ◦C 34 % were incorporated as ring opened units. Furthermore, it is also stated that large alkyl acrylates also favor the ring opening of the MDO [8]. The literature reports about the incorporation of MDO (or other CKAs or monomers like cyclic vinyl acetals [5], and biphenilic cyclic thionolactones [10,11]) into waterborne polymeric dispersions are really scarce. Landfester et al. [12] were the first ones copolymerizing the cyclic ketene acetal 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) with methyl methacrylate (MMA) and styrene by miniemulsion polymerization to obtain biodegradable drug carriers. They showed that * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Aguirre), [email protected] (J.R. Leiza). 1 Currently at tesa SE, Hugo-Kirchberg-Straße 1, 22848 Norderstedt, Germany. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer https://doi.org/10.1016/j.polymer.2024.127285 Received 19 February 2024; Received in revised form 4 June 2024; Accepted 10 June 2024 Polymer 307 (2024) 127285 2 depending on the amount of BMDO in the copolymer, a significant decrease of the weight average molar mass of the copolymer after treatment with either the enzyme Lipase PS or concentrated potassium hydroxide solution (50 %–5 % residual Mw after 48 h) was observed. Furthermore, the feasibility of the nanoparticles to act as drug carriers was proven by the incorporation and release of the cytostatic drug paclitaxel. Apart from the medical application as drug carrier, latices with backbone degradable particles are also of interest in other fields [13]. One of them is the usage as coatings for single use paper products, which are often used for the packaging of food, such as paper boxes, wrappers, etc. These are in many cases coated to increase the mechanical properties and the oil/grease resistance. However, when these food-packaging products are contaminated, it becomes difficult to recover and recycle the underlying fibers, preventing potential pathways for degradation. Sustainability could be increased by coatings made of (bio)degradable latices [14]. With this motivation Carter et al. described the emulsion copolymerization of MDO with vinyl acetate [15,16]. They reported that mildly basic conditions (pH =8) and low temperatures (40 ◦C) are essential to prevent the rapid hydrolysis of MDO. Furthermore, they demonstrated the degradability of the MDO units containing co-polymers by hydrolysis of the ester groups in a basic aqueous solution with a bicarbonate buffer and a pH value of 10. Within 50 days, they observed a mass loss of ⁓50 % for dried films of VAc-co-MDO copolymer. Recently, Kordes et al. [5] presented an extensive analysis of the hydrolysis kinetics of MDO in water, particularly at standard conditions during an emulsion polymerization process. Furthermore, based on the optimal temperature and pH conditions found to reduce the hydrolysis of MDO, they conducted emulsion copolymerization of MDO and VAc and concluded, contrary to what it was reported by Carter et al. [15], that only small amounts of MDO were incorporated in the copolymer and that the majority was hydrolyzed. They also pointed out that the fragments of the degradation of the polymers under basic pH were too large to be metabolized by microorganisms. Recently, Carter et al. [17] in view of the discrepancies found by Kordes et al. [5] to reproduce their experiments, conducted a series of experiments to proof that the optimal experimental conditions described in their original manuscript (i.e., T =40 ◦C, pH =8 (or higher) and high instantaneous conversions of VAc and MDO), if controlled carefully during the emulsion polymerization process, do guarantee a very limited hydrolysis of MDO (or incorporation of MDO higher than 85 %). Furthermore, they carried out experiments mimicking poor control of temperature and pH and the mode of reductant addition on the incorporation amount of MDO. They found that small deviations in pH, temperature and the addition of the reductant yield lower incorporation of the MDO, emphasizing the necessity of a tight control of these variables for a successful incorporation of ester moieties in the backbone and hence, the production of degradable copolymers by emulsion polymerization. On the other hand, Galanopoulo et al. [18] recently reported the ab-initio emulsion polymerization of BMDO and MMA carried out at 70 ◦C and basic pH (around 8). They found that basic conditions are necessary to incorporate BMDO into the copolymer, and the higher the initial fraction of BMDO the lower its incorporation (between 64 % and 100 % for fractions between 10 mol% and 2 mol% of BMDO in the formulation). Accelerated degradation experiments of the poly(MMA-- co-BMDO) copolymers showed that degradation of at least 80 % of the chains containing BMDO occurred. All works discussed above [5,12,15–17,19] have attempted to incorporate MDO into waterborne polymer dispersions using strategies to overcome the hydrolysis of MDO (low temperatures, alkaline pH, high instantaneous conversions, hydrophobic comononomers and miniemulsion polymerization among others). However, very recently, Mothe et al. [20] have reported that by means of batch emulsion copolymerization of MMA and MDO using alkaline pH (higher than 10), temperatures of 70 ◦C, neutral (i.e., non-ionic) surfactants, and 5 mol% of a hydroxy-functional costabilizing monomer (Hydroxy ethyl acrylate, HEA), degradable latexes can be produced. The authors claim that it is essential to use neutral emulsifiers to incorporate MDO in emulsion polymerization together with alkaline pH. The high pH’s used (pH =10) reduced further the rate of hydrolysis of MDO although this pH can be harsh for some monomers that also suffer hydrolysis in alkaline conditions like VAc. Nonetheless, it is worth noting that more than 50 % of the MDO hydrolysed in their batch emulsion polymerizations, likely due to the relatively high temperatures (T =70 ◦C) employed. On the other hand, D’Agosto et al. reported the emulsion copolymerization of the thionolactone monomer dibenzo[c,e]oxepane-5thione (DOT) [21], which was first proposed by Roth et al. [22] and Gutekunst et al. [23], with butyl acrylate and/or styrene. DOT, similar to MDO, also undergoes rROP leading to in-chain thioester function [10]. Unlike MDO, DOT is assumed to present favorable reactivity ratios with styrene or acrylates, and do undergo complete ring opening (no ring retention). Low contents of DOT between 2–5 mol % were sufficient to obtain degradable copolymers. Degradation through isopropylamine and 1,5,7-triazobicyclo[4.4.0]dec-5-ene (TBD) was demonstrated. Due to the growing interest in industry to produce more sustainable polymer latexes, the synthesis of waterborne degradable copolymers is a hot topic. Therefore, the research on the synthesis of new monomers able to polymerize by emulsion polymerization, and that will readily copolymerize with common oil-based or novel bio-based monomers together with the understanding and process optimization of already described monomers is of paramount importance in the field [24]. In this context, this work assesses the copolymerization of MDO in emulsion polymerization. From the current understanding of the interaction of MDO and water, it is clear that to achieve waterborne polymer particles with a degradable backbone by emulsion copolymerization of MDO with other monomers, the rate of consumption of MDO by polymerization (a function of the radical concentration in the polymerization loci and of the comonomer used) must be faster than the rate of its hydrolysis (which is a function of the temperature and pH or the presence of protic reagents) [5,20]. In this work, the kinetics of the hydrolysis of MDO in aqueous media under different conditions have been evaluated and compared with the results already published in the recent literature. Then, these results are used to guide in the selection of the operation conditions to favor the incorporation of MDO in emulsion copolymerization. 2. Experimental section The experiments for the hydrolysis of MDO and the copolymerization of MDO with other comonomers in waterborne systems are described. Scheme 1. MDO is incorporated to a growing polymer chain either by radical Ring Opening Polymerization (rROP) (a) or by ring retention radical polymerization (b). F. Wenzel et al. Polymer 307 (2024) 127285 3 2.1. Materials Ethylenediaminetetraacetic acid (EDTA, Sigma Aldrich), ammonium hydroxide solution (Sigma Aldrich), ammonium peroxodisulfate (APS, Sigma Aldrich), potassium hydroxide (KOH, Sigma Aldrich), Brüggolit FF6 (Brüggemann Chemical), iron(II)sulfate (Sigma Aldrich), n-butyl acrylate (n-BA, Sigma Aldrich), 2-octyl acrylate (2-OA, BASF), methylene-1,3-dioxepane (MDO, Wacker), Dowfax 2A1 (Dow Chemical), Disponil AFX 1080 (BASF) were used as supplied. d-Chloroform (Sigma Aldrich) and deuterated water (Sigma Aldrich) were used as deuterated solvents in the NMR analysis. A commercial acrylic seed with a particle size of 65 nm was used. 2.2. Characterization techniques The characterization of the copolymer samples withdrawn from the reactor was carried out by 1 H and 13 C NMR spectroscopy using the Bruker spectrometers AVIII 300 MHz and AVII+500 MHz with the sample heads of 10 mm BBO and 5 mm BFO, respectively. Deuterated chloroform was used as solvent. Particle sizes were analyzed by disc centrifuge (Lumisizer) at 10 ◦C at a wavelength of 470 nm. pH values were determined with a pH-meter (F20, Mettler Toledo) and the solids content measurement was carried out by a thermo-balance HB43–S (Mettler Toledo) for the calculation of conversions. 2.3. Hydrolysis studies of MDO The hydrolysis studies of MDO were done adding 100 mg of D 2 O and 600 mg of an aqueous KOH solution adjusted to pH =10 or pH =8 into a Wilmad® NMR tube with a length of 18 cm, and a diameter of 5 mm (wall thickness of 0.43 mm). The tube was then heated to the desired temperature (30, 40 or 50 ◦C), and when the temperature was stable, 50 μ L of MDO were added to the NMR tube with an Eppendorf pipette. The liquid 1 H NMR spectra were acquired via NMR measurements with a 500 MHz Bruker Avance NMR instrument equipped with a Z gradient broadband observe (BBO) probe. Scans were performed every 5 min after 2 min of stabilization of the reaction mixture. The number of scans was one for each of the measurements with a relaxation delay of 10 s, a pulse width of 14 μ s and an acquisition time of 2.23 s. 2.4. Seeded semibatch emulsion copolymerization of MDO Seeded semibatch emulsion copolymerization of MDO with the comonomers nBA and 2OA was carried out in a 0.5 L glass reactor with an anchor type stirrer (200 rpm) and under nitrogen atmosphere. The temperature of the reactor was controlled by a Huber Unistat Petite Fleur thermostat unit and a Pt100 sensor in the reactor. A commercial acrylic seed with a particle size of 65 nm was used as the initial charge together with distilled water, ammonia to adjust the pH and EDTA as one part of the catalyst for the redox initiator system. The redox initiator system consisted of FF6, APS, EDTA and Iron(II) sulfate (FeSO 4 ) in ratios that were already described by Kohut-Svelko et al. [25]. When the desired reaction temperature was reached APS, Iron(II)sulfate and FF6 were added as shots, each of them dissolved in distilled water. Subsequently, two feeding streams were started. One containing neat monomer (which was nBA or 2-OA) and the other one, an aqueous solution consisted of surfactants Dowfax 2A1 and Disponil AFX1080, with further FF6 and distilled water. The feeding times were varied in the range 60–180 min. The pH value was adjusted by the addition of ammonia during the reaction (a part of it was added initially and another part was fed in the aqueous solution) and the evolution of the pH value over the reaction time was measured by a pH-meter. Afterwards, the reaction temperature was kept for further 30 min. Then, the reaction temperature was raised to 80 ◦C for further 30 min. Finally, the reactor was cooled down to 25 ◦C. The detailed formulation of the seeded semibatch emulsion copolymerizations is shown in Table 1. 3. Results and discussion 3.1. Kinetics of the hydrolysis of MDO At first, the hydrolysis of MDO to 4-hydroxybutyl acetate (4-HBA) (Scheme 2) under different conditions was studied to find the best parameters for its prevention and therefore, for the successful incorporation of MDO by emulsion copolymerization. Hydrolysis experiments were carried out by in situ 1 H NMR to track the disappearance of the MDO protons and the appearance of the protons of the hydrolysis product 4-HBA. Fig. 1 presents the spectra for the evolution of the hydrolysis reaction (carried out at 30 ◦C and pH =8) monitored online with the assignment of the main peaks corresponding to MDO and 4-HBA protons. Recently, Kordes et al. [5] have extensively analyzed the hydrolysis reaction of MDO in homogeneous and heterogeneous conditions. They note that due to the low water solubility of MDO, monitoring the progress of the hydrolysis can lead to large inaccuracies due to phase separation. Therefore, in order to get homogenous conditions, they used ethylene carbonate as solvent, not only because it guarantees homogeneity, but also because it does not react with MDO, unlike other solvents. They found that in the range of temperatures and pH used (25–40 ◦C and 7–9, respectively), increasing temperature hydrolysis accelerates and increasing pH slows down the hydrolysis reactions. Another interesting remark from this work is that the hydrolysis reaction is more complex than the one depicted in Scheme 2; namely, that although the main product of the hydrolysis is 4-HBA, they also detected (not in the 1 H NMR analysis, but in 13 C NMR and gas chromatography measurements) the formation of acetic acid, 1,4-diacetoxybutane and 1,4-butanediol. Although, they proposed a plausible reaction mechanism for the formation of these compounds due to the very low amounts or their complete absence in certain conditions, the hydrolysis of MDO was monitored by tracking the evolution of the 4-HBA peaks (1, 62-1,74 ppm) and MDO (3,0–3,1 ppm). The peak shifts of Fig. 1 corresponding to the hydrolysis of MDO in a mixture of D 2 O/H 2 O carried out in this work are in reasonable agreement with the peaks reported by Table 1 Formulation for the seeded semibatch emulsion copolymerizations of MDO and nBA and 2-OA acrylate monomers. Component Amount [g] Initial charge Seed Dispersion 20 EDTA 0.025 Ammonia 0.5 Water 110 Shot at reaction temperature APS 0.84 FF6 0.21 FeSO 4 0.01 Water 20 Monomer feed MDO 7/14 Co-monomer 133/126 Aqueous solution Dowfax 2A1 3.11 Disponil AFX1080 0.44 FF6 0.63 Ammonia 0.65 Water 70 Scheme 2. Hydrolysis of MDO to 4-HBA. F. Wenzel et al. Polymer 307 (2024) 127285 4 Kordes et al. [5], but slightly shifted to higher ppms, which can be due to the absence of ethyl carbonate solvent in our experiments. Therefore, the evolution of the conversion was calculated by either the peaks of MDO that decreased or by the new peaks that appeared that correspond to 4-HBA. Further details of the calculation of the hydrolysis extent from the 1 H NMR spectra is provided in the supporting information (SI, section 1). Fig. 2 displays the extent of hydrolysis at the different conditions investigated (for instance, the temperature was varied between 30 and 40 ◦C and the pH between 8 and 10). It can be seen that after 1 h, at pH =8 and 30 ◦C, all the MDO was converted to 4-HBA, or in other words, all the MDO was hydrolyzed. On the other hand, for the cases in which pH =8 and 40 ◦C and pH =10 and 30 ◦C, the appearance of 4-HBA was much faster, total hydrolysis was achieved in 40 min. Although at the conditions of the hydrolysis reaction carried out in this work, the concentration of MDO (7.1 wt% in this work and 2.5 wt% in reference Kordes et al. [5]) exceeded its solubility in water, the kinetics of hydrolysis are in reasonable agreement. For instance, in the work of Kordes et al., at pH =8 and at 40 ◦C the MDO was completely hydrolyzed in 50 min while in this work, complete hydrolysis occurs at 40 min. As mentioned by Kordes et al. this is in contrast with the substantially slower hydrolysis rates reported by Carter et al. [15]. It needs to be mentioned that the conditions for the hydrolysis experiment of MDO by Carter et al. were different. They dissolved MDO first in DMSO‑d 6 and did the experiments changing the ratios of DMSO‑d 6 /D 2 O solutions between 10/1, 100/1 and 1000/1. They found out that the rate of hydrolysis of MDO was increasing with the fraction of deuterated water in the solution. Mothe et al. [6] have also comprehensively investigated the reaction mechanism and reaction kinetics of the hydrolysis of CKAs, covering 5-membered to 8-membered rings. They found that hydrolysis occurred at acid, neutral and basic conditions but the rate was lower at basic conditions. For MDO in pure water they found that at pH =10 and 2 h complete hydrolysis required at least 40 ◦C, whereas at 30 ◦C only 55 % was hydrolyzed, which do not agree with our and Kordes et al. [5] observations even though the concentration used was 5 wt%, which is between the one used in this study and the one of Kordes and co-workers. 3.2. Seeded semibatch emulsion polymerization of MDO and acrylate monomers According to the information gathered in this work and reported in the literature on the effect of pH and temperature in the hydrolysis kinetics of MDO in water, it can be concluded that it is possible to slow down the hydrolysis of MDO in water by adjusting the pH to a slightly basic value (pH ≥8) and working at low temperatures (e.g., 30 ◦C). In the case of the emulsion (co)polymerization of MDO, the consumption of MDO is a competition between its hydrolysis (once in contact with water) and its consumption through copolymerization reactions (that occurs in the polymer particles) as described in Scheme 3. Pathway (a) in Scheme 3 shows the hydrolysis of MDO to 4-HBA. In all the other cases, a radical is added to the MDO monomer at first, generating a MDO radical. The radical then has the possibility to Fig. 1. 1 H NMR spectra for one of the in situ hydrolysis experiments of MDO (pH =8 and 30 ◦C). With assignment of the protons of MDO and 4-HBA to the signals (further details can be found in the supporting information, section 1). Fig. 2. Evolution of the molar fraction of 4-HBA determined by 1 H NMR spectra during the hydrolysis experiment of MDO at different conditions (details on the calculation of the 4-HBA fraction provided in the supporting information, section 1). F. Wenzel et al. Polymer 307 (2024) 127285 5 undergo ring opening to form an ester group in the main chain of the active radical (b). In the other two possibilities, the MDO radical cannot open before another vinyl group is added to it, either from another MDO molecule (c) or from a co-monomer molecule (nBA) (d). The polymeric backbone formed in cases (c) and (d) is the one called ring retention. Therefore, in order to favor path (b) (in Scheme 3), it was tried to minimize the hydrolysis rate or to increase the polymerization rate of MDO. Note that increasing the latter might also favor path (c) and (d), which are not desired. According to equation (1), the polymerization rate of MDO in an emulsion copolymerization is proportional to the propagation rate coefficients (kpi j), the MDO concentration in the polymer particles ([MDO]P), the average number of radicals per particle (n), and the total number of particles (NP). RpMDO =(kpMDO MDOPp MDO +kpi MDOPp i)[MDO]P n NA NP(1) Where, Pp i is the probability of the radical to be terminated in a monomer unit of type i, which is a function of the reactivity ratios of the comonomers, and N A is Avogadros’ constant. Thus, the higher the number of particles, the higher the polymerization rate. One approach to ensure a high number of polymer particles in a reproducible way in emulsion polymerization is to use a preformed seed. The smaller the particle size, the higher the number of particles for the same solids content. Carter et al. [17] used a similar strategy but in-situ generating the polymer particles at the onset of the semibatch reaction, including all the redox initiator pair in the initial reactor load. On the other hand, the rate of hydrolysis of MDO is governed by the hydrolysis rate coefficient, khydro, that is temperature and pH dependent and the concentration of MDO in the aqueous phase, Rhydro,MDO =khydro[MDO]w(2) Thus, seeded semibatch emulsion copolymerization experiments of MDO and acrylate monomers were carried out varying the flow rates of the co-monomers, the type of co-monomer, and the temperature while maintaining constant the number of polymer particles and the pH in the experiments. Table 2 presents a summary of the experiments carried out with acrylate co-monomers (nBA and 2-OA) and the most relevant information of the reaction and final latices synthesized. Fig. 3 shows the time-evolution of the instantaneous monomer conversion and pH evolution of three representative experiments. The instantaneous conversions are higher, the higher is the temperature of the reaction (for feeding time of 60 min reactions), and the higher is the feeding time (for the 30 ◦C reactions). Final conversions are above 95 % in all the cases, indicating the good radical generation efficiency of the redox pair at the low reaction temperatures used. Furthermore, the pH of the reactions was maintained around 8 for the whole reaction as displayed in Fig. 3b. 13 C NMR measurements were used to investigate if the MDO was incorporated into the copolymer (path b-d in Scheme 3) and if the incorporation occurred in the ring open or closed forms (path b vs c-d). Representative 13 C NMR spectra for three experiments (one for the homopolymerizaton of nBA and two of the copolymerizations with MDO at the 95/5 ratio carried out at 30 ◦C and feeding times of 60 and 180 min) are shown in Fig. 4. Peaks 1–6 indicate the generation of the hydrolysis product 4-HBA and peaks a-f belong to the nBA units of the polymer (see supporting information section 2 for a detailed assignment of the peaks). Additionally, the fact that the ratio of the intensities between peaks 1,4 and 5 is 1:1:1, indicates that there are no ring-opened MDO units present in the copolymer. Peak 1 exists only in the hydrolysis product, whereas, peak 4 and 5 would also correspond to the ring-opened ester units within the copolymer. The appearance of peaks x and y in the two spectra for the experiments in which MDO was used, indicates the incorporation of MDO into the copolymer in a ring retained form (see Schemes 1 and 3). Table 2 summarizes the information of the percentage of incorporation of MDO for all experimental conditions tested in this work for the nBA/MDO comonomer system. Note that none of the 13 C NMR spectra for the nBA/MDO copolymerizations, show any indication of the incorporation of MDO in the open formed. Hence, the reported values in Table 2 correspond to the amount of MDO incorporated in the closed formed. Table 2 shows that for the same reaction temperature (30 ◦C) decreasing the feeding time (or increasing the flow rate) the incorporation of MDO increased from 26 % to 76 %; namely, the hydrolysis was reduced by increasing the polymerization rate that increased with higher monomer concentrations in the polymer particles during the reaction (in agreement with eq. (1)). Furthermore, the incorporation increased to 86 % for the same feeding time when the reaction temperature was reduced to 20 ◦C. This means that the reduction of the temperature in this range strongly affected the hydrolysis rate and only mildly to the polymerization rate, and hence the MDO incorporation was favored over its hydrolysis. This is also illustrated in the series of experiments carried out with a feeding time of 60 min and temperatures in the range 20–50 ◦C. Increasing the temperature, although increases the polymerization rate, also increases hydrolysis rate in an extent that exceeds the former and therefore, the incorporation of the MDO decreases drastically to values circa 8 % at 50 ◦C. Table 2 also presents an experiment at higher MDO ratio in the copolymerization (nBA/MDO = 90/10) carried out at 30 ◦C and 60 min feeding time. Interestingly, the Scheme 3. Possible pathways which MDO can undergo in the emulsion copolymerization with nBA. Table 2 Reaction temperature, feeding time, final conversion, average particle size, pH and MDO incorporated as acetal units into the copolymer (with respect to the total amount of MDO used as monomer) for the seeded semibatch emulsion copolymerization reactions between acrylate monomers and MDO. Co-monomer ratio T (◦C) Feeding time (min) Conv. (%) d p (nm) pH Acetal units (%) a BA/MDO 95/ 5 30 180 97 123 8 26 BA/MDO 95/ 5 30 60 97 121 8 76 BA/MDO 95/ 5 20 60 96 132 8 86 BA/MDO 95/ 5 40 60 98 126 8 16 BA/MDO 95/ 5 50 60 98 134 8 8 BA/MDO 90/ 10 30 60 95 116 8 56 2-OA/MDO 95/5 40 180 94 160 8 0 a See supporting information section 2 for details of the calculation. F. Wenzel et al. Polymer 307 (2024) 127285 6 incorporation of MDO is lower (it decreases from 76 % to 56 %) likely because the increase on the MDO polymerization rate does not compensate the increase amount of MDO that hydrolyzes (note that increasing the molar fraction of MDO from 5 to 10 mol% implies an increase of 45 % of the polymerization rate of MDO considering the reported reactivity ratios for nBA and MDO [8] and arbitrary propagation rate coefficients for nBA and MDO of 10000 and 1000 L/mol⋅s, respectively). An additional experiment was carried out with a different acrylate monomer (2-OA, that has a longer ester chain than nBA) at 40 ◦C and feeding the monomer during 180 min. This experiment resulted in 100 % hydrolysis of MDO (0 % incorporation in Table 2) confirming what was observed for nBA; namely, higher temperatures and long feeding times favored hydrolysis over polymerization (low flow rates and hence, lower concentration of MDO in the polymer particles). Summing-up the information gathered in Table 2, it can be concluded that in the seeded semibatch emulsion copolymerization of nBA and MDO it is possible to incorporate MDO into the copolymer chains in large extent (above 85 %) by reducing the temperature to 20 ◦C and feeding the monomer at high flow rates. However and unfortunately, the MDO is present in the nBA/MDO copolymer as acetal units and not as ester units, which hinders the degradability of the copolymers. The conditions found to minimize hydrolysis when copolymerizing MDO in semibatch emulsion polymerization are up to certain point comparable to the optimal conditions found by Carter et al. [15,17]. In both cases, well controlled low temperatures (20–40 ◦C) and basic pH’s (around 8), and high instantaneous conversions are necessary. For achieving the latter, a high radical flux (which in Carter’s approach also served to nucleate a large number of particles) is proposed. Under these conditions, the copolymerization of VAc/MDO yields ester units in the backbone, whereas the copolymerization of BA/MDO yields acetals. Apparently, the lower reactivity ratios of acrylates and in particular n-BA with MDO [8,26] are the main reason for the different incorporation of MDO in both copolymerization systems. Although a mathematical model of the emulsion copolymerization process (including the hydrolysis reaction of MDO) would allow to optimize the process to maximize the incorporation of MDO into the copolymer, for this particular acrylate/MDO copolymer, optimized polymerization strategies will not lead to degradable copolymers because the MDO will be incorporated in a ring retention mode. Pesenti and Nicolas [24] have reviewed the radical ring-opening polymerization and particularly the copolymerization behavior of CKAs. They reported substantial differences in the copolymerization behavior (from random to alternating copolymers), but also in the mode Fig. 3. Evolution of instantaneous conversions (a) and pH value (b) for the seeded semibatch emulsion copolymerizations of MDO and nBA with feeding times of 60 min and 180 min and at reaction temperatures 20 and 30 ◦C. Fig. 4. 13 C NMR spectra of the dried polymers from the seeded semibatch emulsion copolymerization of MDO and nBA. The polymerization of 100 % nBA at 30 ◦C with a feeding time of 180 min (black spectrum), the polymerization of 95 wt% nBA and 5 wt% MDO at 30 ◦C with a feeding time of 180 min (blue spectrum) and polymerization of 95 wt% nBA and 5 wt% MDO at 20 ◦C with a feeding time of 60 min (red spectrum). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) F. Wenzel et al. Polymer 307 (2024) 127285 7 of incorporation of the CKAs depending on the comonomer used. Therefore, future research to produce waterborne degradable copolymers containing MDO or other CKA’s should explore operation conditions where the hydrolysis is minimized and the incorporation of the CKA’s is high in the ring opened formed. These can be done by exploring comonomers that unlike the acrylates, do lead to open mode of incorporation of the MDO in copolymerization, as the very recently shown ester crotonates [27] and/or using conditions as recently disclosed by Mothe et al. [20] that were able working at moderate temperatures (around 70 ◦C) to reduce hydrolysis in the batch emulsion copolymerization of MMA and MDO using neutral surfactants and costabilizing monomers. 4. Conclusions The semibatch emulsion copolymerization of MDO and acrylate monomers (particularly, nBA and 2-OA) have been explored with the aim to maximize the incorporation of MDO into the waterborne copolymer formulations. The incorporation of MDO is elusive because of its high sensitivity to water that makes the rate of hydrolysis higher than the rate of copolymerization. Therefore, in this work the polymerization conditions aiming to favor copolymerization versus hydrolysis were explored. Thus, first the effect of the temperature and pH on the hydrolysis rate of MDO in a mixture of water and deuterated water to find the best conditions to delay the hydrolysis reaction were analyzed. The hydrolysis results obtained in this work are in agreement with the most recent and extensive analysis for the hydrolysis of MDO reported in the literature [5,6]. According to these analysis, the hydrolysis reaction can be delayed by working at pH >8 and reducing the temperature below 40 ◦C. Therefore, seeded semibatch emulsion copolymerization of MDO and nBA were carried out at pH =8 and the effect of the reaction temperature (in the range 20–50 ◦C), the feeding rate of the comonomers, and the composition of the comonomer (nBA/MDO =95/5 or 90/10) were analyzed in terms of the percentage of incorporation of the MDO in the copolymer and the way in which the MDO was incorporated (open or closed forms). It was found, that the lower the temperature (20 ◦C) and the lower the feeding time (i.e., the higher the flow rate of monomer addition) the higher the incorporation of MDO in the copolymer (at 20 ◦C and 60 min for monomer feeding time 86 % of the MDO in the composition was incorporated). However, when the mode in which MDO was incorporated was analyzed by 13 C NMR, it was found that the incorporation was as an acetal unit rather than in the desired ester formed, and hence the copolymers were not degradable. Notwithstanding, the information obtained in this work can be very valuable because it is known that the comonomer affects the way the MDO is incorporated in the polymer [24]. Therefore, the experimental conditions optimized in this work, and other conditions recently reported in the literature by other research groups (e.g., reducing the charge density of the latex particles [20]) would help in optimizing the synthetic procedures towards the production of copolymers with degradable moieties in the backbone. CRediT authorship contribution statement Fabian Wenzel: Writing – original draft, Validation, Investigation, Formal analysis, Data curation. Miren Aguirre: Writing – review & editing, Supervision, Methodology, Investigation. Jose R. Leiza: Writing – review & editing, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Formal analysis, Conceptualization. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jose Ramon Leiza reports financial support was provided by Basque Government. 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