1. Introduction

Polyvinyl acetate (PVA, PVAc, poly(ethenyl ethanoate)) has a number of valuable specific properties and is widely used in various fields – from household products to materials for medical and biological purposes. The most important qualities of PVA are its universal adhesive and binding properties, high strength of fibers and film materials made with its use [1].

2. Research methods and principles

The main method for producing PVA is emulsion polymerization. Among emulsion monomers, vinyl acetate (VA) stands out for its good solubility in water in contrast to vinylbenzene, and this what determines the patterns of PVA synthesis [2], [3], [4]. The considerable reactivity of the VA radical results in heightened susceptibility to the existence of impurities, it is active in chain transfer reactions, and usually VA polymerization proceeds with an induction period [5], [6].

In radical polymerization of vinyl acetate, anionic and nonionic surfactants are most often used [7], [8]. A number of studies have demonstrated the existence of nonionic high-molecular surfactants, for instance, pluronics (triblock-copolymers of a polyoxypropylene and two hydrophilic chains of polyoxyethylene). This can lead to promising stability for PVA particles [9], [10], [11]. According to multiple studies, the importance of environmental protection was highlighted by examining various factors, high-molecular surfactants that can decompose under natural conditions to innocuous low-molecular products are promising. These are amphiphilic block-copolymers based on lactic acid [12], [13], [14], [15].

Thus, in this work, we synthesized several biodegradable linear amphiphilic block-copolymers of L-lactide and ethylene glycol. The colloid-chemical properties of those synthesized compounds were evaluated, and the heterophase polymerization of vinyl acetate in their presence were studied.

3. Main results

3.1. Synthesis of the linear diblock-copolymers

Linear diblock-copolymers of L-lactide and poly(ethylene glycol) was produced by ring-opening polymerization of L-lactide using monofunctional poly(ethylene glycol) methyl ether with molecular weight of 2 000 g mol-1 and stannous octoate (Sn(Oct)2) as the initiating system. The synthesis was performed at 150 ⁰C for 24 h, and the obtained copolymers were separated from the catalyst and monomer residues by double reprecipitation in the tetrahydrofuran/(hexane – ethanol) system, followed by drying to a constant weight in a vacuum oven. The synthesis scheme is illustrated in Figure 1.3.2. Identification of copolymers with NMR spectroscopy and its results

The identification of copolymers and their number-average molecular weight ([LATEX_FORMULA]\overline{Mn}[/LATEX_FORMULA]) were accomplished by employing 1H-NMR spectroscopy using «Bruker» NMR spectrometer (600 MHz). CDCl3 was used as a solvent at room temperature and tetramethylsilane was used as an internal standard. The weight-averaged molecular weight ([LATEX_FORMULA]\overline{Mw}[/LATEX_FORMULA]) and the PDI ([LATEX_FORMULA]\overline{Mw}/\overline{Mn}[/LATEX_FORMULA]) were estimated by using gel permeation chromatography (GPC), which was conducted on a «Knauer» analyzer system (PL-GEL 5u MIXC 300×7.5 mmcolumns). The eluent was tetrahydrofuran (THF) flowing at a rate of 1.0 mL/min at 40 °C. The chemical structure of copolymers was determined according to the literature data [13], [17], [18], [19]. The copolymers' characteristics are presented in Table 1.3.3. Investigation of colloidal chemical characteristics

The colloidal chemical characteristics ofPLLA-MPEG block-copolymers were investigated. The interfacial tension and surface tension of the solutions were determined at room temperature by using a KRŰSS K9 surface tension tensiometer. It has been discovered that all copolymers decrease the interfacial tension to levels below 20 mN/m, aligning with the information found in the literature regarding copolymers containing polylactide [20], [21], [22].

3.4. The use of synthesized copolymers in the polymerization of vinyl acetate as surfactants

Linear and hyperbranched poly(ethylene oxide)-containing copolymers with high biocompatibility, obtained on the basis of aliphatic copolyesters, whose hydrophilic blocks are formed by polyethers, and hydrophobic blocks are formed by polymers of hydroxyacids (glycolic, lactic, hydroxybutyric, etc.), can be ranged among such biodegradable surfactants. It is known that the hydrolysis of such amphiphilic macromolecules leads to decomposition into environmentally harmless natural hydroxyacids and biocompatible oligomers [23], [24], [25], [26].

The use of water-insoluble surfactants in the heterophase polymerization of vinyl monomers is seen as a promising approach to producing polymer suspensions with a narrow particle size distribution (PSD). Biodegradable polyesters have been demonstrated to be exceptional stabilizers of polymer microspheres and hold a distinctive position among water-insoluble surfactants [12], [14].

The PLLA60-MPEG45 block copolymers were employed as a surfactant in the emulsion polymerization of vinyl acetate with potassium persulfate (PPS) as the initiator. Vinyl acetate (Fluka) with a basic substance content of ≥99 % was used as a monomer, and PPS (Sigma-Aldrich) with a basic substance content of 99.9% was used as an initiator. The vinyl acetate polymerization took place at a temperature 60±0.5 ⁰C using a monomer-to-water volume ratio of 1:9. The initiator concentration was 1 wt % based on VA, while the surfactant concentration was 1.0 wt % per monomer.

3.5. Evaluation of the polymerization process using dilatometry method

Dilatometry is based on the fact that the density of a polymer is slightly higher than the density of its constituent monomer, causing the polymerization medium to contract over time. By employing a polymerization vessel equipped with a narrow capillary at the upper part, this shrinkage can be precisely tracked by assessing the variation in meniscus elevation over time [12], [27]. Polymerizations were monitored to about 99.9% conversion. The kinetic curves in the coordinates' conversion – time of VA polymerizations are clearly seen in figure 2. The complete conversion of VA is achieved within from 150 to 250 min.3.6. Investigation of the sizes and ξ-potential of the obtained PVA particles

The sizes of the obtained nanoparticles were estimated by the DLS method. The photomicrographs of PVA particles show that the PSD is narrow (figure 3). One notable aspect of this polymerization process is the creation of stable polymer suspensions that are distinguished by a narrow PSD. This allowed for the synthesis of a PVA suspension with particle sizes of approximately 0.7 μm.

4. Conclusion

Thus, the synthesized biodegradable linear amphiphilic block-copolymers of L-lactide and ethylene glycol were comprehensively investigated. The high surface activity of polylactide-poly(ethylene glycol) block-copolymers and their capacity to generate thick interfacial adsorption layers on the polymer particle surfaces provide a rationale for the system's stability during polymerization.

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