Competition between Hydrolysis and Radical Ring-Opening Polymerization of MDO in Water. Who Makes the Race?

Abstract

A major challenge in modern society is the reduction of microplastics created by polymers having stable C–C backbones. The chemistry of radical ring-opening copolymerization of cyclic ketene acetals (CKAs) with vinyl monomers in introducing degradable ester units into the C–C backbone is highly promising. Although the corresponding reaction in an aqueous medium should provide biodegradable primary dispersions, the bottleneck is the hydrolytic instability of CKAs. Therefore, in-depth hydrolysis kinetics of CKA (2-methylene-1,3-dioxepane, MDO) at different pH values and temperatures under homogenous and heterogenous conditions are studied to get a “sweet spot” under which emulsion polymerization of MDO might be possible. Depending on the pH, the hydrolysis of MDO undergoes three different mechanisms with slowed hydrolysis kinetics under alkaline conditions. Besides 4-hydroxy-1-butylacetate (4-HBA), other co-hydrolysis products were detected, leading to the autocatalysis effect. The fast MDO hydrolysis during emulsion copolymerization with vinyl acetate led to the formation of polymers with extremely less incorporation of ring-opened MDO units. Degradation tests of the corresponding emulsion copolymers compared with copolymers prepared in solution confirmed the low incorporation ratio of MDO. The results and discussion presented in this work will be a strong guideline for future emulsion copolymerizations of CKAs.

Degradable vinyl polymer particles by radical aqueous emulsion copolymerization of methyl methacrylate and 5,6-benzo-2-methylene-1,3-dioxepane

“…This can be explained by the high sensitivity of CKAs to protic species since CKAs are known to rapidly hydrolyze in aqueous media. 18 , 27…”

“…This however requires very fine adjustments in terms of pH and feeding rates, as well as the additional use of hydrophilic charged comonomers, which may not be suitable for a wide range of applications and may generate reproducibility issues. 27…”

“…14 Nonetheless, these copolymerizations have been extensively investigated in bulk and in solution. In contrast, only a very few examples concerning the use of CKAs in an aqueous dispersed media have been reported, [15][16][17]27 despite the tremendous importance of polymerization in dispersed media. This can be explained by the high sensitivity of CKAs to protic species since CKAs are known to rapidly hydrolyze in aqueous media.…”

“…This can be explained by the high sensitivity of CKAs to protic species since CKAs are known to rapidly hydrolyze in aqueous media. 18,27 Miniemulsion copolymerization in water of BMDO with either MMA or styrene was described in 2012. 17 The ultimate objective justifying the use of a miniemulsion polymerization process was the encapsulation of drugs in a degradable polymer matrix.…”

“…This however requires very fine adjustments in terms of pH and feeding rates, as well as the additional use of hydrophilic charged comonomers, which may not be suitable for a wide range of applications and may generate reproducibility issues. 27 Copolymerization of BMDO with MMA, which also present favorable reactivity ratios (r BMDO = 0.53 and r MMA = 1.96), 13 has never been investigated in aqueous emulsion copolymerization. In this paper, we look at developing very simple protocols for the aqueous emulsion copolymerization of BMDO with MMA to yield degradable vinyl polymer particles.…”