Free Radical Copolymerization of Hydroxy-Functional Monomers: Kinetic and Semibatch Studies
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Acrylic resins used as polymeric binders in automotive coatings are complex copolymers containing reactive functional (often hydroxyl) groups. A better understanding of the copolymerization kinetics of these monomers is required in order to ensure uniform distribution of the functional groups among the polymer chains over the course of production. Free radical copolymerization propagation kinetics of styrene (ST) with 2-hydroxyethyl methacrylate (HEMA) and 2-hydroxyethyl acrylate (HEA) have been investigated both in bulk and solution, using pulsed-laser polymerization (PLP) combined with size exclusion chromatography (SEC) and proton NMR. All of the solvents examined (n-butanol, toluene and DMF) affect ST/HEMA copolymer composition relative to bulk polymerization, while the effects on propagation rates suggest that hydrogen bonding interactions need to be explicitly considered. Semibatch reactions of ST/HEMA, butyl acrylate (BA)/HEMA and butyl methacrylate (BMA)/HEMA have been carried out in xylene, DMF and 1-pentanol at 110 and 138 °C. The variation in monomer composition for the three solvents agrees with the kinetic studies. It was found that polymer molecular weight is strongly affected by solvent choice and operating conditions, partially due to branching reactions caused by impurities from commercial HEMA monomers. PLP and 13C-NMR analysis indicate that no backbiting occurred during polymerization of HEA, and it is shown that H-bonding disrupts the backbiting mechanism found for other acrylates. Thus, semibatch production in n-butanol can reduce branching and increase molecular weight of BA homopolymers by a factor of five compared to polymerization in xylene.