Solvent and Temperature Effects in the Photoiniferter RAFT Polymerisation of PEG Methacrylate

Photoiniferter (PI)-RAFT polymerization is a promising approach to synthesise a broad range of (meth)acrylic and styrenic polymers because of its highly ‘living’ nature. The lack of an imbalance between initiating and chain-transfer fragments minimises the inherent bimolecular termination of conventional RAFT. Poly(poly(ethylene glycol) methyl ether methacrylate) (P(PEGMA)) is a potential biocompatible material for biomedical applications, but the highly reactive free radical of PEGMA makes control of its polymerisation challenging. In this study, we investigated the synthesis of P(PEGMA) through PI-RAFT. Current studies on the PI-RAFT mechanisms are limited and the effect of solvents on kinetics has not been reported. We varied several reaction conditions: excitation wavelengths, monomer concentrations, temperatures, and solvents. The propagation constant (k_p) values were affected by the RAFT main equilibrium. We calculated the Arrhenius parameters, enthalpy of activation (Δ𝐻^‡), and entropy of activation (ΔS^‡) for polymerization in various solvents. Regression analysis was conducted to fit the results with extinction coefficients of CTA in seven common solvents, solvent physical properties, and solvatochromic scales. The effective collision factor A had a good fitting with an exponential regression model of the extinction coefficients, indicating a strong relationship between the reaction rate and excitation of the CTA. Solvent polarity scales, such as Kalmet-Abraham-Taft (KAT) and Catalan parameters, failed to predict kp, Arrhenius parameters, Δ𝐻^‡, and ΔS^‡. A chain transfer constant C_tr > 1 for all syntheses indicated relatively good control over the polymerization through degenerative chain transfer with CTA radicals. In general, C_tr decreased with increasing temperatures, a result of the rate of excitation by photon absorption being constant, but the kp being increased by the temperature. Anisole was the best solvent, able to keep Ð = 1.30 even at 40 °C.