Pulsed laser polymerisation studies of methyl methacrylate in the presence of AlCl3 and ZnCl2 – evidence of propagation catalysis†
A series of pulsed laser polymerisation (PLP) experiments in conjunction with size exclusion chromatography (SEC) was carried out to investigate the effect of Lewis acids AlCl3 and ZnCl2 on the propagation rate constants at a variety of conditions. The Arrhenius parameters were obtained from kinetics data over a temperature range of ∼0–60 °C at 5 mol% Lewis acid loading in bulk methyl methacrylate (MMA). The results showed that the Lewis acids are excellent propagation catalysts with catalysis increasing with the charge on the metal ion. The Lewis acids AlCl3 and ZnCl2 enhanced the propagation rate by around 80% and 40% respectively at 5 mol% loading, reducing the activation energy for the propagation step by 5.8 kJ mol−1 and 2.2 kJ mol−1. Propagation rate coefficient enhancements of up to 100% were obtained when AlCl3 was used at a loading of 10 mol%. The presence of Lewis acid also affects the radical initiation process by complexing with the photoinitiator. This results in both catalysis of initiation and increased triplet lifetimes. These problems could be overcome through optimization of the initiator concentration and laser flashing rate to ensure that IUPAC quality criteria for PLP are met. Despite its strong influence to the kinetics of free radical polymerisation, limited evidence of stereocontrol was observed for both Lewis acids.