Multiblock Copolymer Synthesis via Reversible Addition–Fragmentation Chain Transfer Emulsion Polymerization: Effects of Chain Mobility within Particles on Control over Molecular Weight Distribution
Glenn K. K. Clothier, Thiago R. Guimarães, Graeme Moad, Per B. Zetterlund
Abstract
Synthesis of multiblock copolymers using seeded reversible addition–fragmentation chain transfer (RAFT) emulsion polymerization has been explored with a view to elucidate how certain experimental conditions influence the control over molecular weight distribution (MWD). Two separate parameters have been explored in detail: (i) the ratio of monomer concentration to the RAFT end group concentration within particles and (ii) the glass transition temperature (Tg) of the particles. The parameters (i) and (ii) are interrelated as an increase in the ratio [monomer]:[RAFT] leads to a lower Tg because of the increased plasticization of the polymer particle by the monomer. Three different monomers were employed, each giving a polymer of different Tg values: n-butyl methacrylate (Tg = 20 °C), iso-butyl methacrylate (Tg = 57 °C), and tert-butyl methacrylate (Tg = 118 °C). The results show that the level of control over the MWDs via the RAFT mechanism is markedly reduced under conditions where Tg of the polymer particles is high. This is attributed to the high Tg value, leading to low radical penetration rates (low diffusion rates) of radicals generated via initiation in the aqueous phase, preventing propagating radicals from reaching the core region of the particles before bimolecular termination occurs. In the present system, the RAFT end groups are predominantly (but not at all exclusively) located in the core region of the particles.