Effect of silica nanoparticle loading and surface modification on the kinetics of RAFT polymerization

S-(thiobenzoyl) thioglycolic acid was used to synthesize poly(methyl methacrylate) via reversible addition-fragmentation chain transfer (RAFT) polymerization. To study the polymerization kinetics, in situ polymerization reactions were performed with different loading of nanoparticles. To investigate the effect of surface modification on the polymerization kinetics, similar reactions were performed with 3-methacryloxypropyldimethylchlorosilane-modified nanoparticles. Conversion, reaction rate, molecular weight and polydispersity index (PDI) were monitored during polymerization. According to results, pseudo-first order kinetics is achieved, but the rate constant of chain transfer reaction to the RAFT agent (C-tr) has a very small value. Adding nanoparticles causes no considerable change in the kinetic curves, while there is an optimum value for nanoparticles loading in which the polymerization rate reaches its maximum level. A similar trend is observed for molecular weight; however, increasing silica content results in an increase in PDI values. In comparison with pristine silica nanoparticles, the polymerization rate increases slowly in the case of modified particles. Also, molecular weight and PDI for free and graft chains are studied separately. The molecular weight of free chains increases with increasing nanoparticles loading up to 7 wt% and then decreases, while PDI values increase continually by adding nanoparticles. However, for graft chains, molecular weight and PDI values increase with increasing nanoparticle content.