Branching Copolymerization of Styrene and Methyl Methacrylate with Divinylbenzene

The branching copolymerization of styrene (St) and methyl methacrylate (MMA)with divinylbenzene (DVB) as branching agent was conducted using atomtransfer radical polymerization (ATRP). The kinetics and extent of branching withrespect to monomer conversion were studied in details. Gas chromatography (GC),proton nuclear magnetic resonance (1H NMR) spectroscopy and dual detection sizeexclusion chromatography (DD-SEC) were used to monitor the reactions and analyzethe resulting copolymers. The analysis of reactants conversion revealed that theconversion rate of DVB, having two double bonds, is only slightly faster than those ofthe monomers, with no donor/acceptor interactions between DVB molecules and themonomers. The molecular weight increases slowly which is still somehow larger thanthe theoretical value in the early stages; then deviates significantly from the itstheoretical estimation beyond 40% conversion and increases steeply at later stages of70% monomer conversion. These results suggest that branching takes place in theearly stages of the reaction regardless of any possible donor/acceptor interactionsbetween the monomers and DVB molecules, and the coupling reaction originatesmainly from the primary chain resulting in small chain branching. Coupling reactionbetween the branched chains becomes significant in the later stages of thecopolymerization which produces highly branched chains. The reaction systemcontains three components throughout the reaction: the primary chains, the slightlybranched chains comprising of two primary chains, and the highly branched chainsconsisting of 3 or more primary chains. Although the weight fraction of the branchedchains increases with monomer conversion, there is finally 20% primary chains stillleft in the reaction system which gives rise to limited molecular weight and highpolydispersity of the branched polymers prepared through ATRP using divinyl monomeras the branching agent.