Determination of head addition incidence of (meth)acrylate and styrene in radical polymerization by RAFT block polymerization derivation and gradient polymer elution chromatography

Regioselective addition of nonsymmetrical monomers in radical polymerization is a basic issue for control of the kinetics and structure of the resulting products. In RAFT polymerization, head addition generates primary alkyl adducts trapped by RAFT agents. Due to the low leaving ability of primary alkyl radicals, this kind of adduct in RAFT polymerization will become non-active macro-RAFT agents, i.e., dead dormant species (DDS). The contents of non-active macro-RAFT agents in RAFT polymerization of three acrylates (ethyl, butyl, and 2-ethylhexanyl acrylate), two methacrylates (ethyl and butyl methacrylate) and styrene were determined by a combination technology of RAFT block polymerization derivation and gradient polymer elution chromatography. The head addition incidences of these monomers were estimated according to our previously proposed formula between the DDS content and head addition incidence. The results showed that the number or type of side chains both had significant influence on head addition incidence, with incidence of methacrylates and styrene about one order of magnitude smaller than that of acrylates due to steric repulsion effects. The results also showed that DDS contents would accumulate with increased degree of polymerization. Therefore, head addition played an important role in acrylate RAFT polymerization when product composition was considered, especially for polymers of high molecular weight.