Synthesis of block copolymers by a low-temperature free-radical mechanism using systems of the polystyrylaluminium-peroxide type

Abstract

Block copolymers of styrene and a polar monomer (acrylates, methacrylates and acrylonitrile) can be synthesized at low temperatures (20° to minus 20°C) by a free-radical mechanism using initiating systems of the type: polystyrylaluminium (poly St-Al)-peroxide. The degree of polymerization ( P ̄ n ) of the styryl substituent in the polySt-Al component had values 30, 300, 450 and 800. Benzoyl peroxide and dicyclohexylperoxydicarbonate were used as peroxide components. The effect of the concentration and nature of the components of the initiating system, catalytic amounts of the additional electron donor component, pyridine, and temperature on the kinetics of the process were investigated. The kinetic dependence makes it possible to consider the formation of free radicals in terms of the donor-acceptor reactions in which all the components of the system including the monomer take part. Synthesis of a block copolymer with MMA was taken as an example to show the relationships between the conditions of the process and the characteristics of the product. The data on molecular weight distribution ( MW D) and the compositional distribution according to MW (pyrolysis and gel permeation chromatography) show that the main product is the ABA block copolymer where A is the styrene block and B is the MMA block. This structure is formed owing to the conditions of the process: initiation by polystyryl radical and recombination as the dominant chain termination mechanism. This structure cannot be formed under the conditions of anionic or radical (polyperoxide) initiation.