Using kinetics and thermodynamics in the controlled synthesis of low molecular weight polymers in free-radical polymerization

A simple way of controlling molecular weights in the free-radical copolymerization of styrene and a α-methylstyrene (AMS) is presented and investigated by simulation via the program package PREDICI®. It is shown that the molecular weight of the product copolymers may be varied in a wide range (from ̄_w = 500 to 2 · 10⁶) by variation of the comonomer feed ratio and the reaction temperature. The reasons for this simple molecular weight control are associated with the AMS comonomer and are threefold: (i) AMS has a low propagation rate coefficient , due to the increased steric requirements of the monomer, (ii) AMS has a transfer to monomer constant (C_M) in comparison with styrene and (iii) AMS has a low ceiling temperature, so that the effective propagation rate coefficient decreases with increasing temperature. In addition to the styrene/AMS system, other comonomer systems showing similar kinetic and thermodynamic features (e. g. the styrene/methyl ethacrylate (MEA) system) may also be used to generate a wide range of molecular weights. The possibilities for controlling molecular weight and end group functionalities by replacing the slowly propagating monomer by a functional monomer are discussed.