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 alpha -methylstyrene (AMS) is presented and investigated ly simulation via the program package PREDICI(R). It is shown that the molecular weight of the product copolymers may be varied in a wide range (from (M) over bar (w) = 500 to 2.10(6)) 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 high 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 ems 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.