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 copolymer ization of styrene and alpha -methylstyrene (AMS) is presented and investig ated 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 w eight control are associated with the AMS comonomer and are threefold: (i) AMS has a low propagation rate coefficient, due to the increased steric req uirements 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 increa sing temperature. In addition to the styrene/AMS system, other comonomer em s showing similar kinetic and thermodynamic features (e.g. the styrene/meth yl ethacrylate (MEA) system) may also be used to generate a wide range of m olecular weights. The possibilities for controlling molecular weight and en d group functionalities by replacing the slowly propagating monomer by a fu nctional monomer are discussed.