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 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 M?C_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 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 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.