ON THE VALIDITY OF THE STEADY-STATE APPROXIMATIONS IN HIGH CONVERSIONDIFFUSION-CONTROLLED FREE-RADICAL COPOLYMERIZATION REACTIONS

Chain addition polymerization reactions involve active intermediates ( i.e. free radicals) whose total concentration at all times is very sma ll. In copolymerization, in contrast to homopolymerization, one can id entify two radical populations, 'P' and 'Q', ending in an M1 or in an M2 monomer unit, respectively. As a result, two distinct quasi-steady- state approximations (QSSAs) can be made. The first QSSA is applied to the total radical concentration and implies that the total rate of ra dical initiation is approximately equal to the total rate of radical t ermination. The second QSSA is applied to the separate radical populat ions P and Q and states that the rate of reaction of a P-type radical with an M2 monomer is equal to the rate of reaction of a Q radical wit h an M1 monomer. However, whether or not both approximations are appli cable to a given copolymerization system is a point of concern. In the present study, the validity of the two QSSAs is examined. The conditi ons under which both approximations can be applied to a given copolyme rization system are identified, through the application of the method of moments to several experimentally investigated free-radical copolym erization systems leading to linear and branched copolymers. It is sho wn that the validity of the QSSAs in free-radical copolymerization dep ends on the polymerization conditions, namely, the polymerization proc ess (e.g. bulk versus solution) and the importance of diffusion-contro lled reactions at high monomer conversions. However, the effect of bot h approximations on the final molecular properties is, in general, not significant.