Design Criteria for Star Polymer Formation Processes via Living Free Radical Polymerization
journal contributionposted on 19.09.2006, 00:00 by Hugh Chaffey-Millar, Martina H. Stenzel, Thomas P. Davis, Michelle L. Coote, Christopher Barner-Kowollik
The kinetics of reversible addition−fragmentation chain transfer (RAFT), R-group approach star polymerizations have been studied via a combined experimental and theoretical approach. From the improved understanding herein developed, design criteria have been suggested to aid in future syntheses of RAFT, R-group approach star polymer. The suggested criteria are as follows. To minimize the quantity of linear polymer in the system, it is important to have a high rate of monomer propagation but a small delivery of radicals to the system. Crucial to the prevention of star−star coupling and resulting molecular weight distribution (MWD) broadening is the minimization of radical termination events between star molecules. Noting that the number of termination events is directly correlated to the number of decomposed initiator molecules, this might be achieved via several methods. A slow rate of initiator decomposition, a fast rate of propagation, or use of a rate-retarding RAFT agent can all lead to a reduction in star−star coupling events. Additionally, simulations reported herein demonstrate that the use of a star-forming RAFT agent substrate which has a fewer number of arms will lead to a reduction in the concentration of star−star coupled products. Ab initio calculations have been used to study intramolecular RAFT equilibria occurring early in the preequilibrium. These calculations have shown that highly stable intramolecular adduct radicals might be formed due to the close proximity of radicals and SC bonds. The effect of these on the kinetics is studied.