Advanced computational strategies for modelling the evolution of full molecular weight distributions formed during multiarmed (star) polymerisations

A novel computational strategy is described for the simulation of star polymerisations, allowing for the computation of full molecular weight distributions (MWDs). Whilst, the strategy is applicable to a broad range of techniques for the synthesis of star polymers, the focus of the current study is the simulation of MWDs arising from a reversible addition fragmentation chain transfer (RAFT), R-group approach star polymerisation. In this synthetic methodology, the arms of the star grow from a central, polyfunctional moiety, which is formed initially as the refragmenting R-group of a polyfunctional RAFT agent. This synthetic methodology produces polymers with complex MWDs and the current simulation strategy is able to account for the features of such complex MWDs. The strategy involves a kinetic model which describes the reactions of a single arm of a star, the kinetics of which are implemented and simulated using the PREDICI® program package. The MWDs resulting from this simulation of single arms are then processed with an algorithm we describe, to generate a full MWD of stars. The algorithm is applicable to stars with an arbitrary number of arms. The kinetic model and subsequent algorithmic processing techniques are described in detail. A simulation has been parameterised using rate coefficients and densities for a 2,2′-azoisobutyronitrile (AIBN) initiated, bulk polymerisation of styrene at 60°C. A number of kinetic parameters have been varied over large ranges. Conversion normalised simula tions were performed, leading to information regarding star arm length, polydispersity index (PDI) and the fraction of living arms. These screening processes provided a rigorous test for the kinetic model and also insight into the conditions, which lead to optimal star formation. Finally, full MWDs are simulated for several RAFT agent/initiator ratios as well as for stars with a varying number of arms. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Design Criteria for Star Polymer Formation Processes via Living Free Radical Polymerization

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 S=C bonds. The effect of these on the kinetics is studied

Thioketone spin traps as mediating agents for free radical polymerization processes

Thioketones are demonstrated to be suitable agents for controlling free radical polymerization processes: the polymerizations carry (pseudo) living characteristics indicating that the control process is induced by a persistent radical effect