| Reversible addition-fragmentation chain transfer (RAFT) radical polymerization has been widely used for synthesis and preparation of functional polymers with controllable molecular weight, narrow molecular weight distribution and well-defined sequence and architecture, and is one of the most versatile and promising living/controlled polymerization techniques. The rate retardation effect during RAFT polymerization process has drew much attention of polymer chemistry researchers. Several side reactions and corresponding models are promoted to explain the effect during a long-time scientific research and discussion. The existence of cross termination between propagating radicals and intermediate radicals has been proved in a large number of research work, and it’s at least one of the causes of rate retardation during RAFT polymerization process. Accordingly, Intermediate Radical Termination (IRT) model was promoted. But until now, there is no effective means to restrain cross termination and reduce rate retardation effect in RAFT polymerization. Moreover, there is a need for further work to explore other possible mechanisms leading to rate retardation, role of cross termination and role of other side reactions in rate retardation. Based on magnetochemistry and radical pair theory, kinetics and mechanism of RAFT polymerization are studied under an externally applied magnetic field in this work. Results obtained in this work offer us a versatile tool to reduce rate retardation and a quantitative way to kinetics simulation of RAFT polymerizations. Results of bulk polymerization can be further used to probe quantitatively cross termination in various RAFT polymerizations. Main points of this work is introduced in detail as follows:1. Based on the analysis of thermodynamics distribution of electrons and radical pair theory, the feasibility of magnetic field effect on RAFT polymerization kinetics is studied. The calculation results from two theoretical perspectives are compared thoroughly with available experiment equipment. Dilatometer with special temperature controlling, externally applied magntic field by solenoid and dilatometry experiments are designed according to the experiment requires.2. Dithiobenzoate (CPDB) and trithiocarbonate (DDMAT) are synthesized to mediate bulk RAFT polymerization of St as RAFT agents. Kinetics of RAFT polymerization with different RAFT agents are examined under various strength of magnetic fields in dilatometer, respectively. Results show that RAFT polymerizations mediated with above RAFT agents show an obvious rate retardation compared with typical free radical polymerization. Rate retardation effect can be reduced by externally applied magnetic field and has a dependence on its strength. RM is defined to probe the rate retardation brought by cross termination quantitatively. And RM of CPDB- and DDMAT-mediated bulk RAFT polymerization of St is respectively calculated to be 0.382 and 0.534, respectively. This work offers us an easy and versatile way to reduce rate retardation brought by cross termination in situ.3. Based on Stationary State Model, a kinetic approach based on EAMF is established, the kinetic parameters are estimated through the simulation of the curve between apparent polymerization and concentration of RAFT agent. Both cross termination and self termination are proved to exist in this work. The effective reduction of kt,cross under externally applied magnetic field can explain the effect of magnetic field on RAFT polymerization at a quantitative and kinetic level. On the other hand,kt,cross and k, are of the same order of magnitude, kt,cross much smaller, that is, cross termination causes more rate retardation than self termination. The results demonstrate a new tool to modeling kinetics of RAFT polymerization, and shed a light on the research of mechanism of the rate retardation effect during RAFT process. |
