The Study Of Relationship Between Oxidation Stability&Chain Transfer Ability Of RAFT Agents And The Molecule Structure

The Reversible addition-fragmentation chain transfer (RAFT) polymerization has become one of the most versatile controlled polymerization methods that enabled to design and synthesis of a wide range of macromolecules with well-defined complex polymeric architectures. However, the mechanism of the RAFT process still has not reach agreement and the kinetics of the RAFT polymerization is not completely established. In this paper, we discussed the oxidation stability and chain transfer ability of the RAFT agents and put forward our own viewpoint based on our experiments. It was found that the oxidation rate depended on the structure of the RAFT agents. Moreover, the addition constant and fragmentation constant of a series of dithioesters were measured by the model cross reaction between alkoxyamine and dithioesters, which was helpful to establish the kinetics of the RAFT polymerization and understand the mechanism of the RAFT process deeply. In addition, the Monte Carlo simulation method was used to investigate the sequence distribution of the copolymer. The evolution of the sequence distribution with the conversion was obtained. The main contents are as follows:Radical induced oxidation of RAFT agents. It was found that the RAFT agent was easy oxidized in present of the radical. The oxidation reaction would happen in incompletely degassing condition of RAFT polymerization. A remarkable feature of the reaction is the high selectivity preferring radical induced oxidation rather than RAFT process between radical and RAFT agents. The characteristic results of oxidized product indicated all of the RAFT agent undergoed the same oxidation in which carbonyl sulfur was replaced by oxygen. Meanwhile, it was interesting to find that the reaction rate depended on chemical structure, i.e., the structure of leaving (R-) and stabilizing (Z-) groups. For dithioesters with identical Z-phenyl substituent, the oxidation rate decreases in the order of cyanoisopropyl (-C(Me)2CN)> cumyl (-C(Me)2Ph)> phenylethyl (-CH(Me)Ph)> benzyl (-CH2Ph). For dithioesters with identical R substituent, the stability of the RAFT agents towards oxidation increases in the order Z=phenyl-benzyl-<RS-(trithiocarbonates)<RO-(xanthates). Importantly, incomplete deaeration would lead to reduced controllability of the RAFT polymerization. The RAFT polymerization of the styrene mediated by dithioester PEDB was carried out in the presence of air. The result showed the RAFT process exhibits uncontrolled character in which the molecular weight remains nearly constant after an induction period of4h. It seems that the RAFT agent has been oxidized during the induction period. We speculated that termination by oxidation of the living chains may occur in normal RAFT polymerizations if the system is not well deaerated.The measurement of the addition rate constant kadd and fragmentation rate constant kfrg of the RAFT process We had designed a cross reaction between alkoxyamines and dithioesters as a model reaction of the RAFT process, in which alkoxyamine plays the role of radical source. Thus, the advantage of the model cross reaction is that it enable the evaluations of the rates of addition and fragmentation reactions in the RAFT process. Since the RAFT process involved four unknown coefficients, the difficult of the theoretic deduction and parameters monitoring would be increased. For simplification, we first consider a system with R1=R2, which is a dithioester and an alkoxyamine with identical leaving groups. That means the attacking and leaving radicals in the RAFT process are identical. The kinetic model is therefore reduced to a system containing two reversible reactions and one termination reaction, in which only two unknown parameters (kadd and kfrg) are left in RAFT process. On the other hand, HPLC could not distinguish the starting and resulting dithioesters, which appeared in the same position, so the perdeuterated dithioster was used in this system, which can be monitor by in situ NMR method. Therefore, in such a system, the measurable parameters are the concentrations of the starting and resulting dithioesters as functions of time. So the addition rate constant kadd and fragmentation rate constant kfrg can be reasonably evaluated by fitting the experiment results monitored by in situ NMR and the Monte Carlo simulation results. The second model cross reaction is the cross reaction system with a large excess of alkoxyamine, which could be monitored by high performance liquid chromatography (HPLC). The addition coefficient kadd and fragmentation coefficient kfrg can be reasonably evaluated by fitting the experiment results monitored by HPLC and the Monte Carlo simulation results. From the kadd and kfrg value, it is clearly that the chain transfer ability of the dithioesters is dependent on the nature of the dithioesters. The chain transfer ability decreases in the order of cyanoisopropyl (-C(Me)2CN)> cumyl (-C(Me)2Ph)>1-(Benzoyloxy)-2-phenyl (-CH(Me)Ph-C(=O)OPh)>2-(Ethoxycarbonyl)prop-2-yl (-CH(Me)2-C(=O)OCH3)> benzyl (-CH2Ph). It was surprisingly to find that the same order of chain transfer coefficients of dithioesters is as CSIRO group reported.The investigation of the sequence distribution of copolymer by Monte Carlo simulation For the investigation of the behavior of RAFT copolymerization process, a new Monte Carlo algorithm was developed, which was firstly applied in the conventional radical copolymerization. Three types of copolymerization behavior are simulated by developed Monte Carlo algorithm. The evolution of monomer conversion, molecule weight, PDI and copolymer composition with the reaction time is obtained. It is importantly that the evolution of sequence distribution of the copolymer with the conversion was investigated, which is helpful to deeply understand the copolymerization mechanism. Finally, the Monte Carlo algorithm of the RAFT copolymerization was developed by combining the algorithm of the conventional radical copolymerization and RAFT process.