Mechanism And Kinetics Of Multifunctional Peroxy Initiators Thermal Decomposition And Related Initiated Polymerization

Multifunctional peroxides are new types of initiators for free radical polymerization. They can initiate vinyl monomer to obtain polymers with high molecular weight at high polymerization rate and can also be used to prepare block copolymers and graft copolymers. The polymerization processes initiated by multifunctional initiators are more complicated since there are more than one group with different activity in them. The investigation on multifunctional peroxides is of importance.Three types of peroxide, such as triperketal with a cycle in main chain 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane (TETMTPA), diperketal with a cycle in side chain 1,1-di(t-butyl peroxy) cyclohexane (DTBPCH) and diperester with linear chain 2,5-dimethyl-2,5-di(2-ethylhexanoyl peroxy)hexane (DEDMHPH), were fundmentally studied in the following three aspects: decomposition mechanism and decomposition kinetics, polymerization kinetics and their applications in the preparation of vinyl polymers.Decomposition products of these three types of peroxide were dissimilar each other from the analyses by gas chromatograph- mass spectrum (GC-MS). Different decomposition mechanisms were proposed based on the structure and property analyses using nuclear magnetic resonance (NMR), differential scanning calorimeter (DSC) and gel permeation chromatography GPC. On the basis of decomposition mechanisms, initiation mechanisms of three types of peroxide were proposed: Radicals from DEDMHPH decomposition initiated monomer to polymerize directly; radicals with peroxide groups from DTBPCH decomposition took the p-scission reaction of carbon-carbon bond, and then initiated monomer to polymerize; some radicals with peroxide groups from TETMTPA decomposition initiated monomer to polymerize directly, while the others took the p-scission reaction and then initiated.Conventional DSC method for determining decomposition kinetic constant of initiator was modified. The improved method is more preferable to the determination of new initiator by confirming the reaction order firstly. The decomposition kinetic constant of benzoyl peroxide was determined by the improved method and agreed with the value reported. The decomposition kinetic constants of DEDMHPH and peroxide groups in the polymers initiated by DEDMHPH were determined at various scanning rates for the first time. The decomposition activity energy and frequency factor are 129.4 kJ/mol and 1.137×10¹⁵ s^-1 for the former, while the values are 130.1 kJ/mol and 1.196×10¹⁵ s^-1 for the latter.The elementary reactions were presented for the polymerization with both combination and disproportionation termination in the microcosmic kinetic studies. Based on the elementary reactions and material balances, the uniform rate expressions of initiation and polymerization for vinyl monomer initiated by monofunctional and difunctional initiator were induced. The effects of monomer and initiator concentrations on methyl methacrylate (MMA) and styrene (St) polymerization initiated by DEDMHPH showed that the polymerization rates were directly proportional to the square root of peroxide group concentration and to monomer concentration.The activity energy were obtained from the experimental results of polymerization rate at different temperatures according to Arrhenius expression, which were 81.4 kJ/mol for MMA polymerization and 92.0 kJ/mol for St polymerization initiated by DEDMHPH. The initiator efficiency values were also obtained from the experimental results of polymerization rate initiated by DEDMHPH, which were 0.42±0.02 in MMA and 0.55±0.03 in St. Based on the decomposition mechanism and initiation mechanism, the concentration expressions of initiator fragments were induced. And then, the polymerization degree expression for disproportionation termination system was obtained from its definition. The functions of peroxide group distribution were induced for combination termination system through statistics. And then the polymerization degree expression was obtained for combination termination system. So the polymerization degree expression for the system including both combination termination and disproportionation termination was acquired. The transfer constants to initiator and to monomer for methyl methacrylate and styrene polymerization were obtained from the expression and experimental results of polymerization rate and polymerization degree. The transfer constant to MMA was 1.85xlO"5 (60°C), while that to St was 0.59XKT4 (60°C) and 0.75X10"4 (70°C). The transfer constant to initiator was 0.0097 (60°C) in MMA, but 0.59* 10"4 (60°C) and 0.75* 10"4 (70°C) in St. Furthermore, the values of polymerization degree at other conditions were calculated taking into account chain transfers and agreed with the experimental results.The kinetic model of polymerization initiated by difunctional initiator DEDMHPH was established from the presented elementary reactions according to the study of diffusion-controlled theory, free-volume theory and entangled-chain theory on physical parameters. The results including monomer conversion, molecular weight and polydispersity of polymer calculated from this model agreed with the experimental ones very well. The model predicted the changes of various radical (including monoradical with one peroxide group, monoradical without peroxide group and diradical without peroxide group) concentrations and polymer (including polymer with two peroxide groups, polymer with one peroxide group and polymer without peroxide group) concentrations. The relationship of polymerization system volume, initiator efficiency, propagation rate constant and termination rate constant with conversion were also acquired. All relationships to conversion except polymerization system volume were affected by reaction temperature and initiator concentration. Results showed that difunctioanl initiator could increase polymerization rate and molecular weight simultaneously compared with monofunctional initiator.Finally, studies on the multifunctional perketals application include synthesizing homopolymer and preparing block copolymer. Effects of reaction temperature and initiator concentration on monomer conversion, molecular weight and polydispersity for styrene polymerization initiated by TETMTPA and DTBPCH were investigated. High temperature or high initiator concentration resulted in high polymerization rate while low molecular weight. The effects of initiator concentration and reaction temperature on polydispersity were very unique. Compared with the system initiated by monofunctional peroxide, multifunctional peroxide canoptimize the relationship of molecular weight and polymerization rate. Researches showed that the peroxide groups in multifunctional peroxide decomposed successively and then initiated the monomer to produce the polymer with undecomposed peroxide groups. The existence of these undecomposed peroxide groups in polymer chains influenced the properties of the polymer. Block copolymer was obtained through two-step polymerization methods utilizing the undecomposed peroxide groups in polymer and the composite in block copolymer was affected by the reaction time.