Studying The Mechanism Of Allylthioketone-Mediated Controlled Radical Polymerization

Controlled/living radical polymerization is one of the hotspots in the research of polymer chemistry.Through controlled/living radical polymerization,the polymers with definite structure and controlling molecular weight can be obtained.Compare to the anionic polymerization,the monomers used in the controlled/living radical polymerization are more versatile,a wide range of topological polymers with various polymers including functional polymers can be tailor-made.In this paper,we discussed the polymerization mechanism in order to know how and why the radical polymerization in the presence of allylthioketone can control over the growth of polymer chain and the polymerization behaviors.The results are as follows:1.By combination of high trapping free radical efficiency of the thioketone and resonance of the allylic radical,a new type of control agent,1,3,3-triphenylprop-2-ene-l-thione(TPPT)has been successfully synthesized,and is used to study controlled free radical polymerization of methacrylates.Very stable TPPT radicals at the end of PMMA are detected in the polymerization of MMA using TPPT and AIBN as control agent and initiator,the EPR studies reveal increase of the TPPT radical concentration with reaction time.The MALDI-TOF MS spectra are used to identify terminal groups of the resultant PMMA,and two polymer structures,(CH3)2(CN)C-PMMA-TPPT and(CH3)2(CN)C-PMMA are attained.The former is formed by isobutyronitrile radical initiating polymerization of MMA and subsequently terminated by TPPT.The latter is produced by disproportionation termination of PMMA radicals.Chain extension reaction tests ascertain the formation of dead polymers during the polymer storage and purification process of the crude polymers.Owing to very slow fragmentation reaction of the TPPT-terminated PMMA,that is,equilibrium between the dormant radicals and the growing radicals is difficult to establish,the polymer chains formed through AIBN or AIBME initiating polymerization are much more than that formed through initiation of the regenerated radicals,so,unobvious change of the molecular weight with conversion is observed.The molecular weights of polymers can be controlled by the ratios of monomer/initiator and TPPT/initiator.However,the first order kinetics of the polymerization and the polymers with narrow polydispersity are obtained,and these phenomena are discussed.This study provides useful information on how to design a better controlling agent.2.By combination of high trapping radical efficiency of thioketone with resonance of allylic radicals,a novel mediated agent,1,3,3-triphenylprop-2-ene-l-thione(TPPT)has been successfully synthesized.1H NMR method is used respectively to trace the TPPT-mediated radical polymerization of benzyl methacrylate(BzMA)at 70℃ and styrene(St)at 70℃ and 100℃,and the results reveal that they display different polymerization behaviors.In the case of BzMA polymerization,disappearance rates of the BzMA,the TPPT and the initiator,dimethyl 2,2’-azobis(2-methylpropionate)(AIBME)have the following order:AIBME>BzMA>TPPT,monomer conversion increases gradually with proceeding of the polymerization and approximately 60%of conversion is obtained within 12 h of polymerization.However,relatively high molecular weight of the PBzMA is formed at the beginning of polymerization,and then the molecular weights are not varied obviously with the conversion,but the polydispersity indices are relatively low.For the St polymerization at 70℃,the three components have the following order of disappearance rates:AIBME>TPPT>St,the monomer conversion is very low and less than 5%,the molecular weights do not change obviously with the conversion and their degree of polymerizations(DPs)are also low.When the polymerization of St is conducted at 100℃,the polymerization displays first-order kinetics and linear increase of the molecular weight with conversion.Electron paramagnetic resonance(EPR)and MALDI-TOF MS studies reveal interesting mechanisms of the three polymerizations.The TPPT can efficiently capture the growing PBzMA radicals to form PBzMA-TPPT· radicals,but the highly stable radicals are not be able to cross-terminated by the growing PBzMA· radicals in the polymerization of BzMA at 70℃.Although the growing PS radicals are efficiently trapped by TPPT to form PS-TPPT-radicals,the latter radicals are cross-terminated by the propagating PS radicals to produce the PS-TPPT-PS chains,but this termination reaction is irreversible,and the PS-TPPT-PS chains cannot be cleaved to regenerate the propagating radicals in the polymerization of St at 70℃.When the polymerization of St is conducted at 100°C,cross-termination reaction of the PS-TPPT· radicals with the growing PS· radicals is reversible and the growing PS radicals are generated from the dormant PS-TPPT-PS chains.The factors influencing the polymerization of St are also studied.3.The controlled free radical polymerization of butyl acrylate was studied using 1,3,3-triphenylprop-2-en-l-thione(TPPT).The use of TPPT and AIBME as control agents and initiators,respectively,owing to very slow fragmentation reaction of the TPPT-terminated PBA,that is,equilibrium between the dormant radicals and the growing radicals is difficult to establish,the polymer chains formed through AIBN or AIBME initiating polymerization are much more than that formed through initiation of the regenerated radicals,so,unobvious change of the molecular weight with conversion is observed.The molecular weight of the polymer can be controlled by the ratio of monomer/initiator and TPPT/initiator.And the polymerization process and product were characterized.4.A free radical polymerization thioketone control agent DMOTBP was synthesized.In the free radical polymerization,the thione compound can effectively capture the free radicals and form a dormant chain free radical with stable and no monomer initiating ability.The polymerization process of methyl methacrylate(MMA)and styrene(St)under different reaction conditions such as DMOTBP concentration and temperature were studied.Using DMOTBP and AIBME as control agents and initiators,due to the very slow fracture reaction of DMOTBP-terminated PMMA/PSt,that is,the balance between the dormant group and the growing free radicals is difficult to establish and the polymerization is initiated by AIBN or AIBME The polymer chains are much larger than the polymer chains initiated by regenerating free radicals,and thus the observed molecular weight does not change significantly with increasing conversion.The molecular weight of the polymer can be controlled by the ratio of monomer/initiator and TPPT/initiator.And the polymerization process and product were characterized.