Organocatalyzed Anionic Alternating Copolymerization

Alternating copolymer features a perfect periodic sequence structure,thus can be regarded as a special category of homopolymer which exhibits significantly different properties than the respective homopolymers of the comonomers and their random/gradient/block copolymers.Alternating copolymerization of epoxides and compounds with high reactivity but unable to homopolyerize due to kinetic and/or thermodynamic factors such as steric hindrance,pendantgroup conjugation effect,instability of the product,etc.has become an indispensable strategy to expand the variety of monomers and enrich the structures and properties of polymers.Similar to classic ring-opening polymerization,catalyst is the key factor for epoxide-based alternating copolymerization,and organometallic complex has been the main class of catalysts studied and used in this area for decades.The unprecedentedly fast development of metal-free/organocatalyzed polymerization in recent years has infused vigorous vitality into epoxide-based alternating copolymerization.However,insights into catalytic mechanisms and structureactivity relationship of the catalysts are critically inadequate,and there is still a profound lack of catalytic activity optimization strategies.The study of this thesis mainly focuses on the alternating copolymerization of epoxides with 3,4-dihydrocoumarin catalyzed by organobases,and alternating copolymerization of epoxides with carbon disulfide catalyzed by Lewis acidbase type two-component organocatalysts.Sepcial efforts are made to explore for the impacts of catalyst structures on the copolymerization mechanism,efficiency,and selectivity.The insights gained from the results are important for further advancement of epoxide-based controlled anionic alternating copolymerization.The main contents are summarized below.(1)Copolymerization of 3,4-dihydrocoumarin and ethylene oxide is performed using relatively mild and non-nucleophilic phosphazene bases.A "proton-shuttling" mechanism is proposed.Namely,when the basicity is appropriate,the catalyst can deprotonate the phenolic hydroxyl group,endowing it with adequate activity to react with the epoxide.Subsequently,the highly basic alkoxide anion,generated by the ring opening of the epoxide,can recapture the proton from the phosphazenium cation to reduce chain-end nucleophilicity.In this way,the propagating species always maintains a suitable activity to ensure the copolymerization proceeds in a strictly alternating manner,to avoid self-propagation of epoxide,and to suppress transesterification reaction.Compared to the copolymerization catalyzed by a phosphazene superbase,the alternating copolymers obtained by use of mild phosphazene base may have higher molecular weights and narrow molecular weight distribution.In addition,low-molarmass alternating copolymer with a perfect linear structure and defined ?,?-dihydroxyl terminal functionalities can be obtained and used as macromolecular precursors to react with a classic cyclic ester monomer or a diisocyanate without changing the catalyst.Therefore,novel block copolyester and poly(ether-ester)-based polyurethane can be one-pot tandemly synthesized.(2)Mild phosphazene bases are used for the alternating copolymerization of 3,4-dihydrocoumarin and styrene oxide,and the scope of catalyst is expanded to weakly nucleophilic organobases including cyclic guanidine(TBD)and cyclic amidine(DBU).Efforts are made to disclose the effects of the catalyst nucleophilicity on the copolymer structure and copolymerization mechanisms.When catalyzed by a phosphazene base,the alternating copolymerization solely starts from the exogenous initiator to afford higher monomer conversion and higher molar mass of the alternating copolymer with intra-macromolecular transesterification(back-biting reaction)effectively suppressed.These results further validate the "proton-shuttling" mechanism and its significance for improving the selectivity and controllability of alternating copolymerization.In contrast to the in situ chain-end activation and chain-growth mechanism followed by non-nucleophilic phosphazene bases,more complicated mechanisms are inferred for the ring-opening alternating copolymerization catalyzed by the weakly nucleophilic organobases.Without an exogenous initiator,the catalyst can directly attack the epoxide causing its ring opening,so that the alternating copolymerization is initiated and proceeds in a zwitterionic mechanism.In this case,the products are dominantly macrocylclic structures due to the prevalence of intra-macromolecular transesterification caused by the high nucleophilicity of the propagating chain end.On the other hand,when the exogenous initiator is present,the alternating copolymerization trends to occur from the exogenous initiator,and the final products are mainly linear structures.(3)On the basis of clarified reaction mechanisms,attemps have been made to develop novel alternating copolymer structures and their corresponding synthetic strategies utilizing the organobase-catalyzed alternating copolymerization of 3,4-dihydrocoumarin and epoxides.1)A facile synthetic method for cyclic polymers has been proposed based on the fact that intra-macromolecular transesterification occurs much more extensively than inter-macromolecular transesterification during the alternating copolymerization.Alternating copolymerization of 3,4-dihydrocoumarin and epoxide is carried out with a multi-hydroxyl poly(ethylene oxide)as a macroinitiator.In this way,the cyclic alternating copolymer generated during the alternating copolymerization can be easily isolated by precipitating the product in a selective solvent owing to the large molar mass and solubility difference from the poly(ethylene oxide)-containing starshaped product.2)Terpolymerization of 3,4-dihydrocoumarin with two different epoxides,one of which is cyclohexene oxide,is performed to obtain gradient-alternating type dual sequence distribution based on the different ring-opening reactivity of the epoxides.This method is expected to be used for adjusting the properties of alternating copolymers such as crystallinity and glass transition temperature.Kinetic analysis of terpolymerization shows that the reactivity of cyclohexene oxide is significantly lower than that of ethylene oxide and mono-substituted epoxides so that the structural unit constituted by 3,4-dihydrocoumarin and cyclohexene oxide are enriched in the second half of terpolymer chain.(4)Organobases or Lewis acid-base type two-component catalysts comprising an organobase and an alkylborane are used for the copolymerization of carbon disulfide and epoxide.The influences of the nature of organobase(phosphazene base,cyclic amidine)and the structure of alkylborane(triethylborane,tributylborane)on the copolymerization efficiency,selectivity,copolymer structure,and molar mass are investigated.It is found that nucleophilic DBU leads to alternating copolymerization of carbon disulfide and epoxide when used alone,while nonnucleophilic phosphazene base shows no catalytic effect.When DBU is used together with excess triethylborane or tributylborane,the rate of the copolymerization and the molar mass of the product are noticeably enhanced,but the reaction mechanism appears different for the two Lewis acids.The NMR results of small molecule mixtures indicate that DBU,tributylborane,and carbon disulfide form stable coordinated structure,which might be the reason for higher activity of the two-component catalyst,despite higher steric hindrance of Lewis acid,and also responsible for the suppressed formation of five-membered cyclic thiocarbonate.