Cyclic Polystyrenes:Synthesis,Purification And Their Properties

The relationship between topological structure and property has received continuing attention from research community.As one of the unique topological architectures,cyclic polymers demonstrate different properties compared to their linear precursors due to the lack of free chain ends,which have attracted immense attention.However,cyclic polymer is hard to synthesize.Recently,cyclic polymers with improved performance or more complicated structures have been explored according to the development of controlled living polymerization and efficient“click”chemistry.The approach of combining atom transfer radical polymerization(ATRP)and Cu(I)-catalyzed azide-alkynyl cycloaddition(CuAAC)has been one of the widely used methods to prepare cyclic polymers.However,not all the linear precursors undergo ring-closure reaction in this approach and even a trace amount of linear precursor will significantly affect the properties of cyclic polymers,which has become the obstacle for our deeply understanding of the cyclic topology-property relationship.Hence,the critical purification approach becomes another issue in the development of cyclic polymer field.In this thesis,we designed and synthesized cyclic polystyrenes via combining ATRP and CuAAC reaction:1)the synthetic condition effects on the cyclization yield were systematically investigated and the optimum cyclization conditions for the quantitative synthesis of cyclic polystyrene were explored;2)highly purified cyclic polystyrenes with various molecular weights were obtained by liquid chromatography at the critical condition(LCCC);3)the glass transition temperatures(Tg)of highly purified cyclic polystyrenes were systematically investigated?and the results were compared with those of linear precursors with different chain ends to further understand the cyclie topological effect.The detailed researches were summarized as following:(1)Fully chain end functionalized(>94%)linear polystyrenes were prepared by stopping ATRP at low conversion.According to the quantitative chain end substitution,a-alkynyl-?-azido linear precursors were obtained and their chain end functionalities were quantitatively analyzed by 1H NMR.Cyclic polystyrenes were synthesized by intra-molecular ring-closure reaction via CuAAC,where chain end functionality is proved to play an important role in the preparation of cyclic polymers.Furthermore,the effects of molecular weight,linear precursor concentration and solvent on the synthesis of monocyclic polystyrenes were systematically investigated.By tuning the reaction conditions,the optimum cyclization conditions for quantitatively preparation of cyclic polystyrenes were revealed and the successful eyclization was eonfirmed by eomprehensive characterization including SEC,]H NMR and FTIR spectra.The topological purity of cyclic product was proved to be higher than 98%by LCCC at the critical adsorption point of linear polystyrenes.(2)Cyclic polystyrenes with various molecular weights(3400 g/mol to 13400 g/mol)were obtained via combining ATRP and CuAAC reaction.As revealed by LCCC at the critical adsorption point of linear and cyclic polystyrenes,there were still some linear contaminates in the as-synthesized cyclic polymers.The MALDI-TOF results prove that the linear contaminates is inevitable during the preparation of cyclic polymers via combining ATRP and CuAAC reaction.The elution behaviors of linear and cyclic polystyrenes were systematically investigated by LCCC at their corresponding critical adsorption points,and the resolution between linear and cyclic polystyrenes were calculated and compared.As a result,the resolution is higher at the critical adsorption of cyclic polymers,which is satisfactory for the purification of low molecular weight cyclic polymers.For the first time.LCCC fractionation at the critical adsorption point of cyclic polymers was applied to putify cyclic polymers.Cyclic polystyrenes with ultra-high purities(>99.6%)were obtained via a two-step tandem-coupled LCCC fractionation at the critical adsorption point of linear and cyclic polystyrenes,respectively.(3)Linear polystyrenes with different chain ends were obtained via ATRP and CuAAC reaction.These linear chains possess high chain end functionalities due to the low polymerization conversion.The chain end effect on the properties of linear polystyrenes was revealed by comparing the molecular weight dependence of Tg.The infinite glass transition temperature(Tg,?)and empirical parameter in Fox-Flory euqation were calculated and the free volumes of different chain ends were compared accordingly.In addition,the molecular weight dependence of Tg for cyclic polystyrenes with ultra-high purities was investigated and compared with that of linear precursors.It reveals that they have the similar Tg,?,while cyclic polystyrene possesses higher Tg and lower molecular weight dependence.By blending linear and cyclic polystyrenes with various ratios,the linear contamination effect on the Tg of cyclic polystyrene was systematically investigated and the linear contamination effect on the Tg of cyclic polystyrene can be satisfactorily described by Fox theory.