Synthesis And Characterization Of Cyclic Topological Copolymers

Cyclic topological polymers including single and multiple cyclic polymers have gained growing interest due to their unique properties unlike branched and linear topological polymers.In contrast with the significant achievement obtained in the investigation of branched polymers,an efficient and practical synthesis of a variety of cyclic topological copolymers is still a challenge due to the inherent synthetic difficulty.Most of the studies on cyclic polymers focused on the synthesis and characterization of single-loop cyclic polymers and only a few literatures reported the preparation of multi-loop cyclic copolymers.The most general synthetic approach of cyclic topological polymers is intramolecular cyclization of multifunctional precursor bearing two series of complementary functional groups under high dilution.All the synthetic methods mentioned above are based on anionic,cationic or ring-expansion polymerizations,rigorous operation conditions are needed.Controlled radical polymerization techniques and "click" reaction facilitate the preparation of polymers with predetermined molecular architectures,such as block,grafted,star,cyclic and hyperbranched polymers at less stringent conditions and offers exceptional versatility of monomers comparing with living ionic polymerizations.Based on the researches of the precursors,this dissertation described several outspread works in the synthesis of cyclic topological copolymers:a series of multifunctional initiators were prepared at first,and they were used as the initiators for the synthesis of linear or star copolymers bearing two different kinds of functional group by the combination of controlled radical polymerization and ring-opening polymerization;after the intermolecular click reaction of the precursors under high dilution,a series of multiple cyclic copolymers, such as tadpole-shaped,figure-of-eight-shaped,theta-shaped copolymers were prepared and their physical properties were primary investigated.All these facts are the origin and impetus of this thesis.The main results obtained in this thesis are as follows:1.The tadpole-shaped amphiphilic linear poly(N-isopropylacrylamide)- block-cyclic polystyrenes(LPNIPAAM-b-CPS) have been successfully synthesized by combination of reversible addition-fragmentation chain transfer(RAFT) polymerization and "click" reaction.A linear acetylene-terminated PNIPAAM-b-PS with a side azido group at the junction between two blocks was prepared at first;and then intramolecular cyclization reaction produced the cyclic PS block using "click" chemistry under high dilution.The difference of surface property between linear-cyclic block copolymer and its linear precursor was observed,and the water contact angles on the former surface are larger than that of the latter surface.2.A synthetic route to well-defined eight-shaped polystyrenes(PSs) with controlled molecular weight and narrow polydispersities has been developed.The synthesis involves two steps:1.Preparation of a linear tetrafunctional PS with two azido groups one at each end of the polymer chain and two acetylene groups at the middle of the chain;2.Intramolecular cyclization of the linear tetrafunctional PS at very low concentration through a "click" reaction to produce the eight-shaped polystyrenes.The glass transition temperatures(T_gs) were determined by differential scanning calorimetry(DSC) and it was found that the decrease in chain mobility by cyclization resulted in higher T_gs for 8-shaped polystyrenes as compared to their corresponding precursors.3.Well-defined hetero eight-shaped copolymers composed of polystyrene(PS) and poly(ε-caprolactone)(PCL) were successfully synthesized,a tetrafunctional PS and PCL star copolymer with two PS and two PCL arms was prepared at first using the tetrafunctional initiator bearing two hydroxyl groups and two bromo groups;and then the terminal hydroxyl and bromo groups were translated to acetylene and azido groups through the reaction with 4-propargyloxybutanedioyl chloride and NaN₃ respectively;After the intramolecular cyclization reaction,the hetero eight-shaped copolymers was produced using "click" chemistry under high dilution.Their thermal behavior was investigated by DSC,and their crystallization behaviors of PCL were studied by polarized optical microscopy. The decrease in chain mobility of the eight-shaped copolymers restricts the crystallization of PCL and the crystallization rate of PCL is slower in comparison with their corresponding star precursors.4.Cyclic polystyrene(PS)-b-cyclic poly(ε-caprolactone)(PCL)s,which is denoted as miktocycle figure eight-shaped copolymers have been successfully synthesized. The synthetic process involves three steps:(1) preparation of a tetrafunctional initiator with two acetylene groups,one hydroxyl group and a bromo group;(2) synthesis of diblock copolymers with two azido groups,one at each end of the two chains and two acetylene groups at the junction of two blocks, N₃-PCL-(CH≡C)₂-PS-N₃;(3) synthesis of cyclic PS-b-cyclic PCLs by the intramolecular cyclization of the linear diblock copolymers through a "click" reaction under high dilution.Their thermal behavior was investigated by differential scanning calorimetry and it was found that the decrease in chain mobility of cyclic-b-cyclic copolymers restricts the crystallization of PCL.5.A synthetic route to well-definedθ-shaped copolymers has been developed successfully.The AB₂ miktoarm star copolymers,which contain one PCL chain terminated with two acetylene groups and two PS chains with two azido groups at their one end,(α,α'-diacetylene-PCL)(ω-azido-PS)₂,were synthesized by ROP,ATRP and the terminal group transformation at first;and then well-defined pureθ-shaped copolymers were prepared by intramolecular cyclization of AB₂ miktoarm star copolymers using "click" chemistry under high dilution.Their thermal behavior was investigated by DSC.The mobility decrease of PCL chain across PS ring in the theta-shaped copolymers restricts the crystallization ability of PCL segment.