Non-linear Graft Copolymer And Star Polymer Molecular Design, Synthesis And Characterization

Since the concept of living polymerization was proposed by Szwarc, its importance in polymer chemistry and applications is becoming more and more manifest. The living polymerization can enable us to synthesize the polymers with predetermined structures through selecting polymerization systems and polymerization conditions. Considering that more than 60% of synthetic polymer products on the market are produced by radical polymerizations, the living radical polymerizations are of more important in science and applications.Recent years, the living radical polymerizations, including atom transfer radical polymerization (ATRP), nitroxide-mediated stable free radical polymerization (SFRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization have been extensively applied to synthesize the block copolymers. For SFRP, the polymerization temperature is relatively high, and polymers obtained by ATRP are usually contaminated with metal salt. In comparison with these methods, the RAFT polymerization has the following advantages: versatile monomers, relatively gentle polymerization conditions, no metal in the obtained polymers etc.It is well known that solution and bulk properties of polymers are strongly influenced by their chain architecture. Polymers with narrow molecular weight distribution and homogeneous compositions (in the case of copolymer) and with well-defined architecture are essential for establishing structure/properties relationship. It is necessary to fulfill the design of macromolecules for obtaining the polymer materials with predetermined properties. In the past few decades, most of our fundamental understanding has been gained by studying well-defined and nearly monodisperse linear diblock and, to a lesser extent, triblock copolymers. However, very little is known about the influence of chain architecture on properties until now due to the difficulties associated with the synthesis of well-defined polymeric materials with special architecture. Controlling polymer properties through design and synthesis of copolymers and macromolecular architectures is a continuing theme for polymer chemistry. For probing relationship between the architecture and properties of polymers, and further designing the polymers with predetermined properties, it is necessary to apply the known polymerization mechanism to synthesize all kinds of polymer with special architecture. The special properties of nonlinear polymers, such as graft copolymers, star polymers, H-shaped, macrocyclic copolymers and comb-shaped polymers attracted much attention in recent years. Among them, the graft polymers have many applications in engineering plastics, thermoplastic elastomers, surface modifiers, compatibilizers, and surface active agents. Graft copolymers are composed of a main chain, and one or more side chains which are connected to the main chain via covalent bands. Generally, the branches are randomly distributed along the polymer backbone. Controlling the number of branches and their position on the backbone are very difficult. For example, how to synthesize theÏ€-shaped polymer, in which two branches are connected to a polymer chain. Based on our knowledge, because of the critical experiment condition and the limitation in monomers that can be applied in the living anionic polymerization, some special graft copolymers, such asÏ€-shaped and H-shaped polymers have been prepared from only a few kinds of polymers including polystyrene (PSt), polyisoprene (PI) and polybutadiene (PBD). Therefore, finding a new, convenient, and more versatile synthetic strategy for preparing these special graft copolymers is a very interesting project.All these facts are the origin and impetuses of this thesis. The main results obtained in this thesis are as follows:1. A more convenient synthetic route for preparation ofÏ€-shaped copolymers by the combination of RAFT mechanism and esterification reaction has been developed. The polymerization is of controlled nature, and varying the chain length of the centered PSt can adjust the branches position on the polymer backbone. The advantage of this method is the molecular weight and distribution of two branches in theÏ€-shaped copolymers can be controlled.2. The H-shaped copolymers, (PLLA)₂PSt(PLLA)₂, have been successfully synthesized by ATRP of St and followed ROP of LLA. Divergent reaction of Br-PSt-Br with diethanolamine formed four hydroxyl-terminated polymers, (HO)₂PSt(OH)₂ successfully. The (HO)₂PSt(OH)₂ obtained can be used as macroinitiator in the ROP of LLA with the aid of Sn(Oct)₂ to afford H-shaped copolymers, (PLLA)₂PSt(PLLA)₂.3. The heteroarm H-shaped terpolymers. [(PLLA)(PS)]PEO[(PS)(PLLA)] have been successfully synthesized by combination of RAFT polymerization and ROP. The PEO capped with two different terminal functional groups, dithiobenzoate and hydroxyl group at every chain end was used as macro RAFT agent in the RAFT polymerization of St, the triblock copolymer, [(HOCH₂)(PS)]PEO[(PS)(CH₂OH)] was obtained. The molecular weight of PSt segment could be controlled by feed molar ratio of St to RAFT agent and conversion. The heteroarm H-shaped terpolymers, [(PLLA)(PS)]PEO[(PS)(PLLA)] were finally obtained by the ROP of LLA using [(HOCH₂)(PS)]PEO[(PS)(CH₂OH)] as macroinitiator and Sn(Oct)₂ as catalyst. By changing the molar ratio of monomer to the macro initiator, the PLLA segments with various molecular weights were prepared. The molecular weight distribution of heteroarm H-shaped terpolymers is narrow (M_w/M_n<1.2).4. Double comb-shaped copolymers P(MA-MPEO)-block-PNIPAM-P(MA-MPEO) have been synthesized successfully by two steps polymerization. The ⁶⁰Coγirradiation polymerization of MA-MPEO in the presence of DBTTC at room temperature afforded a polymer, P(MA-MPEO)-SC(=S)S-P(MA-MPEO), which was subsequently used as macro RAFT agent in the RAFT polymerization of NIPAM to obtain a double comb-shaped copolymer, P(MA-MPEO)-block-PNIPAM-block-P(MA-MPEO). The RAFT polymerization is of controlled nature, the handle length of double comb-shaped copolymer is easier to be adjusted by varying polymerization degree of the PNIPAM.5. PSt stars with very reactive N-succinimidyl ester groups on their surface have been successfully synthesized by atom transfer radical polymerization of DVB using NSI-PSt-Br as macroinitiator. The NSI ester group is easily substituted by amine compounds. Thus the block copolymers, PSt-b-tetraaniline stars were prepared by substitution reaction of NSI groups on the surface of PSt stars with linear amine-capped tetraaniline. This novel strategy developed should be highly useful to synthesize various functional stars.6. PSt stars with very reactive N-succinimidyl ester groups on their surface have been successfully synthesized by RAFT polymerization of DVB using C₁₂H₂₅SC(S)S-PSt-NAS as macro RAFT agent. The N-succinimidyl ester group is substituted easily by amine compounds. Thus the block copolymers, PSt-b-tetraaniline stars were prepared by substitution reaction of N-succinimidyl groups on the surface of PSt stars with linear amine-capped tetraaniline. The substitution reaction efficiency of the surface N-succinimidyl groups is very high, and the density of tetraaniline on the surface of PSt stars is determined by the density of N-succinimidyl groups on the surface.