Synthesis Of Block Copolymers Based On Polymerization Mechanism Transformation

Anionic polymerization is a truly living polymerization method. It has priority in synthesis of (co)polymers with low polydispersity, well-defined architecture, precise composition or ultrahigh molecular weight. The monomers susceptible to anionic polymerization are styrenic, dienic, vinylpyridines and (meth)acrylates. Particularly, anionic polymerization has been the most approximate method for polymerization of dienes such as isoprene and butadiene. In contrast, polymerization of monomers with polar substituents, such as carbonyl, cyano and nitro, involves tedious protection and deprotection processes. On the other hand, these polymers could be readily polymerized by controlled/"living" radical polymerizations. In addition, controlled polymerization of cyclic monomers like epoxides, lactones and cyclic siloxanes could only be conducted using ring-opening polymerization (ROP). Therefore, the combination of different polymerization mechanisms facilitates the synthesis of block copolymers from mechanistically incompatible monomers. This thesis focuses on exploring a directly efficient transformation from anionic polymerizaiton to RAFT polymerization, and utylized the transformation approach to prepare linear and dendrimer-like star copolymers. The alternative combination of anionic polymerization and ROP was also used to produce dendrimer-like star copolymers comprising PS and PEO components. Finally, globular dendrigraft star copolymers of ultrahigh molecular weights were prepared after iterative grafting polymeric anions to epoxy groups, which were introduced by epoxidation reaction.Direct transformation of anionic polymerization into RAFT-based polymerization. Styrene or isoprene was polymerized by anionic polymerization, the anionic propagating center was directly transformed into thiocarbonylthio moiety by sequential additions of1,1-diphenylethylene (DPE), carbon disulfide (CS2) and alkyl bromides (R-Br). The efficiency of the end-group transformation could reach as high as95%. The obtained macro chain transfer agents (macro-CTAs) were in-situ used to mediate RAFT polymerizations of a second monomer, including N-isopropyl acrylamide (NIPAM) and2-hydroxyethyl acylate (HEA). Thus amphiphilic dibock and ABA-type triblock copolymers were successfully prepared. These block copolymers are cleavable into homopolymer components due to the presence of the thiocarbonylthio moieties at their joint points, and also "clickable" as a whole onto Au nanoparticles through Au-sulfur complexation.Synthesis of dendrimer-like star copolymers by direct transformation of anionic polymerization into RAFT-based polymerization. Living dendrimer-like star polystyrene was first prepared by anionic polymerization in a continuous manner. The process involves stoichiometric reaction between polystyryllithium and "inimer"(prepared from monoaddition of MDDPE with s-BuLi), and the in-situ initiation of styrene polymerization from the intermediate alkyl diphenyllithium. The terminal anionic growing centers were directly transformed into thiocarbonylthio moieties after sequential additions of CS2and1-phenylethyl bromide to give a multifunctional dendrimer-like star polystyrene macro-CTA. The obtained macro-CTA was further used to mediate RAFT polymerization of either styrene or NIPAM to give well-defined dendrimer-like star (co)polymers. The amphiphilic copolymers formed nanofibre micelles in water, a selective solvent for PNIPAM.Synthesis of amphiphilic dendrimer-like star copolymers by tandem anionic polymerization and ROP. Living dendrimer-like polystyrenes of different generations were prepared by anionic polymerization in a "one-pot" method by alternative additions of "inimer" and styrene. End-capping with ethylene oxide (EO) provided dendrimer-like star polystyrenes with different number of terminal hydroxyl groups after terminating by methanol. These multifunctional polystyrenes were then used as macroinitiators for ROP of EO, resulting in formation of amphiphilic dendrimer-like star copolymers with PS in the core and PEO in the shell. The prepared amphiphilic diblock copolymers showed sperical micelles in selective solvents (methanol or H2O) and emulsification effect to help form stable emulsion in mixture of xylene and water.Synthesis of globular dendrigraft star-like copolymers by iterative epoxidation and anionic grafting reactions. Star copolymers with "Y-shaped" miktoarms were prepared by crosslinking off-chain living star using DVB as crosslinkers. The PI segment in the "Y-shaped" arms was subject to sequential epoxidation reaction to introduce epoxy groups, which were attacked by anionic living polymers (PSLi or PILi) to give dendrigraft star-like (co)polymers of first generation (G1.0). Repeating the cycles of epoxidation and anionic grafting reactions leads to formation of high-generation dendrigraft star-like copolymers of ultrahigh molecular weights. The molecular weights of polymers increased exponetially with increasing generations, while the intrinsic viscosity decreased for high-generation polymer due to its highly branched and compact structure. The light scattering results of these polymer solutions and AFM observation confirm the sperical structure of polymers.