The Preparation Of Block Copolymers Combining Ring-opening Metathesis Polymerization And Enzymatic Ring-opening Polymerization

The increasing need of polymeric materials for various applications will dramaticallyrequire the construction of materials with tailor-made structures and properties, and thus itwill be necessary to develop novel catalysts or synthetic methods. In recent years, numerouschemical synthetic methods or catalytic systems have been developed for preparing polymericmaterials, while most of these catalysts are highly specific and selective, which will limit thegeneral application of a catalyst in multistep reactions. This constraint of chemical methodscan be overcome through the combination with other catalysts or methods, especiallyenzymatic polymerization. In the past two decades, enzymatic ring-opening polymerization(eROP) has rapidly developed and become an important synthetic method in polymerchemistry. Compared with a conventional chemical route, enzymatic polymerization hasmany advantages, including:(1) mild reaction conditions,(2) high control of chemo-andregioselectivity,(3) recyclability of biocatalysts,(4) few by-products and (5) high efficiencyto catalyze the ring-opening polymerization of macrocyclic lactones. By utilizing theseadvantages of enzymes, chemoenzymatic methods have been greatly developed forsynthesizing new polymeric materials that are otherwise difficult to prepare. More importantly,chemoenzymatic polymerization could further increase the diversity and complexity ofsynthesizing macromolecules via multistep reactions and cascade reactions.1In thechemoenzymatic polymerization, the most prominent example is the combination of eROP oflactones with radical polymerization to construct the designed copolymers, e.g., atom transferradical polymerization (ATRP), nitroxide mediated polymerization (NMP) and reversibleaddition fragmentation chain transfer (RAFT). In the above chemoenzymatic polymerization,bifunctional initiators with hydroxyl groups were usually employed fo r the proceeding ofthese reactions.In an attempt to expand this combination strategy to non-radical polymerization, ring-opening metathesis polymerization (ROMP) is undertaken to combine with eROP in thepresent research. Due to its versatility, effectiveness and functional group tolerance, ROMPusing Grubbs catalyst has been widely employed to construct functional polymers. Theseruthenium alkylidene catalysts could allow a variety of monomers bearing polar, apolar, andcharged functional groups to be successfully polymerized. To our knowledge, there were noprevious reports on the combination of ROMP and eROP, while some research groups haverealized the combination of ROMP and chemical ROP (cROP) to prepare copolymers.Hillmyer once successfully synthesized hydroxyl-terminated polymers through the hydrolysisof acetoxy, and then these hydroxyl-terminated precursors were used to initiate cROP,yielding block copolymers. Jerome constructed the polymer through the ROMP of acetoxylnorbornene, and then the ester groups were hydrolyzed to hydroxyl groups, which couldfurther initiate the cROP to prepare comb copolymers. In these reactions, the nucleophilichydroxyl group was used as initiators to trigger the cROP. However, hydroxyl group is notstable and can be easily oxidized to an aldehyde/ketone (CHO/CO) or a carboxylic acid group(COOH). In addition, it might coordinate into the metal catalytic center, which reduces theactivity of catalysts or even deactivates the catalysts. It will be of great significance todevelop new routes to avoid the tedious protection-deprotection procedure of hydroxyl group.Moreover, utilizing ROMP technique, end-functionalized macromolecular precursors areeasily prepared, which could be used as initiators in the eROP for the synthesis of blockcopolymers under mild reaction conditions. Thus, the combination of these two techniqueswill be a simple and versatile tool for preparing block copolymers, and also other polymericmaterials with tailor-made structures and properties.