Metal-Free Ring-Opening Polymerization Of Epoxides Towards Functional Polyethers

Aliphatic polyethers and their derivatives,especially those based on poly(ethylene oxide)(PEO)and poly(propylene oxide)(PPO),constitute a rich catalog of functional polymers which have found widespread applications in the areas ranging from daily commodities to high value-added chemicals/materials.The burgeoning demands have spurred development on synthesis method aiming at improved efficiency,chemoselectivity,and environmental performance,as well as on functionalization method which embeds active group into the main-chain,side-group,and end-group structure.Ring-opening polymerization(ROP)of epoxides is a universal approach for synthesis of polyethers.In this dissertation,we are focusing on novel metal-free ROP methods to fulfill both high efficiency and chemoselectivity.Several long-standing challenges,eg.chain transfer to monomer,transesterification,and primary amine ethoxylation,are addressed,paving the way to well-defined functional polyethers.(1)There is an increasing awareness that amphiphilic ambiguous surfaces with small-scale morphological heterogeneities are particularly competent for antifouling applications.Hydrophilic PEO has shown great suitability for fabrication of amphiphilic surfaces because of the effects of the hydrated surface layer.Based on these,we design an(AB)_n type amphiphilic multiblock-like polymer constituted by PEO and bio-sourced betulin.A convenient organocatalytic synthetic route is developed and the structural characters are readily tunable.Contrary to common wisdom,protein resistance performance of the polymer thin film is enhanced as the surface hydrophobicity(content of betulin units)increases.Surface morphology study provides the rationale for such a unique phenomenon.When the density of betulin units in the polymer structure is substantially high,phase separation of the two incompatible components is restricted,so that the thin film holds molecular-level surface heterogeneity and effectively resist a broad spectrum of proteins.(2)Achieving both high efficiency and precise control is the constant topic of polymer chemistry,while remains a major challenge for ROP of epoxides.We thus design a metal-free bicomponent catalyst comprising mild phosphazene base and triethylborane(Et₃B).To our knowledge,this is the most powerful organocatalyst that have been developed so far.TOF is up to 6000 h^-1 for ethylene oxide and 2720 h^-1 for propylene oxide,which is the highest ever been reported for room-temperature ROP of these monomers.Weight fraction of the catalyst can be minimized to only 44 ppm,so that the produced PEO shows no cytotoxicity even without purification.PPO with molar mass up to 210 kg mol^-1(the highest ever been reported)with low dispersity(D_M<1.1)can be readily achieved owing to the absence of detrimental transfer reactions.The approach is readily applied on facile and controlled synthesis of end-functionalized,multi-functionalized,block,and non-linear(co)polyethers.Besides,the catalyst is also effective to afford step-growth polymerization of polyether-diol and diisocyanate,allowing(for the first time)one-pot one-catalyst synthesis of PPO-based polyurethane.(3)Aliphatic polyethers and derivatives represent a typical class of non-degradable polymers.Embedding aliphatic ester groups into the main-chain structures is a crucial strategy to convert traditionally non-degradable polymers into(bio)degradable polymers.We thus develop an unusual-initiator strategy,i.e.carboxylic acid-initiated ring-opening polymerization of epoxide,to achieve ester-embedded polyethers with defined number and position of the ester groups as well as controlled molar mass and structures.The long-standing challenge of ester lability under epoxide polymerization conditions is well addressed by a two-component chemoselective metal-free catalytic system.Therefore,this strategy provided a convenient and versatile one-step access to(bio)degradable polyether-based materials as well as?,?-heterobifunctional polyethers with readily cleavable,releasable,or modifiable end groups.Also,a unique slow-initiation controlled/living polymerization mode is exhibited,which contradicts the general knowledge about living controlled/polymerization that initiation should be no slower than the propagation.(4)Amino group is a key functionality for conjugation of PEO with a large variety of substrates in organic and aqueous media.However,tedious and costly synthetic detours have been needed for PEO amines because amino group is easily ethoxylated during the ROP of EO.Common route to NH₂-PEO-OH relies on mono-or bifunctional initiators bearing a covalently protected amino group,which can tolerate the ROP conditions and break down afterwards to recover the amino end group.But the synthesis and/or purification of the initiator,as well as the deprotection treatments,are apparently calling for simplification.The amine-Et₃B complexation is found to effectively prevent amino group from ethoxylation.This noncovalent protection aligns perfectly with hydroxyl-initiated ROP of EO using a bicomponent metal-free catalyst.Therefore,a noncovalent protection strategy is coined to allow unprecedentedly facile synthesis of well-defined?-amino-?-hydroxyl PEOs in one step with enhanced productivity.The protective Et₃B is readily removed by precipitation of PEO in ether.The existence of protective Et₃B and the initiator structure considerably impact the storage(oxidation)stability of PEO amines.