Synthesis Of Amphiphilic Block Copolymers Via Controlled Ring-opening Polymerization Catalyzed By Rare Earth Complex

Biodegradable amphiphilic block copolymers have potential application in biotechenology.Exploring new catalysts with high efficiency and low toxicity is crucial to the research of biodegradable materials.In this paper,single component yttrium tris(2,6-di-tert-butyl-4-methylphenolate)[Y(DBMP)₃]has been synthesized. Amphiphilic biodegradable PCL-b-PEG-b-PCL triblock and mPEG-b-PCL diblock copolymers with controlled molecular weight have been successfully prepared by the ring-opening polymerization ofε-caprolactone(ε-CL)employing Y(DBMP)₃ as catalyst with double-hydroxyl capped PEGs(DHPEG)or monomethoxy poly(ethylene glycol)(mPEG)as macro-initiator,respectively.From NMR and SEC analyses,there is only one kind of active center existing in either triblock or diblock system,and the molecular weights of PEG-PCL can be well controlled by adjusting the feeding molar ratio ofε-CL to PEG.From DSC,WAXD and POM analyses, thermal and crystallization properties of the copolymers greatly depended on the type of block copolymers and the length of each block.On the POM micrograph of PEG_10,000-(PCL_8,600)₂ which crystallized at 45℃,a unique morphology of concentric spherulites was observed due to the sequent crystallization of the PCL and PEG blocks.PCL-b-PEG-b-PCL triblock and mPEG-b-PCL diblock copolymers can self-assemble into micelles in water.The critical micelle concentrations(CMCs)were all below 10mg/L determined by fluorescence technique using pyrene as probe.The CMC value decreased with the increasing of PCL block length.The particle sizes determined by DLS were in the range of 30-80 nm increasing with the PCL block length.TEM images showed that these micelles were regularly spherical in shape with narrow distribution.The micelles formed by amphiphilic biodegradable copolymers have potential applications in drug delivery.A new monomer:2-oxepane-1,5-dione(OPD)has been attracting more and more attention in recent years.Dibutyltin dimethoxide(Bu₂Sn(OMe)₂)has high efficiency in initiating ring-opening copolymerization of OPD andε-CL which can get high conversions(＞90%)within a few minutes polymerization.¹H NMR spectrum showed that the content of OPD units in the copolymer coincided with the feeding molar ratio,but the conversion decreased as the increasing of the feeding molar ratio of OPD toε-CL.The molecular weights of the copolymers cannot be controlled by adjusting the feeding molar ratio of the monomer to the catalyst. (2,2-)Bis(1-caprolactone-4-yl)propane)(BCP)which acts as a cross-linking agent was introduced to copolymerize with OPD and CL which significantly increased the molecular weight of the copolymers.Carbonyl group in the main chain can be selectively reduced to hydroxyl group by NaBH₄ providing possibility for further modification.