肌酐乳酸胍催化L-丙交酯活性开环聚合反应研究

合成了一种新型有机胍化合物即肌酐乳酸胍(CGLA),并将其应用于L-丙交酯(LLA)的开环聚合反应.聚合反应动力学研究表明,CGLA引发的LLA开环聚合反应具有典型活性聚合反应特征,lnM]0/M]t-t、Mn-Conversion关系均呈准直线关系.同核去偶1H-NMR法对产物立构规整度的表征证明所合成PLLA具有较高等规度(76%).聚合反应的添加剂效应研究证明了大分子活性物种的非自由基及非离子性本质.以1H-NMR法对聚合反应的跟踪和增长大分子物种结构的表征证明了增长大分子活性端为肌酐胍-烷氧结构(CG-OR),在实验研究的基础上推断CGLA引发的丙交酯开环聚合反应遵循配位插入增长机理.

LIVING RING OPENING POLYMERIZATION OF L-LACTIDE INITIATED BY CREATININE-GUANIDINIUM LACTATE

A new organic guanidine compound,i.e.creatinine-guanidinium lactate(CGLA),was synthesized for the first time by the direct reaction of creatinine with lactic acid under selected conditions(50℃,3 h,water solution).Evaporating off water in vacuo the solid was recrystallized in water for three times,and a snow-white product was obtained.1H-NMR characterization indicated that the synthesized CGLA was of high purity(>99%).Using CGLA as a mono-component organic initiator the ring-opening polymerization(ROP)of L-lactide(LLA)was conducted for the first time.Bulk ROP of LLA was carried out at 100℃ under vacuum producing a poly(L-lactide)(PLLA)with narrower molecular weight distribution(LLA]_0/CGLA]_0=100,t=87 h,M_n=1.20×104～1.3×104,PDI=1.29).Investigation on the kinetics of the polymerization in toluene showed that plots of lnM]_0/M]_t versus time and M_n versus conversion were linear which were the typical features of living polymerization.To know the nature of the propagation species,an experiment on additive-adding effect of the polymerization was carried out as follows.2,2,6,6-Tetramethyl-1-prperidinyloxy(TEMPO;a free-radical capturer),H_2O and CH_3COOH(terminators for cation and anion)were added to the three started polymerization systems after the polymerization was conducted for 24 h(polymerization in toluene at 100℃,M]_0/I]_0=50,24 h,Conv.=30.2%,M_n=4.3×103).Maintaining the original polymerization conditions for another 24 h,then the contents in the three reactors were checked simultaneously.The experimental results revealed that the polymerizations were not stopped,but continued synchronously in the three reactors.The molecular structure characterization of the propagating polymer species by 1H-NMR indicated that the growing polymer bore a creatinineguanidinium-alkoxy bond as its active end.Based on the experimental investigations,the mechanism of the CGLA-initiated ROP of LLA was postulated as follow:initiation occurred through a monomer-activated mode.The association of the creatinine-guanidinium(CG+)with the acyl-oxygen atom in the LLA molecule greatly increased the nucleophilic activity of the lactate anion(LA-)in the CGLA molecule.Initiatin occurred when a lactate anion attacked at the carbonyl carbon causing the cleavage of the RCO-O bond in LLA molecule.Owning to that the reaction of CG+ with the cleavaged LLA residue RO-formed a molecule with polar covalent bond CG-OR,but not an ionic bond,the propagation was postulated to follow the coordination-insertion mode through the repeatedly insertion of the monomer in the CG-O bond in the propagating polymer molecule.Homonuclear decoupled 1H-NMR characterization of the methine in the product indicated that the synthesized PLLA possessed higher isotacticity(76%).