| Ring-opening polymerization of lactones attracts much attention in recent years because the resulting polymers are applicable to biodegradable materials and medicine. Increasing interest in the synthesis of poly (lactones) has caused scientific interest in exploring new initiator systems, understanding the kinetics and mechanism of lactones polymerization. Due to the abundance of rare earth metals in China, it is valuable to explore and expand the applications of the rare earth metals in the synthesis of polymer materials that have new properties, especially in the field of medical polymer materials prepared by ring-opening polymerization using new rare earth metal complexes.In this article lanthanum tris (2,6-di-tert-butyl-4-methylphenolate) (La(OAr)3) was used as a single component initiator in the ring-opening polymerization of -caprolactone (CL). Orthogonal experimental method was used to explore the optimal polymerization condition when La(OAr)3 initiated the ring-opening polymerization of CL. The orthogonal layout L9 (34) was applied to explore the optimal polymerization condition because there are four major factors (temperature, time, monomer concentration and monomer/initiator ratio respectively) that affect the polymerization in the system. Changing levels of the four factors, five orthogonal layouts were designed to find the optimal condition. According to the results of the orthogonal experiments, [CL]=0.6mol/L and [CL]/[La]=1000 is the optimal polymerization condition when CL is initiated by La(OAr)3. Besides the optimal condition, the results of the orthogonal experiments show that temperature is the most important factor among the four factors. Orthogonal results under ten different temperatures (-15 ~80 ) were compared. It is concluded that 20 is the optimal polymerization temperature. Conversion can reach approximately 90% in ten minutes and reaction rate constant is 0.367min-1 under 20 .Characteristics of ring-opening polymerization of CL initiated by La(OAr)3 were investigated under seven different temperatures (-15 ~60 ). The characteristic of living polymerization is found in low polymerization temperatures (-15 and 4 ).Conversion and molecular weight increase linearly by the polymerization time in -15 and 4 The reaction rates under the two temperatures were investigated, finding that Ln([CL]0/[CL]) also increases linearly with the time, the polymerization is first order with respect to monomer concentration. The molecular weight distribute (Mw/Mn) is 1.30 measured by GPC. While in the. range of 20 ~60 , the reaction rate is very fast, conversion can reach approximately 90% in 20 minutes. In the beginning 20 minutes, conversion and molecular weight increase linearly with the polymerization time, the polymerization is first order with respect to monomer concentration The reaction rate constants under different temperatures were carried out according to the kinetic results It is found that the constants increase with the temperatures in the range of 30 -50 , while decrease in the range of 50 ~60 . The overall activation energy of CL polymerization initiated by La(OAr)i amounts to 53.5kJ/mol, which is carried out according to the different reaction rate constants under different temperatures. In the late of polymerization, conversion remains unchangeable when reaches maximum, while molecular weight decreases after reaching the maximum. The higher the temperature, the more the molecular weight decreases, it may be attributed to the influence of chain transfer reaction such as transesterfication to the system. The results of GPC also indicate that in the beginning of the polymerization, molecular weights and Mvv/Mn of the resulting polymers are almost same under different temperatures However, in the late polymerization period, the molecular weights and Mw/Mn appear with a great difference, the higher the temperature, the wider the Mw/Mn. It is concluded that transesterfication is very notable in the late po... |
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