High molecular weight poly (lactic acid) (PLA) was obtained by chain extending with hexamethylene diisocyanate (HDI). The influences of the amount of chain extender, reaction time, and molecular weight changes of prepolymers on the poly(lactic acid) were investigated. PLA prepolymer with a viscosity, average molecular weight ( ) of 2 × 104？g/mol was synthesized from L-lactide using stannous octoate as the catalyst. After 20？min of chain extension at 175°C, the resulting polymer had of 20.3 × 104？g/mol and of 10.5 × 104？g/mol. Both FT-IR and 1H-NMR verified that the structure of PLA did not change either before chain extending or after. The optically active characterized that the chain extending-product was left handed. DSC and XRD results showed that both the and the crystallinity of PLA were lowered by chain-extension reaction. The crystalline transformation happened in PLA after chain extending, crystalline form to form. 1. Introduction At present, plastics waste disposal has become a serious problem worldwide. There is a strong need to provide plastic materials suitable for packaging, which will also be degradable and result in products that are environmentally safe [1]. Concerning the feasibility of the use of renewable raw materials and production process, the poly (lactic acid) (PLA) has become a very promising biodegradable polymer [2, 3]. High molecular weights are needed for PLA to have good physical properties. Until now, high molecular weight PLA was synthesized by ring-opening polymerization of the lactide [4], which is relatively complicated and expensive. Direct polycondensation of lactic acid is a low-cost process to produce PLA [5]; however, it is hard to increase the molecular weight enough because of the difficulty of removing the water from the system. A large number of investigations have been made to improve PLA properties via plasticization, copolymerization, and blending with elastomers [6–14]. Among them, low-cost, nontoxic HDI as chain extender is the simplest and most effective way. Addition of a chain extender to improve the molecular weight of PLA, it can solve the problem of degradation time of polylactic acid being uncontrollable [15]. But most of the researches were focused on the molecular weight of the products, optical activities and the transition of crystal form of the products effect of chain extension reaction were ignored. In this paper, higher molecular weight PLLA was produced by ring-opening polymerization of lactide followed by chain extension. Hexamethylene diisocyanate (HDI) was used as the chain extender.
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