Synthesis and Crystallization Behavior of Well-Defined Poly(L-lactide)-b-Polystyrene Copolymers

Synthesis and Crystallization Behavior of Well-Defined Poly(L-lactide)-b-Polystyrene Copolymers

Tao Liu, Dongdong Hu, Zhiqi Gu and Ling Zhao^*

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People's Republic of China

*To whom correspondence should be addressed. E-mail: zhaoling@ecust.edu.cn.

Keywords: ring-opening polymerization, atom transfer radical polymerization, block copolymers, crystallization

  Well-defined poly(L-lactide)-b-polystyrene (PLLA-b-PSt) was synthesized by using ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP) with 4-hydroxybutyl-¦Á-bromoisobutyrate as a dual initiator in a sequential two-step procedure. The results of ¹H NMR spectroscopy and gel permeation chromatography (GPC) showed that either the ring-opening polymerization of lactide or the radical polymerization of St was controlled so that PLLA-b-PSt with well-defined structure and controlled molecular weight could be obtained.

The crystallization behavior of PLLA in the PLLA-b-PSt was significantly affected by the PSt fraction, which was confirmed from the differential scanning calorimetry (DSC) and polarized optical microscopy (POM) results of the PLLA-b-PSt. As shown in Figure 1, the PLLA crystals may become more irregular, smaller and less with increasing the PSt fraction. Figure 2 showed that the melting temperature (T_m), cold crystallization temperature (T_cc) and crytallization temperature (T_c) decreased with increasing PSt fraction. The cold crystallization peak disappeared when the PSt fraction was over 60%. T_c firstly increased and then decreased with increasing PSt fraction. It was supposed that a small fraction of PSt acted as a nucleating agent promoting the crystallization of PLLA while large amount of PSt inhibiting that of PLLA. The crystallinity of PLLA-b-PSt was lower than that of pure PLLA and decreased with increasing PSt fraction.

Figure 1. DSC results of PLLA and PLLA-b-PSt (a) heating, 10 K/min; (b) cooling, 2 K/min

  

(a)                              (b)

  

(c)                              (d)

Figure 2. POM images of PLLA-b-PSt after non-isothermal crystallization at cooling rate of 5 K/min£º

(a) pure PLA; (b) 10% PS; (c) 60% PS; (d) 80% PS