(217ew) 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