Chapter 3
Stimuli-Responsive Block Copolymers by RAFT and Their Micellization Behavior
Downloaded by YORK UNIV on July 3, 2012 | http://pubs.acs.org Publication Date: April 12, 2007 | doi: 10.1021/bk-2007-0961.ch003
Yuting L i , Brad S. Lokitz, and Charles L . McCormick
Department of Polymer Science, The University of Southern Mississippi, 118 College Drive, Hattiesburg, MS 39406-0076
Here, we review our recent progress in the synthesis of thermally responsive block copolymers by reversible additionfragmentation chain transfer polymerization. In aqueous solution at room temperature these block copolymer chains exist as unimers but form micelles when the solution temperature is raised above the lower critical solution temperature. Incorporation of an active monomer Nacryloxysuccinimide into the triblock copolymer allows for facile formation of uniform shell cross-linked micelles by reaction with ethylenediamine in aqueous media. When cystamine was used as the crosslinker, the cross-linking process is fully reversible: the cleaved S C L micelles can be reformed using cystamine as a thiol-exchange compound. These S C L micelles can be used as potential nanoscale drug delivery carriers, and the rate of drug release can be easily controlled.
© 2007 American Chemical Society
In Cosmetic Nanotechnology; Morgan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
73
74
Introduction Controlled/"living" radical polymerization including nitroxide mediated polymerization, atom transfer radical polymerization, and reversible additionfragmentation chain transfer (RAFT) polymerization have been the research of focus in the last few decades since they are effective for preparing complex architectures such as blocks and stars. ' Although each controlled/"living" radical polymerization technique has its own characteristics and advantages, R A F T is arguably the most versatile since it is compatible with most functional monomers under conditions that are similar to conventional free radical polymerization. ' R A F T operates on the principle of degenerative chain transfer. Rizzardo et al. proposed a mechanism for R A F T polymerization as outlined in Scheme l . The consumption of C T A 1 and reversible fragmentation of species 2 to yield the reinitiating R fragment are often referred to as the "preequilibrium" shown in eq b of Scheme 1. Eventually, the "main" equilibrium (eq d) is reached in which active (kinetic) radical chain ends add to monomer or reversibly to dormant chains (termed macro-CTAs). The pseudo-first-order kinetics, the linear evolution of molecular weight with time, narrow molecular weight distributions, and the ability to prepare block copolymers under appropriate conditions attest to significant elimination of chain termination events such as radical coupling and chain transfer common to conventional free radical polymerization shown in e of Scheme 1. 1
2
3
4,5 6
7 8
Downloaded by YORK UNIV on July 3, 2012 | http://pubs.acs.org Publication Date: April 12, 2007 | doi: 10.1021/bk-2007-0961.ch003
3
a.
b.
Initiator
-
I·
+
Monomer
P*
+
S-C>-S-R
• 21·
^zz:
Ζ
3
1 c.
d.
R- +
Monomer
P„
Pn +
Monomer
ρ·
Prf
-f
S~C-S-P
n
P -S-
