Chapter 9
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Using Atom Transfer Radical Polymerization in Environmentally Benign Processes Scott Gaynor, Jian Qiu, and Krzysztof Matyjaszewski Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213
Atom
Transfer
Radical
Polymerization
(ATRP)
is
a
controlled/"living" radical polymerization system that allows for the synthesis o f well-defined polymers with predetermined molecular weights, functionalities, and architectures.
The
ability to control these parameters enables the preparation o f polymeric materials with novel, and heretofore
unknown,
utility. Materials prepared by A T R P perform more efficiently than
comparable
manufacture,
and
polymers, generate
conventional processes.
require less
less
catalytic
material
to
waste
than
This paper describes the
latest
progress i n the development o f more efficient catalyst systems for A T R P , the extension to more environmentally
friendly
media, and some o f the advanced materials prepared.
Introduction A t o m transfer radical polymerization ( A T R P ) is a metal-catalyzed process through w h i c h a reversible activation/deactivation o f growing polymer chains suppresses termination and other chain breaking reactions (1, 2 ) , Scheme 1.
© 2 0 0 2 A m e r i c a n C h e m i c a l Society
In Advancing Sustainability through Green Chemistry and Engineering; Lankey, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
113
114 A T R P allows for the preparation of smart materials that can be used more efficiently to perform the same application as other polymers, if they exist, but with less material. Additionally, the research that is being pursued is designed to use less transition metal catalyst, or to provide for economically feasible catalyst recovery, which prevents the generation of catalyst waste streams which would then require disposal. The A T R P activation reaction involves the homolytic cleavage of a carbonhalogen bond ( P - X ) at the end of a polymeric chain (P ) or a small molecule (R), that is, an initiator molecule, by oxidation of the transition metal catalyst ( M / L ) . The moiety adjacent to the halogen stabilizes the radical, either through resonance or through induction. Some examples (and the polymers they mimic) include benzyl, 1-phenylethyl (styrenes), perfluoroalkyl (PTFE), 2-propionitrile (acrylonitrile), 2-propionate (acrylates), and 2-isobutyrate (methacrylates). Heteroatom-halogen bonds can also be activated and used for initiation, such as in sulfonyl chlorides (3).
Downloaded by PENNSYLVANIA STATE UNIV on September 3, 2012 | http://pubs.acs.org Publication Date: July 19, 2002 | doi: 10.1021/bk-2002-0823.ch009
n
n
n
t
^ P — X
n
+
M /L
*
t
v^P
#
+
n
X—M
n + 1 t
/L
dead chains P = polymer chain, small organic molecule (R) n
H
F C-CF - H C ^ / 3 C y
R =
3
2
3
c n
w
H Cjp 3
>o
μ
0
Scheme 1
The radical (R*) formed as a result of the activation reaction follows three routes: initiates and continues propagation by addition of monomer (M); reacts with the transition metal halide ( X - M / L ) and reforms the dormant organic halide and the original catalyst (deactivation); or terminates via coupling or disproportionation. The fate of the radicals can be described by equation 1. n+1
t
^
n
at
=k [M /L][RX]-k [R.][X-M a
t
d
n + 1 t
/L]-k [R*]
2
t
In Advancing Sustainability through Green Chemistry and Engineering; Lankey, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
(1)
115 The stationary concentration of radicals is established by a balance between rates of activation and deactivation, rather than the balance between rates of termination and initiation, as in conventional radical polymerization. Through a process called the persistent radical effect (PRE) (4), an equilibrium between the radical and the dormant species is established in A T R P during the early stage of the polymerization, with the dormant species dominating (the stationary concentration of which is on the order of 10" to 10" M , versus 10" to ΙΟ" M for radicals). Because the reaction between the radical and the transition metal halide is designed to be so fast that only a few monomer units are added before the active radical is converted to the dormant alkyl halide, the polymer chains grow gradually and continually. The net result is the formation of well-defined polymer chains. These chains have the same end groups (from the R of the original initiator) and the halogen. The degree of polymerization (DP ) of the polymer chain is described by the molar ratio of consumed monomer to added initiator (DP = A[M]/[R-X] ), and the distribution of molecular weights is relatively narrow ( M / M
