Chapter 5
Solvent-Free Stable Free Radical Polymerization: Understanding and Applications
Downloaded by UNIV OF GUELPH LIBRARY on September 6, 2012 | http://pubs.acs.org Publication Date: January 28, 1999 | doi: 10.1021/bk-1998-0713.ch005
Peter G . Odell, Nancy A . Listigovers, Marion H . Quinlan, and Michael K . Georges Xerox Research Centre of Canada, Mississauga, Ontario L5K 2L1, Canada
The kinetics of Stable Free Radical Polymerization permits the polymerization to be conducted without solvents. The use of nitroxide as a reversible chain terminator provides control over the polymer architecture in a free radical polymerization. The concentration of the unbound nitroxide capping agent controls the kinetics. The difference in the nitroxide concentration behaviour between styrene and acrylates has been studied over the course of the polymerization. A variety of block copolymers have been synthesized and new GPC techniques developed to elucidate their structure. The polymerizations have also been conducted in supercritical carbon dioxide.
Stable Free Radical Polymerization (SFRP) offers the advantages of free radical polymerization, while sharing many of the attributes of other living polymerization systems, such as the ability to synthesize block, dendrimeric, and hyperbranched copolymers. Unlike other "living" polymerizations, such as anionic or group transfer polymerization (GTP), SFRP is inexpensive, robust, can be performed without any prior purification of monomers, and is compatible with a wide variety of functional groups. In addition, GTP is restricted by its applicability to (meth)acrylates alone. Anionic polymerization, although applicable to both styrenics and (meth)acrylates is limited to homo and block copolymers of finite architecture, since cross-initiation between disparate monomers is not possible. In SFRP, cross-initiation is possible, which substantially broadens its synthetic use. The key to SFRP is the use of nitroxide stable free radicals to reversibly terminate the propagating chains, thus substantially reducing premature irreversible termination, and enabling monomer to continue to add in a controlled fashion over the course of the polymerization. The replacement of conventional, irreversible termination with reversible termination by a small molecule in the polymerization eliminates
80
©1998 American Chemical Society In Solvent-Free Polymerizations and Processes; Long, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
81
the prospect o f an autoacceleration, or Trornmsdorf effect (7). This allows the reaction to be readily carried out in a bulk or solventless manner. Because solventless polymerizations are easy to perform, and do not necessitate the eventual removal o f solvent, they were used extensively in the development of the S F R P process, and continue to be used for the synthesis o f homopolymers and block copolymers. This chapter examines the chemistry o f the S F R P process, under solventless conditions, along with recent developments i n the synthesis and characterization o f block copolymers. Preliminary results o f the S F R P process under supercritical C 0 polymerization conditions, an alternative route to freeing a polymerization of conventional solvents, are also presented. A l l living free radical polymerizations employ a small molecule to reversibly terminate the propagating polymer chain. The first example o f this was reported by Borsig et aL who showed that methyl methacrylate oligomers, terminated with primary radicals derived from l,l,2,2-tetraphenyl-l,2-diphenoxyethane, could initiate a conventional polymerization o f methyl methacrylate by the homolytic cleavage o f the terminal group from the oligomer chain (2). Otsu et al subsequently introduced the use of iniferters to provide a more controlled polymerization (3-4). Dithiocarbamate radicals were shown to reversibly terminate growing polymer chains such that the polymer chains increased in molecular weight in a linear fashion with conversion. However, the propensity o f the dithiocarbamate radical to initiate new chains during the course o f the polymerization, or to lose CS2, prevented the polymerization from behaving in a welldefined, living manner (5). In the 1970's, a series of papers were published showing that alkoxyamines were thermally unstable and would regenerate the nitroxide, or a hydroxylamine, upon continued heating (6-7). In 1984, Rizzardo and Solomon successfully used the l-(2-cyano-2 -propoxy)-2,2,6,6-tetramethylpiperidine adduct to initiate the synthesis of acrylate oligomer, although in low yields (8-9). Georges et al built o n this initial result to synthesize polystyrene resins with polydispersities narrower than what was considered theoretically possible (
