Chapter 2
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Comparison of Controlled Living Carbocationic and Radical Polymerizations Krzysztof Matyjaszewski Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213-2683
New controlled/"living" carbocationic and radical polymerizations are compared from the point of view of basic mechanisms including spontaneous and catalyzed equilibration between dormant covalent species and growing species, equilibration between onium ions / carbenium ions and persistent radicals / growing radicals, as well as the corresponding degenerative transfer reactions. Similarities and peculiarities of carbocationic and radical systems are reviewed. Fundamental requirements for the synthesis of controlled polymers such as quantitative initiation, fast exchange and low contribution of chain breaking reactions are discussed.
For a long time, carbocationic and radical polymerizations have been considered to be very difficult, if not impossible, to control at the level attainable for anionic polymerization of styrene and dienes. Polymers with unpredictable molecular weights, broad polydispersities and uncontrolled end-functionalities are usually obtained in both systems. Different reasons, related to the very fundamental nature of carbocations and radicals, are responsible for such behavior. Carbocations react extremely rapidly with alkenes ( k p ^ l 0 moW-L-S" ) and readily participate in transfer reactions by loss of β-protons, especially in the presence of basic impurities (1,2). On the other hand, radicals recombine and disproportionate with rates close to the diffusion controlled limits (3). Thus, in typical carbocationic and radical polymerizations, the concentration of growing species (carbocations or radicals) must be very low (k PP. (ki PP=kj K Q , where ki and KQ refer to the absolute rate constant of addition of the initiating radical to the alkene and the equilibrium constant for the initiating species, respectively; in the same way the apparent propagation rate constant is defined, kp PP=kp K ) . If k i P P « k P P , initiation is incomplete, molecular weights are higher than expected and polydispersities are high. A
P
A
A
A
A
e q
P
A
There is a multitude of initiators for A T R P . Any alkyl halide with activated substituents on an α-carbon such as an aryl, carbonyl, or allyl can be used, in addition to polyhalogenated compounds (CCU, HCCI3) and those with a weak X bonding such as N - X , S-X, O-X, etc. This includes not only low molar mass compounds but macromolecular species as well, used to form the corresponding block/graft copolymers. Group X must rapidly and selectively migrate between the growing chain and transition metal. Bromine and chlorine function best in this exchange. Fluorine is bound too strongly to the growing chain, and although iodine is a
In Cationic Polymerization; Faust, Rudolf, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
CATIONIC POLYMERIZATION
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good leaving group for acrylate polymerizations, it is involved in side reactions in styrene polymerization (heterolytic cleavage). Some pseudohalogens, thiocyanates for example, have also been used successfully for the polymerization of acrylates and styrenes.
IV. Other effects There are a few additional requirements for controlled radical and carbocationic polymerizations. The latter, as with any ionic process, must be carried out in the absence of moisture. Water may act not only as the transfer/termination agent but also as a coinitiator, especially when a large excess of Lewis acid is used. Protons generated under such conditions may be scavenged with proton traps such as hindered pyridines, however the resulting pyridinium ions may produce various salt effects which in turn affect polymerization. In radical processes, moisture is well tolerated and many of them are in fact carried out in aqueous media. On the other hand, compounds with mobile groups/atoms can act as transfer agents and should be avoided. Oxygen, which may not affect carbocationic processes, is a dreadful inhibitor to the radical reaction and must be carefully removed from all reaction mixtures. If the ion pairs described in systems 1 . l . A and 1.2.A dissociate to free ions it may be necessary to trap them using salts with common ions or reduce their concentration by using less polar solvents. It is not yet clear what the proportion of free radicals to caged radicals is in new controlled systems but the addition of an extra scavenger is very beneficial for such systems, indicating the presence of some free radicals.
Conclusions The above discussion demonstrates similarities between new controlled/"living" carbocationic and radical systems. Below, in Tables 1 and 2, a comparison between conventional and the new controlled cationic and radical systems is presented. Although conventional processes show many similarities to the new systems, there are significant differences between the conventional and controlled systems. Most conventional carbocationic systems polymerize faster than radical systems due to higher propagation rate constants and higher concentration of active sites. Initiation is relatively much slower than propagation for both systems which are very sensitive to either oxygen (radical) or moisture
In Cationic Polymerization; Faust, Rudolf, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
2. MATYJASZEWSKI
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Carbocationic & Radical Polymerizations
(cationic). The main differences are related to reaction temperatures, chain breaking reaction and monomers polymerizable via certain mechanism.
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Table 1 Comparison of Carbocationic & Radical Polymerization I. Conventional Systems Mechanism Propagation
Cationic k *10 mol^Ls-* +
5±1
1
p
*10"
[PI Main Chain Breaking
«10"
3±1
6±2
mol/L
3
7±1
=10" mol/L
Transfer
Termination
Τ 0 ;H 0
low (e.g.
