Stable free radical polymerization propagation reactions

Scheme 1. The four propagation reactions based on the terminal model for random copolymerization by the stable free radical polymerization process.

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

Anionic and later cationic pol3Tnerization gave most of examples of living pol3rmerization systems until recently, when more sophisticated methods of manipulation with free-radical polymerization processes become available. These methods are based on the use of the compounds which reversibly react with propagating radical and convert it to the so-called dormant species . When the equilibrium between the active and dormant species is regulated by special catalysts based on a transition metal, this process is called atom transfer radical polymerization (ATRP). If this equilibrium is provided by stable radicals such as nitroxides, the process is called stable free-radical polymerization (SFRP). In the case when dormant species are formed via a chain transfer rather than reversible termination reactions, this process is referred to as reversible addition fragmentation chain transfer (RAFT) polymerization. All these techniques allow to produce macromolecules of desired architecture and molecular masses. 

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

In this mechanism of polymerization, a small amount of free radicals is generated. These attack the carbon-carbon double bonds of monomer molecules, bond to one carbon, and produce the more stable free radical this is the initiation step. Since few chains are initiated, the free radical attacks yet another monomer, adds to the double bond, and forms another free radical that, in turn, continues the process this is propagation. Eventually two developing free radical chains may bond together and terminate the chain reaction. 

It should be noted that, whereas the preceding discussion has been cast in terms of free-radical polymerizations, the thermodynamic argument is independent of the nature of the active species. Consequently, the analysis is equally valid for ionic polymerizations. A further point to note is that for the concept to apply, an active species capable of propagation and depropagation must be present. Thus, inactive polymer can be stable above the ceiling temperamre for that monomer, but the polymer will degrade rapidly by a depolymerization reaction if main chain scission is stimulated above T.. 

Free-radical polymerizations of certain monomers exhibit autoacceleration at high conversion via an additional mechanism, the isothermal gel effect or Trommsdorff effect (23-26). These reactions occur by the creation of a radical that attacks an unsaturated monomer, converting it to a radical, which can add to another monomer, propagating the chain. The chain growth terminates when two radical chains ends encounter each other, forming a stable... 

SFRP is particularly suitable for styrene polymerization. Its elementary reartions are the same as in the conventional free-radical polymerization. All the reactions of eqns [18]-[25] can possibly occur. Peroxides are often used as the initiator. In addition, nitroxide is added as the mediator. The nitroxide molecule bears a stable free-radical center that can temporarily stabilize the propagating radical... 

Like free radical polymerization, cationic polymerizations undergo chain transfer and termination reactions. Chain transfers can take place by proton transfer to monomer, by hydride transfer from a chain, and by a variety of alkylations and cyclizations. Termination by coupling two cationic centers is not possible, but termination can occur when propagating centers react with water or other basic contaminants to give stable products. 

Propagation is the second step in the free radical chain reaction. This step leads to new radicals but also to the formation of small stable molecules. Because the stability of a long chain free radical is usually higher than that of a small free radical, if small radicals are formed in the first step, they usually react with the polymer, forming polymeric chain radicals and small molecules. This type of propagation is known as radical transfer reaction. Using the notation Pn for the polymer and Rn for the polymeric radical, the radical transfer reactions can be indicated as follows ... 

The key feature of the use of a dormant species may be seen in the following general scheme (Scheme 1.32) that involves complexation of the propagating species by means of a stable nitroxide radical (Hawker et al, 2001). The P -0 bond of the alkoxy amine P -0-NR is thermally labile at the polymerization temperature, so this becomes the site for the insertion of monomer. Propagation then occurs at a rate that is much slower than for a simple free-radical addition reaction since the propagating radical concentration (which is governed by the position of the equilibrium with the alkoxy amine... 

The active center involved in the propagation reaction may be a free-radical, ion, or metal-carbon bond (see Chapters 6-10). A propagating species will be more stable if the unpaired electron or ionic charge at the end of the chain can be delocalized across either or both substituents X and Y. Such resonance stabilization is possible in (VII) but not in (Vm). Moreover when X and/or Y is bulky there will be more steric hindrance in reaction of Eq. (P2.7.2) than in the reaction of Eq. (P2.7.1). So, in general, head-to-tail addition as in Eq. (P2.7.1) is considered to be the predominant mode of propagation in all polymerizations. 

The nltroso monomer, formed by Irradiation of the dimer with short UV light. Interferes with the normal free-radlcal-lnduced polymerization process by reaction with the free radicals or with the photoactlvated nltroso monomers to form stable nltroxlde radicals v4ilch are not able to propagate the radical chain process and, hence, serve as efficient chain terminators. The process Is illustrated In the following scheme ... 

The activation of bonds is not the only requirement for a successful polymerization. In fact, the resulting free radical must be sufficiently stable so that it can add on monomer in a propagation reaction before any possible decomposition reaction or other side reactions take place. Theoretically, for example, the free radical activation of the carbonyl double bond should be possible ... 

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