Chain-transfer, free-radical polymerization

Ignatz-Hoover, F., Petrukhin, R., Karelson, M. and Katritzky, A.R. (2001) QSRR correlation of free-radical polymerization chain-transfer constants for styrene. J. Chem. Inf. Comput. Sci., 41, 295-299. 

In a free radical polymerization, chain transfer is an important reaction. Chain transfer to a monomer, solvent, mercaptan, or other growing chain can take place. When a chain transfer reaction to another chain takes place, it creates a radical, which acts as a site for further chain growth and grafting (see Chapter 2 for additional details) ... 

Branching is a special case of a process called chain transfer that is operational in free-radical polymerization. Chain transfer simply means the transfer of the radical from the growing polymer chain to another speeies. Effectively chain transfer eurtails polymer growth. For example, ehlorin-ated solvents are efiBcient ehain transfer agents (see Eq. 2.12). 

Scheme 1 The underlying initiation, propagation, activation/deactivation, and termination reactions for living and free-radical polymerization (chain transfer is not...

Chiefari, J. andE. Rizzardo, Control of Free Radical Polymerizationby Chain Transfer Methods, Chap. 12 in Handbook of Radical Polymerization, K. Matyjaszewski and T. P. Davis, eds., Wiley, New York, 2002. 

There are three ways that can be used to regulate the molecular weights of telechelic polymers produced by free radical methods chain transfer, control of the initiator-monomer ratio, and the temperature of polymerization. 

The transfer process does not change the radical concentration, but the chain length of the polymer. Without changing the free radical concentration, chain transfer processes remain hidden in any experiment measuring the rate of polymerization alone. The kinetic chain length is also unaffected by transfer, because the growing free radical centre stays active after the transfer, although... 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

The molecular weight distribution for a polymer like that described above is remarkably narrow compared to free-radical polymerization or even to ionic polymerization in which transfer or termination occurs. The sharpness arises from the nearly simultaneous initiation of all chains and the fact that all active centers grow as long as monomer is present. The following steps outline a quantitative treatment of this effect ... 

The minimum polydispersity index from a free-radical polymerization is 1.5 if termination is by combination, or 2.0 if chains ate terminated by disproportionation and/or transfer. Changes in concentrations and temperature during the reaction can lead to much greater polydispersities, however. These concepts of polymerization reaction engineering have been introduced in more detail elsewhere (6). 

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent ... 

There is less information available in the scientific literature on the influence of forced oscillations in the control variables in polymerization reactions. A decade ago two independent theoretical studies appeared which considered the effect of periodic operation on a free radically initiated chain reaction in a well mixed isothermal reactor. Ray (11) examined a reaction mechanism with and without chain transfer to monomer. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

Free radical polymerization of MMA is a well understood process. The kinetic mechanism neglecting the chain transfer reactions is given as follows (Odian (1970), Rudin (1982)). 

Phosphoranyl radicals can be involved [77] in RAFT processes [78] (reversible addition fragmentation transfer) used to control free radical polymerizations [79]. We have shown [77] that tetrathiophosphoric acid esters are able to afford controlled/living polymerizations when they are used as RAFT agents. This result can be explained by addition of polymer radicals to the P=S bond followed by the selective p-fragmentation of the ensuing phosphoranyl radicals to release the polymer chain and to regenerate the RAFT agent (Scheme 41). 

Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

The hazards of a rigid classification of substances which may modify the course of a free radical polymerization are well illustrated by the examples of inhibitors and retarders which have been cited. The distinction between an inhibitor or retarder, on the one hand, and a co-monomer or a transfer agent, on the other, is not sharply defined. Moreover, if the substance is a free radical, it is potentially either an initiator or an inhibitor, and it may perform both functions as in the case of triphenylmethyl. If the substance with which the chain radicals react is a molecule rather than a radical, three possibilities may arise (i) The adduct radicals may be completely unreactive toward monomer. They must then disappear ultimately through mutual interaction, and we have a clear-cut case of either inhibition or retarda-... 

The most common poly(alkenoic acid) used in polyalkenoate, ionomer or polycarboxylate cements is poly(acrylic acid), PAA. In addition, copolymers of acrylic acid with other alkenoic acids - maleic and itaconic and 3-butene 1,2,3-tricarboxylic acid - may be employed (Crisp Wilson, 1974c, 1977 Crisp et al, 1980). These polyacids are prepared by free-radical polymerization in aqueous solution using ammonium persulphate as the initiator and propan-2-ol (isopropyl alcohol) as the chain transfer agent (Smith, 1969). The concentration of poly(alkenoic add) is kept below 25 % to avoid the danger of explosion. After polymerization the solution is concentrated to 40-50 % for use. 

Scheme 36.1. Co -catalyzed chain transfer of methacrylate free radical polymerization.

Alkyl Co oxime complexes have been used as chain transfer catalysts in free radical polymerizations.866,867 Regioselective hydronitrosation of styrene (with NO in DMF) to PhCMe=NOH is catalyzed by Co(dmg)2(py)Cl in 83% yield.868,869 Catalytic amounts of the trivalent Co(dmg2tn)I2 (192) (X = I) generate alkyl radicals from their corresponding bromides under mild reaction conditions, allowing the selective preparation of either saturated or unsaturated radical cyclization products.870... 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

Figure 2,3 Chain growth polymerization exemplified by free radical polymerization of polyethylene a) initiation, b) propagation, c) chain transfer, and d) termination...

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