Free-radical copolymerization, transfer reactions

In stepwise reactions, all functional groups take part in bond formation. Their reactivity can be considered independent of the size and shape of the molecules or substructures they are bound to (Flory principle). If such a dependence exists, it is mainly due to steric hindrance. In chain reactions only activated sites participate in bond formation if propagation is fast relative to initiation, transfer and termination, long multifunctional chains are already formed at the beginning of the reaction and they remain dissolved in the monomer. Free-radical copolymerization of mono- and polyunsaturated monomers can serve as an example. The primary chains can carry a number of pendant C=C double bonds... 

As is well known from free radical copolymerization theory, the composition of the copolymers will depend only on the propagation reaction. The relative ability of monomer to add to a growing chain is influenced by the nature of the last chain unit and by the relative concentration. Generally, chain transfer to monomer by polymer radicals will occur to an appreciable extent, and the final product will be made up of homopolymers, multisegment block copolymers, and branched and grafted structures. In the presence of two or more monomers,... 

A zwitterionic tetramethylene initiates ionic homopolymerization, while a diradical tetramethylene initiates free radical copolymerization. As initiating species, zwitterions are likely to remain in the coiled gauche-conformation and collapse to small molecules. Diradicals, on the other hand, are easily transferred to the trans-conformation. Accordingly, diradicals are more effective initiators and more radical copolymerizations occur than ionic homopolymerizations. Addition of solvent will also influence the reaction of polar tetramethylene. A polar non-donor solvent may permit carbenium ion polymerization, while a polar donor solvent impedes it. 

Quantum chemistry thus provides an invaluable tool for studying the mechanism and kinetics of free-radical polymerization, and should be seen as an important complement to experimental procedures. Already quantum chemical studies have made major contributions to our understanding of free-radical copolymerization kinetics, where they have provided direct evidence for the importance of penultimate imit effects (1,2). They have also helped in our understanding of substituent and chain-length effects on the frequency factors of propagation and transfer reactions (2-5). More recently, quantum chemical calculations have been used to provide an insight into the kinetics of the reversible addition fragmentation chain transfer (RAFT) polymerization process (6,7). For a more detailed introduction to quantum chemistry, the interested reader is referred to several excellent textbooks (8-16). 

Polarity effects can also result in solvent effects in free-radical copolymerization. Recent theoretical studies (43,55-57) of small-radical addition reactions suggest that in a wide range of cross-propagation reactions, the transition structure is stabilized by the contribution of charge-transfer configurations. When this is the case, the extent of stabilization (and hence the propagation rate) will be... 

For the styrene (Sty)-vinyltriethoxysilane (VTES)-MA system, it was pointed out that didonor-acceptor combinations may polymerize via both a CTC-donor and/or CTC-CTC mechanism, depending on the initial monomer feeds.Studying the Sty-VTES-MA reaction, from both perspectives, it was concluded that the experimental data explained satisfactorily some of the assumptions in copolymerizations involving charge-transfer complexes. However, the data did not provide convincing proof that CTC formation preceded polymerization and whether all of the monomer molecules add to the growing chain in the form of a CTC. Also, the data did not provide the possibility of judging the effect of CTC formation on the overall mechanism of free-radical copolymerization. 

A number of metal chelates containing transition metals in their higher oxidation states are known to decompose by one electron transfer process to generate free radical species, which may initiate graft copolymerization reactions. Different transition metals, such as Zn, Fe, V, Co, Cr, Al, etc., have been used in the preparation of metal acetyl acetonates and other diketonates. Several studies demonstrated earlier that metal acetyl acetonates can be used as initiators for vinyl polymeriza-... 

Anionic polymerization of polystyrene takes place very rapidly- much faster than free radical polymerization. When practiced on a large scale, this gives rise to heat transfer problems and limits its commercial practice to special cases, such as block copolymerization by living reactions. We employ anionic polymerization to make tri-block copolymer rubbers such as polystyrene-polybutadiene-polystyrene. This type of synthetic rubber is widely used in the handles of power tools, the soft grips of pens, and the elastic side panels of disposable diapers. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

As the amount of polymer in the system increases, the probability of interaction of growing free radicals with polymeric (inactive) species either by transfer or, sometimes, by copolymerization through residual double bonds must increase. Because such reactions automatically give rise to branched and crosslinked species, reliable experimentation aimed at studying these processes is difficult to achieve. This follows from the fact that the region of the reaction in which study is essential is precisely that region in which there are rapid changes in the characteristics of the polymer produced, particularly in solubility. This is of... 

When unsaturatcd polymers have hydrogen or halogen atoms in a-position to the double bonds, they are especially sensitive to chain transfer by a free radical attack. Therefore in these cases, the graft copolymerization may involve a combination of two initiation processes which occur simultaneously and compete with each other, one by chain transfer, the other by addition copolymerization. The relative importance of both processes is again dependent on the nature of the polymerizing monomer and of the backbone polymer involved in the reaction. 

These investigations have demonstrated the successful application of cyclodex-trins in polymer synthesis in aqueous solutions via free radical polymerization or via a oxidative recombination mechanism. Some special aspects of cyclodextrins were found concerning the kinetics, chain transfer reaction, and copolymerization parameters [63],... 

Paramagnetic centers containing a sulfur atom in different oxidation states, (=Si-0)3Si-0-S = O, (=Si-0)3Si-0-S 02, (=Si-0)3Si-0-S02-0, and (=Si-0)3Si-0-S02-0-0, were obtained in Ref. [118]. Their radio-spectroscopic parameters were determined, and the mechanism of free radical oxidation of S02 molecules in this system was established. The mechanism of the initial steps of free radical polymerization and copolymerization of hydrogen- and fluorine-substituted unsaturated hydrocarbons was studied in Ref. [117]. The pathways were found and the kinetic parameters were determined for reactions of intramolecular H(D) atom transfer between r (CH3, CD3, CH2-CH3) and r (CH2-CH2, CD2-CD2), in the structure of (=Si-0)2Si(r)(rI) [120]. 

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

---