Transfer in radical polymerizations

A certain kind of radical transfer can be modelled by the transfer of a hydrogen atom from an alkane molecule to a small alkyl radical. This reaction was studied in detail in the gas phase. With hydrocarbon partners, heats of reaction are a fairly safe measure of the relative rate of transfer, as the pre-exponential Arrhenius factors remain approximately constant for a series of transfers to a given radical. Tabulated thermodynamic data indicate, however, [31, 32] that the correlation between the heat of reaction and the transfer rate is not valid for reactions of a radical with polar substrates [32, 33], In condensed phases, transfer reactions have not been sufficiently studied. Polymerizations themselves are the source of the most valuable, though incomplete, information. 

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In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

Radical polymerization, including the question of the dependence of chain termination rate constant on the length of the macroradical chain, the possibility for continuous radical polymerization to be achieved through the complexing and stabilization of free radicals and of catalytic chain transfer in radical polymerization. 

We saw how intramolecular chain transfer in radical polymerization of ethylene creates a four-carbon branch on a polyethylene chain. What branch is created by a comparable intramolecular chain transfer during radical polymerization of styrene ... 

Chain transfer Chain transfer in radical polymerization involves the transfer of the radical center from a polymeric radical to another molecule. It can occur to all of the substances present in the polymerization system, and always causes a reduction in 0 and DP[, . Scheme 4.3 includes only transfer to monomer and solvent (or added transfer agent), but transfer to initiator and dead polymer can also occur. The former is often neglected since [/] is small relative to the concentration of other species, while the latter can be an important reaction impacting MWD and... 

The relative propensity of radicals to abstract hydrogen or add to double bonds is extremely important. In radical polymerization, this factor determines the significance of transfer to monomer, solvent, etc. and hence the molecular weight and end group functionality (Chapter 6). It also provides one basis for initiator selection (Section 3.2.1). 

Various methods for estimating transfer constants in radical polymerization have been devised. The methods are applicable irrespective of whether the mechanism involves homolytic substitution or addition-fragmentation. 

Traditionally thiols or mercaptans are perhaps the most commonly used transfer agents in radical polymerization. They undergo facile reaction with propagating (and other) radicals with transfer of a hydrogen atom and form a saturated chain end and a thiyl radical (Scheme 6.6). Some typical transfer constants are presented in Table 6.2. The values of the transfer constants depend markedly on the particular monomer and can depend on reaction conditions.4"1 44... 

The molecular weight and chain-end structure of polymers can be modified using the chain transfer reaction [65-68]. When an appropriate chain transfer agent, X-Y, is used in radical polymerization, two types of oligomers or telomers having different end groups, 4 and 5, are formed depending on the value of the chain transfer constant, Ctr> of X-Y used. 

Until recently, the most detailed kinetic investigations of phase transfer free radical polymerizations were those of Jayakrishnan and Shah (11, 12). Both of these studies have been conducted in two phase aqueous/organic solvent mixtures with either potassium or ammonium persulfate as the initiator, and have corroborated our earlier conclusions (2, 3)... 

Scheme 2 The mechanism of the chain transfer free radical polymerization of HPMA in the presence of alkyl mercaptans [26].

The low reactivity of a-olefins such as propylene or of 1,1-dialkyl olefins such as isobutylene toward radical polymerization is probably a consequence of degradative chain transfer with the allylic hydrogens. It should be pointed out, however, that other monomers such as methyl methacrylate and methacrylonitrile, which also contain allylic C—H bonds, do not undergo extensive degradative chain transfer. This is due to the lowered reactivity of the propagating radicals in these monomers. The ester and nitrile substituents stabilize the radicals and decrease their reactivity toward transfer. Simultaneously the reactivity of the monomer toward propagation is enhanced. These monomers, unlike the a-olefins and 1,1-dialkyl olefins, yield high polymers in radical polymerizations. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

Secondary reactions usually proceed in addition to template polymerization of the system template-monomer-solvent. They influence both kinetics of the reaction and the structure of the reaction products. Depending on the basic mechanism of reaction, typical groups of secondary reactions can take place. For instance, in polycondensation, there are such well known reactions as cyclization, decarboxylation, dehydratation, oxidation, hydrolysis, etc. In radical polymerization, usually, in addition to the main elementary processes (initiation, propagation and termination), we have the usual chain transfer to the monomer or to the solvent which change the molecular weight of the product obtained. Also, chain transfer to the polymer leads to the branched polymer. 

Fig. 39 Micrograph of polymerized uniform submicrostructure with 9 p.m line width and 50 xm line spacing. The structure was written by two-photon initiated electron-transfer free radical polymerization of diacrylate monomer Sartomer SR 349 at 775 nm via direct excitation of dye 5,7-diiodo-3-butoxy-6-fluorone(H-Nu 470) for (A) and dye 3 for (B) in the presence of Ar,Ar-dimethyl-2,6-diisopropylaniline...

Much of the work with labelled initiators in radical polymerizations is quite independent of determinations of molecular weights, attention being focussed on kinetic chain lengths instead of on molecular chain lengths (10). In a sensitized polymerization, the rate of initiation is identified with the rate at which initiator fragments are incorporated in the polymer to calculate the rate of initiation, it is necessary only to determine the empirical formula shown previously for the recovered polymer and the overall rate of polymerization. No assumptions are required concerning the mechanism of termination or the frequency of transfer processes involving monomer, solvent or polymer. Errors can arise from three causes ... 

Norrish and Smith [29] and later Tromsdorff et al. [30] described a polymerization of methyl methacrylate, the rate of which increased from a certain conversion. The number of monomers of similar behaviour was extended by methyl acrylate [31 ], butyl acrylate [32] and other acrylates [33] and methacrylates [34], and vinyl acetate. The effect was explained by the reduction of the termination rate caused by hindered macroradical mobil-ity in viscous medium it was called the gel effect, or the Norrish-Tromsdorff effect. The gel effect is clearly manifested in radical polymerizations of weakly transferring monomers in bulk. It is significant also in the presence of a good solvent. The gel effect is suppressed by the presence of poor solvents++ and by... 

Transfer to polymer was observed, for example, in the polymerization of p-isopropyl-a-methylstyrene initiated by sodium naphthalene. The macromolecule can eliminate a proton (in the presence of cationic initiators a hydride ion, in radical polymerizations of this monomer a hydrogen atom). An active centre is generated... 

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