Polymerization transfer

Hepuzer et al. [91] have used the photoinduced homolytical bond scission of ACPB to produce styrene-based MAIs. These compounds were in a second thermally induced polymerization transferred into styrene-methacrylate block copolymers. However, as Scheme 24 implies, benzoin radicals are formed upon photolysis. In the subsequent polymerization they will react with monomer yielding nonazofunctionalized polymer. The relatively high amount of homopolymer has to be separated from the block copolymer formed after the second, thermally induced polymerization step. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

For very active transfer agents, the transfer agent-derived radical (T ) may partition between adding to monomer and reacting with the polymeric transfer agent (Pn 1) even at low conversions. The transfer constant measured according to the Mayo or related methods will appear to be dependent on the transfer agent concentration (and on the monomer conversion).40 2 A reverse transfer constant can be defined as follows (eq. 20) ... 

These thiohydroxamic esters have seen use in grafting of PAN onto PE,iM of PS, PAM and I MPAM onto cellulose127128 and of PS, PMMA, PVP and PAM onto poly(arylene ether sulfone).12 7 The process involves derivitization of a parent carboxy functional polymer to form the thiohydoxamic ester 82 (R=polvmcr) which then behaves as a polymeric transfer agent and/or radical generator. 

Various side reactions may complicate RAFT polymerization. Transfer to solvents, monomer and initiator occur as in conventional radical polymerization. Other potential side reactions involve the intermediate radicals 165 and 167. These radicals may couple with another radical (Q ) to form 271 or disproportionate with Q to form 270. They may also react with oxygen. The intermediate radicals 165 and 167 are not known to add monomer. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

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-... 

Mickley et al. have proved that immobilization of radicals by occlusion predominates over transfer to polymer even in the polymerization of the strongly transferring vinyl chloride [49]. Currently, the occlusion theory is preferred for explaining autoacceleration in precipitating radical polymerizations. Transfer to polymer is of little importance for autoacceleration. 

Chain transfer to monomer is perhaps the most important chain breaking reaction in cationic polymerization. Transfer to monomer involves transfer of a /3-proton from the carbocation to a monomer molecule. This results in the formation of terminal unsaturation in the polymer molecule. Since in isobutylene there are two different types of j9-protons, two different unsaturated groups are possible ... 

It is particularly interesting to note that the transfer film thickness builds up as the counterface becomes smoother and that it exceeds the counterface roughness at a value of (Ra) of about 0.1 ym, which is close to the optimum surface roughness observed for dry conditions. It is suggested that it is the polymeric transfer film, and particularly its thickness relative to the surface roughness of the counterface, that is responsible for the observed minimum wear factor under dry conditions. 

Tip 8 Terminal double bond polymerization. Transfer to monomer and termination by disproportionation lead to dead polymer molecules with a TDB. This TDB may react with a polymer radical, thus forming a radical center somewhere along the chain of the combined molecules. This radical center, on propagation with monomer, will evenmally form a trifunctionally branched chain. 

Polymeric Transfer Reagents for Organic Synthesis with Self-Control... 

In such a reaction, which may be termed a "transfer reaction", an active species A, is transferred to the acceptor B. In practice, an excess amount of one of the components in such a reaction often is used, in order to effect complete conversion of reagent B to the required product BA, and to shorten the reaction time. At the aid of the reaction, the resulting product BA, must be separated from the excess reagent By using polymeric transfer reagents the separation is facilitated by a simple filtration step ... 

The halogen abstraction ability decreases according to I > Br > Cl. For example, in styrene polymerization, transfer coefficient for iodoacetates is C = 0.8, for bromoacetates C = 0.043, but only C = 0.0001 for chloroacetates. The C value increases with number of halogens, for example for dibromoacetates C = 0.27 and C > 10 for tribromoacetates. Among the most often use alkyl halides are halomethanes and aforementioned halogenated esters. Both of them—including... 

Several chain-transfer mechanisms are operative in coordination polymerization transfer by j8-hydride elimination transfer by yS-methyl elimination transfer to monomer transfer to cocatalyst and transfer to chain-transfer agent - commonly hydrogen - or other small molecules. The type of termination reaction determines the chemical group bound to the active site and the terminal chemical group in the polymer chain. The first three types produce unsaturated chain ends, while the last two types produce saturated chain ends. Figure 8.11 illustrates these five transfer mechanisms. 

While Equation 1.35, in combination with Equation 1.32, can give the number-average degree of polymerization, it is important not to ignore the role of the transfer reactions. Even in the case where transfer to initiator and solvent is nonexistent (presumably by careful initiator choice and a solvent-free polymerization), transfer to monomer can never be avoided entirely. Another way to approach the problem is to consider the simplest definition of dp, that is, the total number of polymerized monomers units divided by one-half the number of chain ends. Here, it is worth considering the number of chain ends produced by each of the processes [17]. Neither propagation nor termination by combination produce any chain ends (n=0), while both initiation and termination by disproportionation produce one chain end ( =1), and transfer reactions actually create two chain ends (n=2). The steady-state approximation again allows the absolute number of each of these processes to be substituted by the overall rate of each ... 

The steps of radical chain depolymerizaton (28) are presumably analogous to those in polymerization. Transfer should be more important because of the high temperature involved. 

Patchornik carried out many studies on polymeric transfer agents " they are insoluble polymeric agents which react with a soluble substrate to produce a soluble product. The product results from the transfer of a functional group from the polymer to the substmte or from the substrate to the polymer. 

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