Chain transfer mechanism

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

Suitable catalysts are /-butylphenylmethyl peracetate and phenylacetjdperoxide or redox catalyst systems consisting of an organic hydroperoxide and an oxidizable sulfoxy compound. One such redox initiator is cumene—hydroperoxide, sulfur dioxide, and a nucleophilic compound, such as water. Sulfoxy compounds are preferred because they incorporate dyeable end groups in the polymer by a chain-transfer mechanism. Common thermally activated initiators, such as BPO and AIBN, are too slow for use in this process. 

R may be a radical formed by the decomposition of an initiator or a growing radical chain. Similarly, grafting by the chain-transfer mechanism occurs when the branched part consists of another monomer. Since cellulose is a poor transfer agent [8], the efficiency of grafting is quite poor. Incorporation of—SH groups into cellulose enhances the probability of chain transfer. This can be achieved as follows ... 

As shown by the data in Fig. 31, the chain transfer constant of this initiator, Q = 1.0. In this context it is of interest to remember that the effect of initiator concentration on the molecular weight of HSi-PaMeSt was negligible, probably because of unfavorable thermodynamics (Sect. III.B.3.b.iv.). In contrast, with isobutylene chain transfer from the propagating carbenium ion to initiator is thermodynamically favorable (see Sect. IH.B.4.b.i.). Thus it is not surprising to find a large Q. The chain transfer mechanism has been illustrated in Scheme 5. 

Two competing chain-transfer mechanisms in copolymerization of CO and ethene catalyzed by Pd11 acetate/dppp complexes were found. One involves termination via an isomerization into the enolate followed by protonation with methanol the rate of this reaction should be independent of the concentration of the protic species. The second chain-transfer mechanism comprises termination via methanolysis of the acylpalladium species, and subsequent initiation by insertion of ethene into the palladium hydride bond.501... 

In this study, molecular weight of the produced polymers will not be tracked over the course of the reaction. Thus, in order to simplify the model, chain transfer mechanisms will not be considered, along with side reactions, such as the production of carbon disulfide. Each of these reactions, as well as the molecular weight, plays a significant role in the iniferter polymerizations however, to simplify the system, it is essential to examine only the core reactions which contribute significantly to the mechanism. 

Numerous papers have appeared on further detailed studies of the mechanism, including chain transfer mechanisms, epimerization of the stereocenter shortly after its formation, the role of agostic interactions, etc. A special issue of Chemical Reviews gives a complete overview [29],... 

It is important to note that in methanol as the solvent the reaction is much slower and also the molecular weight is much lower. Apparently a major part of the palladium complex occurs in an inactive state and the termination reaction is relatively accelerated by methanol. This suggests that ester formation is the dominant chain transfer mechanism in methanol, although P-hydride elimination will still occur. 

The percentage of diester, keto-ester and diketone end groups in the copolymers depends on the relative rates of alcoholysis and protonolysis. For example, when these two termination processes occur at a comparable rate, the diester, keto-ester and diketone end groups are in a 1 2 1 ratio, respectively [11], The use of oxidant co-reagents may increase the amount of ester end groups [lb], while the chain-transfer mechanism controls the nature of the termination metal product (Pd-OR, Pd-OH and Pd-H). 

Hessen and co-workers discovered that the reaction of ethylene with [Cp 2La(C4H3S)]2 (prepared from [Cp 2LaH]2 and thiophene) in the presence of thiophene formed thienyl-capped PE, H(CH2CH2)42-C4H3S) (e.g., 80 °C, ethylene 7.5 atm, activity = 27 g-polymer mmol-La h , 4/ =1300, PDI = 1.4). They revealed that C-H activation of thiophene was the only chain-transfer mechanism that is in operation, and that all of the PE chains produced under the conditions they examined were capped on one side by a thienyl group. 

To explain the results obtained in our researches, we could take into consideration a chain transfer mechanism, such as the following one (at... 

If the reaction takes place in the presence of monomer, grafting occurs in the usual manner. The fact that some homopolymerization also occurs in the redox-catalyzed grafting system can be explained by a chain-transfer mechanism (Reaction 10). The growing polymer radicals can abstract hydrogen atoms from the monomer, forming monomer radicals and thereby initiating homopolymerization. 

In this type of system, the backbone rubber acts as a chain transfer site for the monomer, and an initiator is chosen which preferably attacks the a-hydrogen position on the vinyl polymer backbone. Efficiency of the grafting reaction based solely on the chain transfer mechanism depends on several competing reactions (5). 

The molecular weight values of poly(TMC) obtained from oxetane and C02 in the presence of a (salen)CrCl/n-Bu4NN3 catalyst system were generally lower than the theoretical values (Mn = 11050 Mn (theoretical) = 85 000). This situation was most likely due to a chain transfer mechanism arising from the presence of trace amounts of water in the system. However, when the catalytic runs were carried out under rigorously anhydrous conditions, the molecular weights more closely tracked the predicted values. 

Pentadienyl-terminated poly(methyl methacrylate) (PMMA) as well as PSt, 12, have been prepared by radical polymerization via addition-fragmentation chain transfer mechanism, and radically copolymerized with St and MMA, respectively, to give PSt-g-PMMA and PMMA-g-PSt [17, 18]. Metal-free anionic polymerization of tert-butyl acrylate (TBA) initiated with a carbanion from diethyl 2-vinyloxyethylmalonate produced vinyl ether-functionalized PTBA macromonomer, 13 [19]. 

Resconi, L., Piemontesi, F., Franciscono, G., Abis, L. and Fiorani, T., Olefin Polymerization at Bis(pentamethylcyclopentadienyl)zirconium and -hafnium Centers Chain-transfer Mechanisms , J. Am. Chem. Soc., 114, 1025-1032 (1992). 

Hexene Polymerization Polymerization of 1-hexene (and also propylene) by the Kaminsky catalyst [(Cp2ZrCl2/(MeAlO) /toluene] differs fundamentally from that of ethylene in that beta hydrogen elimination is the only detectable chain transfer mechanism. Insertion of 1-hexene into the Zr-C bond in Cp2ZrCH3+ produces Cp2ZrCH2CH(CH3)C4H9+. The electron donating... 

Musaev DG, Froese RDJ, Morokuma K, Molecular Orbital and IMOMM Studies of the Chain Transfer Mechanisms of the Diimine-M(II)-Catalyzed (M = Ni, Pd) Ethylene Polymerization Reaction, Organometallics, 17, 1850-1860 (1998)... 

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