Transfer with monomer

In this reaction a hydrogen atom is transferred from the propagating chain to the mo. omer, with the formation of a stable molecule and a new 

This mechanism is in agreement with the presence of terminal vinylidene groups in polypropene [77], and the dependence of molecular weight on monomer concentration. 

Molecular weights of polymers from styrenes deuterated in the side chain are the same as those of polystyrene prepared under the same conditions [78], and this is true also of polymers from ethylene and deuteroethylene [79]. If hydride ion transfer were rate determining an isotope effect would be expected with higher molecular weights in the deuterated polymers. The rate determining step would therefore appear to be coordination of monomer followed either by rapid transfer or insertion into the polymer chain. 

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This quadratic in Rp is of the form required by the data for styrene-benzoyl peroxide shown in Fig. 14. The first term, corresponding to the intercept, represents the creation of chain ends through transfer with monomer. It occurs to an extent which is independent of the polymerization rate. The second term corresponds to 1/2 according to Eq. (27) it represents the pairs of ends created at the initiation step. Its coefficient is given by the initial slope of the line in Fig. 14. The third term, which accounts for the curvature at higher rates, represents the contribution of chain transfer with benzoyl peroxide. This becomes more prominent at higher rates because of the larger amounts of the initiator which are present. The marked rise in the curves for... 

Hence, by comparison with Eq. (36), evidently Cm = 0.60X10 , i.e., chain radicals at 60°C prefer to add monomer rather than undergo transfer with monomer by the factor (1/0.60) X10. Knowing the value of Cm, one may replot the quantity l/Xn CM)/Rp against Rp. The slope of the resulting linear plot is equal to the coefficient of the last term in Eq. (36). In this way, the complete expression... 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

From the radioactivity of the polymer and of the original methyl chloride it was found that if there was not more than one 14C atom per polymer molecule, 0.27 of the polymer molecules contained a methyl group derived from the solvent. The authors concluded that the formation of this fraction of molecules had been started by initiation according to equation 10, and that the remainder had been started by transfer with monomer. It has been admitted by the authors (private communication, and in press) that this conclusion is not warranted on this evidence, since alkyl halides such as methyl chloride, are known to act as transfer agents by a reaction which can be represented by Equation 7. At best, the evidence shows that methyl chloride was involved in starting - by initiation and/or transfer - about a quarter of the polymer molecules. The results of further studies with 14CH3C1 and CH336C1 are in process of publication [12]. 

Molecular weight is regulated to some degree by control of the chain transfer with monomer and with the cocatalyst, plus internal hydride transfer. However, hydrogen is added in the commercial processes to terminate the reaction because many systems tend to form longer chains beyond the acceptable balance between desired processing conditions and chain size. 

Operating at low temperature (<80°), ki can be neglected with respect to k Pc st Therefore, the relationship (29) is equal to the ratio between the number of polymeric chains interrupted by the chain transfer processes, depending on the catalyst concentration, and the number of polymeric chains interrupted by the chain transfer with monomer. 

The similarity between polymers and homologous low molecular weight compounds may be sometimes disturbed on account of the presence of some highly reactive sites, e.g. unsaturated end groups due to chain termination by disproportionation (a) or to chain transfer with monomer (6) ... 

Our kinetic work (10) showed that the small molecule radical produced by chain transfer with monomer had to be a stable radical. This was confirmed in the present paper by analysis of the isotope effect on the bulk polymerization rates. The isotope effect on molecular weights and rates unequivocally showed that almost 100% of the chain transfer involved the vinyl hydrogen. There is some evidence in the literature to support the idea of a stable vinyl radical. Phenyl acetylene acts as a retarder when copolymerized with styrene or methyl methacrylate (25). Thus the phenyl vinyl radical is very stable compared to the growing styryl or methacrylyl radical. 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

Initiation can be most effectively studied in systems with the smallest number of elementary steps in the polymerization reaction. This requirement is fulfilled in living systems (without termination and transfer) with monomers which do not generate polymer chains. By eliminating propagation (or suppressing it to dimerization) kinetically relatively simple systems can be obtained which are suitable for the application of convenient analytical methods. 

For all catalysts studied under the usual conditions of slurry polymerization of ethylene (75-80 °C, [C HJ = 0.1-0.6 mol/1) chain transfer with monomer predominates. This is directly illustrated by the independence of the polyethylene molecular mass ofmonomer concentration in the case of TiCIj and TiCl4/MgCl2 - - AlEtg... 

After reaction with the monomer to form a new propagating chain the position is formally the same as transfer with monomer. However, the two mechanisms can be distinguished kinetically if realkylation of the catalyst is slow compared with propagation. There is no direct evidence for this reaction although it is well established that the relatively stable alkyls of ms nesium and aluminium form metal hydride bonds on decomposition at elevated temperatures [83]. The existence of spontaneous termination has been deduced from a consideration of the kinetics, and by analogy with the effects of hydrogen on the polymerization. 

Binary or ternary catalyst systems from nickel compounds with Group I—III metal—alkyls have many features in common with those from cobalt and it may be inferred that a similar type of catalytic entity is involved. The composition for optimum activity may be different, however, and in the soluble catalyst Ni(naphthenate)2/BF3. EtjO/AlEtj (Ni/B/Al = 1/7.3/6.5) [68] the ratio of transition metal to aluminium is much higher than in cobalt systems. Rates were proportional to [M] and [Ni], molecular weight was limited by transfer with monomer and catalyst efficiency was relatively low (Table 5, p. 178). With the system AlEtj/ Ni(Oct)2/BF3—Et2 0 (17/1/15) the molecular weight rose with increase in [M] /[Ni] ratio and 3—9 chains were produced per nickel atom. It was observed that as molecular weight increased so the cis content of the polymer increased — from ca. 50% up to ca. 90% [292]. 

It is seen from Fig. 9.10 that chemisorbed ethylene disappears in initiation, propagation, and transfer with monomer. Therefore,... 

Ordinarily the energy of activation for transfer with monomer, APjr,M> would be greater than the energy of activation for propagation, AEpi so the molecular weight should decrease with increasing temperature. 

Unlike many other polymerization reactions, chain transfer with monomer cannot occur. However, many compounds (proton donors in particular) function as chain-transfer agents when added to the system. In general, the mechanism of the chain-transfer reaction can be depicted this way ... 

Chain Transfer In the absence of any externally added transfer agent, three transfer reactions [Eq. (9.9)-(9.12)] may be considered, viz., chain transfer with monomer, chain transfer with aUcylaluminum, and spontaneous chain transfer. 

Since chemisorbed ethylene disappears by initiation reaction with adsorbed hydrogen, pro-pagation reaction with surface-bound polymer chain, and polymer chain transfer with monomer (Fig. 9.8), one may write (Friedlander and Oita, 1957) ... 

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