Backbiting mechanism

Much is left not undestood in this reaction. It would appear that more than one route leads to the higher MW material, most likely two to account for the appearance of the two higher MW peaks in the GPC. The low molecular weight cyclics are perhaps the products of a simultaneous growth and backbiting mechanism In one of these routes. One of the mechanisms shows some evidence of ionic or alkalimetalsilyl intermediates, and this maybe also the route to the cyclics because this is promoted in THF solution. This reaction would then form most of the products and be the major factor deciding the kinetics of the reaction. 

Peebles et al. [50] found that the formation of cyclic oligomers at 290 °C increased with decreasing Mn of the polymer, and that the cyclization rate depended linearly on the hydroxyl end group concentration. Therefore, a backbiting mechanism (cyclodepolymerization) has been proposed as a probable mechanism for oligomer formation (Figure 2.9). 

Short branching has also been verified in poly(vinyl acetate), poly(vinyl chloride), and various polyacrylates. Branching in poly(vinyl acetate) and polyacrylates involve the intramolecular backbiting mechanism as in polyethylene [Adelman and Ferguson, 1975 Heatley... 

Poly(bisphenol-A-carbonate) under pseudoideal reaction conditions was investigated, and the cyclic polycarbonate was obtained as the main product. In the system, the interface of the water/toluene mixture might have favored the cyclization reaction between the polar end groups [88]. Cyclic carbonates during the (Salen)CrCl catalyzed CCh/cyclohexene oxide copolymerization process in the presence of ionic initiators was also obtained [89]. The cyclic carbonate is produced via the backbiting mechanism, and the process is assumed to take place via a metal alkoxide (polymer chain) intermediate. Subsequent ring-opening of the cyclic carbonate with concomitant formation of polyether and CO2 was fast at the reaction temperatures from 80 to 100 °C). 

Since they found about 0.95 ethoxy end groups in the polymer per mole of HCIO4 used, they argue in favour of tertiary oxonium ions as the propj ating species. (They point out that carboxonium ions seem unlikely on the basis of other studies. Also copolymerization parameters in copolymerization with dioxacycloheptane can be explained by this tertiary oxonium mechanism and not by others.) They suggest that the macrocycles observed by Plesch are formed by a backbiting mechanism. Their proposed reactions [132] are ... 

In fact, Kelen et al. [134] have presented data and a theoretical approach indicating that the backbiting mechanism seems to be the most probable interpretation of ring-formation in the cationic polymerization of 1,3-di-oxolane. 

Figure 2.3 Mechanism of formation of 2-ethylhexyl branch in LDPE. As in backbiting mechanism for n-butyl group formation, homolytic scission of a CH bond occurs down the chain. is a polymeric alkyl group.

Figure 2.2 Backbiting mechanism in free radical ethylene polymerization 27...

Monosulfide polymer 19 is the most stable of the polymers discussed, not only in its resistance to base, oxidizing acids, and light but also in thermal stability. Degradation is slow at 300°C, but at 350°C chain scission results in an unzippering to evolve tetrafluoroethylene and di-thietane 6 (n = 2). The latter gives rise to polydisulfide 16 and may cycloadd to tetrafluoroethylene to form dithiane 7, the third major product. More likely under these conditions, however, is direct formation of 7 by a backbiting mechanism as illustrated below. 

As indicated by eqn [5], with some exceptions, rings produced by a backbiting mechanism are generally of small to medium size. They are typically ranging from dimers to oligomers having less than 10 units for their large majority. 

This reaction also plays a role in the degradation of polysulfides. A backbiting mechanism as shown in equation (6) results in formation of the cyclic disulfide (5). Steam distillation of polysulfides results in continuous gradual collection of (5). There is an equilibrium between the linear polysulfide polymer and the cyclic disulfide. Although the linear pol5uner is favored and only small amoimts of the cyclic compound are normally present, conditions such as steam distillation, which removes (5), drives the equilibrium process toward depolymerization. 

The multiple backbiting mechanism, as outlined in the previous section, can lead to a tertiary radical which undergoes /3-scis n to vinylidene, and it is then only necessary to assume that such a tertiary radical is formed through an additional backbite of a radical formed in the sequence depicted in F. 13. For example, the last step in this sequence could be CH,... 

It is clear from the numerous accounts in literature that DMCs can efficiently catalyze the copolymerization of CO2 and epoxides. DMCs can however also be used to develop systems that selectively catalyze the CO2 cycloaddition rather than the copolymerization (Scheme 1.4) as is illustrated by the work of Dharman et al. [20]. By itself, a Zn-Co-DMC is an efficient catalyst for the copolymerization reaction. However, the addition of a quaternary ammonium salt to the reaction mixture switches the selectivity of the catalytic system toward the exclusive formation of the cyclic carbonate. The quaternary ammonium ion plays two important roles in the catalytic system it accelerates the diffusion of CO2 into the reaction mixture and it favors a backbiting mechanism. As such, it hinders the growth of the polymer chain and it enables the selective cyclic carbonate production. Although most zinc-containing catalysts for this reaction are very sensitive toward water, Wei et al. have shown that, for example, the combination of Zn-Co-DMC with CTAB (cetyltrimethylammonium bromide) could even use water-contaminated epoxides as an epoxide feed [21]. 

Figure 12. Backbiting mechanism for formation of cyclic dimer. M = Mo(N-...

Figure 13.2 Intramolecular chain backbiting mechanisms. Upper scheme illustrates the attack of a chain by its own OH-terminated free end to form a cyclic siloxane. Lower scheme illustrates the case where a closed, continuous chain folds back upon itself and rearrangement occurs to form a new Si-O bond and a free cyclic siloxane...

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