Cyclization during addition polymerization

During incineration of 1 in the polymeric matrix debromination/hydrogenation occur in addition to cyclization process. Tetrabrominated dibenzofuran isomers are the most abundant products formed in the temperature range between 300° or 400° (Figure 6 shows Br-composition at 300° - 800°C). Incineration at 400°C gives tetrabromo-benzofurans in yields up to 13 % (Fig. 6). Besides of PBDF, brominated dibenzodioxins are also formed, but to a much lesser extent (30-90 ppm) (ref. 11). 

In reality, the polydispersity of the hyperbranched polymer even in the absence of core is lower than that predicted for the ideal case. Cyclization and steric hindrance during polymerization can be the reasons. Polydispersity can also be lowered intentionally, for instance, by introduction of core molecules or by programmed addition of the monomers. 

The stereospecificity of the cyclization step has been examined both for model systems" "" and in a few cyclopolymeri/ations. " In formation of either five- or six-membered rings, there is a preference for the polymeric residues to end up cis- to each other. Note that for cyclopolymers with six-membered ring units, the ring stereochemistry is established in the intermolecular addition step (Scheme 4.15). In the case of fivc-mcmbcrcd ring units, ring stereochemistry is established during the intramolecular step (Scheme 4.16). 

The molar mass is additionally fixed by these regulators so that mastication is no longer necessary. The cold polymerization is more favorable than the warm polymerization, since more /rnns-rich structures are produced. Ci5-rich polymers, of course, tend more to cyclization, which produces "stringiness,"" that is, an undesirable increase in viscosity, during subsequent processing. Buna S can be mixed directly with natural rubber. It is primarily used for the running surfaces of car tires. 

Table 3 contains reaction conditions and polymer properties for polybenzoxazoles prepared by one-step thermal polymerizations and by cyclization of the intermediate polyamides. The latter dehydration process was readily followed by IR. The polyamides have strong bands near 1655 cm which gradually disappear during the cyclization. Concomitant appearance of the characteristic benzoxazole band at 1600-1620 cm confirms the process and the product structure. In addition, microanalysis data have been obtained for the polybenzoxazoles from monomers and Calculated and found values for C, H, and N were within 0.3% of each other for the former. The values for the latter were consistent with either equivalent of bound H2O per repeat unit and/or incomplete cyclization. 

Dienes with isolated double bonds can polymerize with the formation of cross-linked polymers, because the two double bonds present in the molecule can react independently of each other. However, in some cases in addition reactions both double bonds of monomer molecules can take part, leading to the formation of cyclic polymers. This reaction occurs particularly when it leads to the formation of rings with five or six-members. During the cyclization reaction side vinyl groups are formed, which pol3mierize with formation of cyclic polymers. 

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