Depolymerization isotacticity

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

The thermal degradation of irradiated isotactic PMMA has also provided information on the structure of the end groups formed at the site of main-chain scission [408]. The weight loss of non-irradiated PMMA at 250°C has been shown by Grassie and Melville [409] to be mainly due to depolymerization initiated at the carbon—carbon double bonds situated at the chain ends. A small proportion of randomly initiated depolymerization also occurs at this temperature. In agreement with this mechanism, the rate of volatilization has been found to be much higher for atactic than for isotactic PMMA, the latter having no double bonds at the chain ends. If 4.3 scissions per chain are produced by 7-irradiation in the isotactic sample, the rate of monomer evolution is identical to that of the initial unirradiated isotactic sample. This proves that chain ends of the type... 

It was observed that isotactic polypropylene decomposes thermally by a mechanism that varies at different temperatures and conditions [455]. Thus, at 340°C the major volatile product is propane, while at 380°C it is n-pentane, and at 420°C it is propylene. The propane is believed to originate from some weak spot in the polymeric chain. Formation of n-pentane involves a radical abstraction and a six-membered ring formation in a backbiting process. Propylene may come from a free-radical depolymerization process or a cyclic six-membered ring formation involving a terminal double bond [455]. 

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