Molecular weight reduction groups

Molecular weight reduction probably plays a role even in pretreatment processes that are not intended to cause this effect (19). The most common method of molecular weight reduction is the addition of dilute acid to hydrolyze the acetal groups (I). The acid may be generated in situ by the liberation of acetic acid from acetate groups by steam or hot water. 

Clementini and Spagnoli (7) patented a method in which polypropylene was first functionalized with a hydroperoxide group before extrusion with acrylic acid. Other reactive sites could also be incorporated into polypropylene during its production. Chung (8) has several patents that discuss the incorporation of borane or joflra-methylstyrene into polypropylene. These functional sites can then be reacted with unsaturated monomers. Both methods supposedly limit both the amount of homopolymerization and the level of molecular weight reduction that occurs compared with other grafting methods. 

For non-transparent or non-translucent specimens, the relative humidity of the interfacial climate on an irradiated surface can be decisive for photo-chemical aging. Tests with PE-HD packaging showed clear influence of relative humidity on molecular weight reduction and carbonyl group formation. Figure 1.32 [87]. 

The improved thermal and chemical stability of the copolymer versus the homopolymer is a result of randomly distributed oxyethylene groups, which is circled in Fig. 5.10. All polyacetals are subject to oxidative and acidic degradation, which leads to molecular weight reduction. Degradation of the copolymer ceases, however, when one of the randomly distributed oxyethylene linkages is reached. These groups offer stability to oxidative, thermal, acidic, and alkaline attack. The raw copolymer is hydrolyzed to an oxyethylene end cap to provide thermally stable polyacetal copolymer. 

The alcohol portion of the ester is a by-product of the reaction. The esters typically reduced by lithium aluminum hydride contain a low molecular weight alkyl group that was introduced in the conversion of an acid to an ester. The alcohol obtained by reduction of the acid portion of the ester is easily separated from the low molecular weight, water-soluble alcohol. 

Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

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