Polypropylene oxide, high-molecular-weight

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

Additive-free film samples of isotactic polypropylene (iPP, 30pm Himont Profax resin) and polyethylenes (LLDPE, 120ym, linear low density DuPont Sclair resin, and UHMW-PE, 120wm, ultra high molecular weight, high density Himont LSR 5641-1B resin) were oxidized by exposure in air to y-radiation (AECL Gamma Cell 220, 1.0... 

Subsequent to the report (9) by Pruitt and co-workers that certain iron catalysts could be used to prepare high-molecular-weight polypropylene oxide), a number of other catalysts were found useful for this purpose (10, 11, 12). These catalysts were necessarily used in amounts larger than would be required, assuming that each metal atom were active. Furukawa (13) and Vandenberg (11) concluded that... 

Catalysts derived from reaction systems such as triethylaluminium-water and triethylaluminium-water-acetylacetone [225], triethylaluminium-triphe-nylphosphine [226], triethylaluminium-pyrogallol [209] and rare-earth metal phosphonate- triisobutylaluminium-glycerol [227] appeared to be effective in the copolymerisation of propylene oxide and carbon dioxide, yielding high molecular weight polypropylene ether-carbonate)s (Table 9.4) but not the respective alternating copolymer which is polypropylene carbonate). 

Poloxamer is the generic name for a series of block copolymers that are composed of one polypropylene oxide block sandwiched between polyethylene oxide blocks. For example, poloxamer 188 can be written as (PEO)75-(PPO)3o-(PEO)75. The poloxamers serve as high molecular weight surfactants because the PEG blocks are hydrophilic, whereas the PPO blocks are hydrophobic. 

Problem 2.10 Explain why polypropylene of relatively high molecular weight is optically inactive despite having an asymmetric center at every other carbon, while, on the other hand, polyfpropylene oxide) is optically active. 

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