Mechanistic study, polypropylene

Fully aware of this conflicting situation, we decided, nevertheless, to perform mechanistic studies on low molecular models of polypropylene, in the hope of achieving on this basis some new information and ideas with respect to the basic processes taking place in this polymer. Isooctane, as a model for the structural unit of polypropylene, seemed to us to be an appropriate substance for this kind of investigation. 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

In preliminary work (19), divinylbenzene (DVB) has been reported to be a useful additive for enhancing the above grafting reactions. These early data (19) indicate that there are possible common mechanistic pathways between the acid effect and the DVB process. More detailed DVB studies are discussed in this paper for enhancing the radiation grafting yields of styrene in methanol to films of polyethylene and polypropylene. The work has been extended to include the use of other polyfunctional monomers such as tri-methylol propane triacrylate (TMPTA) as additives. The possibility of being able to use these additives for copolymerisation of monomers to naturally occurring trunk polymers such as cellulose will also be considered. 

N.S. Allen, A. Parkinson, F.F. Loffleman, and P.V. Susi, Mechanistic action of a new p-hydroxybenzoate light stabilizer in polypropylene film a spectroscopic study, Angew. Makromol. Chem. 1983, 116, 203-219. 

Low monomer concentration can also decrease the isotacticity of polypropylene. The three-coordinate metallocene alkyl species that exists in the absence of a n-coordinated monomer is capable of racemizing the methyl group of the last-inserted monomer unit. Evidence for the occurrence of chain-end epimerization has come from studies using deuterium-labeled propylene " the relevant mechanistic steps are believed to be a series of P-hydride eliminations and subsequent isomerizations with a tertiary alkyl species as an intermediate (Scheme 1.8). 

Allen NS, Parkinson A, Loffelman FF, Rauhut MM, Susi PV. Spectroscopic properties and mechanistic action of a new p-hydroxybenzoate light stabilizer a comparative study in polypropylene and high density polyethylene and anti-oxidant interactions. Polym Degrad Stab 1984 7 153-74. 

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