Sequencing in Polyethylene

Voigt [1] states that the formation of decomposition products of polyethylene (PE) is a result of a primary step in a statistical thermal breaking down of a chain  

The very small amounts of branched molecules which are present even in low-pressure polyethylenes do not make themselves noticeable on the gas chromatogram under the conditions employed by Voigt [1]. Branched molecules are, however, found under other more searching gas chromatographic conditions. In this connection, it should be pointed out that high-pressure polyethylenes produced completely identical gas chromatograms. 

According to Wall and co-workers [2] the pyrolysis of polyethylene proceeds by a radical chain mechanism. The products formed result from the process of random-chain cleavage, followed by intermolecular or intramolecular hydrogen abstraction. Hydrogen abstraction occurs preferentially at tertiary carbon atoms, and product formation results from homolysis of the carbon-carbon bond at the beta position relative to the radical site. The major products formed are the -alkanes and alpha-omega-diolefins. The peaks between the triplets result from chain branching. 

From an examination of the products formed from the pyrolysis of high-density polyethylene and polymethylene, Tsuchiya and Sumi [3,4] proposed that the major hydrogen-abstraction reaction is due to an intramolecular cyclisation. They proposed that, following initial radical formation at Cj, successive intramolecular hydrogen abstractions occur along the chain resulting in the formation of new radicals at C5, C9 and Ci3 as shown in Equations 6.2 to 6.5. 

Cleavage of the carbon-carbon bonds at the beta position to these macroradicals, results in the formation of increased amounts of Cg, Cjq and C14 a-olefins and C3, C7 and Cjj n-alkanes over that which could be predicted from statistical-chain cleavage. 

---

For this cycloalkane, the Tic s of the methylene carbons in the folded sequence are estimated to be 35-39 s at room temperature and become shorter with increasing temperature [66]. Hence, the Tic s for the regularly folded methylene sequence in polyethylene, if it exists, will be appreciably shorter than 35 s at room temperature, and much shorter than ca. 220 s of the inner methylene carbons of polyethylene (cf. Table 1). Hence, if we measure the DD/CP 13C NMR spectrum of the solution-grown polyethylene sample by a single pulse sequence with a rep-... 

---