Investigation of monomer ratios in random and block copolymers of ethylene and propylene using pyrolysis–gas chromatography/mass spectrometry

Abstract

Polyolefins such as polyethylene and polypropylene degrade during pyrolysis to form a wide distribution of oligomers, producing pyrograms which are complex, but characteristic. Polyethylene produces mainly straight-chain hydrocarbons, including alkanes, alkenes and dienes, while polypropylene produces methyl-branched hydrocarbon oligomers. The pyrogram of a block copolymer of polyethylene and polypropylene will contain oligomers of both homopolymers, and simple relationships may be made between peaks for oligomers of each polymer to the relative amount of each component in the copolymer. When a random copolymer of ethylene and propylene is pyrolyzed, however, mixed oligomers are produced which are not seen in the homopolymer pyrolysate. Depending on the relative amounts of the two monomers, homo-oligomers may also be present, especially for short sequences such as trimer and tetramer. Although the pyrograms of random copolymers appear different when different monomer ratios are examined, the relationship of specific peaks to the monomer concentration is not so straightforward as in block copolymers. This paper presents analytical results for random and block copolymers of ethylene and propylene, relating the peaks for various oligomers to the monomer ratio in the polymer. In the case of random copolymers, the mathematical probability of finding three ethylene units adjacent to one another on the polymer molecule is compared with the analytical determination of the ethylene trimer 1-hexene in the pyrograms.