Studies on the degradative behaviour of polymers containing weak-links of group VI A elements

Abstract

Unavoidable interaction of aerial oxygen during free-radical polymerization of vinyl monomers results in the formation of weak peroxy linkages in the backbone which dramatically affect the stability of polymers, even though the peroxy content in the chain may be small. In the present investigation, a novel approach has been made to understand the mechanism of polymer stability by deliberately introducing the weak links into the backbone. The polymers chosen were alternating copolymers of styrene and Group VIA weak linkages namely peroxide [poly(styrene peroxide), PSP], disulfide [poly(styrene disulfide), PSD], tetrasulfide [poly(styrene tetrasulfide), PST] and diselenide [poly(styrene diselenide), PSDSE]. Direct Pyrolysis Mass Spectrometric (DP-MS) analysis of the above polymers clearly revealed that all the polymers undergo degradation at the weak links. Based on the higher molecular weight cyclic compounds, it was found that in PSD, apart from disulfide backbone, to some extent monosulfide linkages are also present. In PST, mono-, di- and trisulfide linkages are present along with the major tetrasulfide backbone. The spectrum of pyrolysis products revealed an interesting transformation down the group. In PSP neither styrene nor oxygen is formed. But in PSDSE only styrene and selenium metal is obtained. The PSD and PST show a mixed product spectrum which contains sulfur compounds in addition to styrene and sulfur. The diverse nature of the products in these Group VIA polymers emanates from the energetics and reactivity of the alkoxy and thiyl radicals. To get complete understanding of the decomposition mechanism of PSD and PST, Py-GC-MS analysis was performed both at 400 and 610°C. In the former case, styrene dimer formation occurring in PSD is completely absent in PST. Also, hydrogen transfer reactions which take place easily in PSD are negligible in PST. This difference is attributed to varied radical recombination reactions. While in PSD both the C-S bonds cleave forming the styrene dimer, in PST, the C-S and S-S bonds cleave forming the monosulfide compounds. At 610°C, both the polymers essentially yield styrene and sulfur. Finally, investigations on the chemical reactivity of both PSD and PST were accomplished using triphenylphosphine. The progress of the reaction was monitored using ³¹P NMR spectroscopy. Product analysis was done using DP-MS technique; the mechanism of degradation involved S-S bond interchange reactions.