The thermal decomposition of some silphenylene-siloxane polymers
Polymers composed of alternating p-silphenylene and siloxane units with either dimethyl or methylphenyl substituted silicon atoms have been prepared in a dilute solution condensation of the appropriately substituted phenylenedisilanol, catalysed by tetramethylguanidine di-2-ethylhexoate, TMG. A range of copolymers has been synthesised with extended siloxane segments where the substituents on the silicon atoms are both methyl groups. Two methods of polymerization have been employed with comparable success. These are the condensation between p-phenylenebis(dimethylsilanol) and tetramethyldisiloxanediol catalysed by TMG and the KOH catalysed polymerization of p-phenylenebis(dimethylsilanol) and octamethylcyclotetrasiloxane. Characterisation of the polymers employed the techniques of infrared and n.m.r. spectroscopy and membrane osmometry. Attempts to prepare poly(tetraphenyl-p-silphenylene-siloxane), TPPS, with a high molecular weight have met with little success, only low molecular weight polymer being obtained. The first feature observed in the thermal degradation of poly(tetramethyl-p-silphenylene-siloxane), TMPS, is the development of insolubility in the residue when the polymer is heated isothermally at 350-400°. Weight loss is slight at this stage and is due to the formation of the cyclic dimethylsiloxane trimer and tetramer and a small quantity of "cold ring" fraction. It is suggested that the mechanism at this stage involves structural rearrangements leading either to chain branching or some production of volatile material. Prolonged heating of TMPS at 450-500° results in up to 80% weight~loss, most of which is collected as "cold ring" fraction. Subsequent separation and analysis using GC, GCMS and infrared spectroscopy identified a series of short chain oligomers terminated by either the Si-H or the Si-Ph group. A mechanism involving direct scission of the silphenylene bond has been proposed to account for their formation. The products of degradation of the silphenylene-siloxane polymers with extended siloxane segments are dependent on the copolymer composition. A complex mixture of cyclic oligomers has been separated and identified from the"cold ring" fraction of those polymers with low silphenylene content. The presence of these compounds at the lower temperatures of degradation has led to the assumption that their formation involves a structural rearrangement not unlike that involved in the degradation of poly(dimethylsiloxane), DMS. Results from infrared analysis demonstrates that Si-H groups are formed at higher temperatures, indicating the occurrence of silphenylene bond scission. As the silphenylene content increases, the characteristics of degradation become increasingly more like those observed for TMPS. Thus both the range and the amount of cyclic oligomers, formed in structural rearrangements, decreases in favour of the formation of linear oligomers via rupture of the silphenylene bond. One feature common throughout the copolymer series is the chain branching reaction occurring between 350 and 400·0 A comparison of the rates of branching, calculated from sol-gel analysis, has shown that the rate is directly proportional to the silphenylene content. Thermal analysis demonstrates that poly(metbylphenyl-p-silphenylene-siloxane), MPPS, is thermally more stable to weight loss than !MPS. Benzene and a mixture of linear oligomers, similar in structure to the original polymer but with Si-H and Si-Ph end-groups, have been analysed and identified as the volatile products of degradation with the aid of GC, GeMS and infrared spectroscopy. No evidence of cyclic compounds has been detected. A comparison of results calculated from sol-gel analysis reveals that chain branching occurs more rapidly in MPPS than !MPS. Two distinct stages again appear to be present in the degradation of MPPS. At the lower temperatures of degradation structural rearrangements are responsible for chain branching and the initial production of benzene •. Increasing the temperature causes the mechanism to undergo a change to one involving scission of either the silphenylene or Si-Ph bond as the first stage. Further production of benzene and the formation of the short chain oligomers occur in the higher temperature range. Results from TG demonstrate that the low molecular weight TPPS is thermally less stable to weight loss than DMS. Apart from benzene all the products of degradation are contained in the "cold ring" fraction. Infrared analysis demonstrates that these products are similar in structure to the original polymer. However in view of the inability to obtain higb molecular weight polymer, further investigations of the thermal degradation properties of TPPS have not been pursued. To conclude this work two dimethyl substituted aryloxysilane polymers have been synthesised in a melt polymerization of dimethyldianilinosilane with either hydroquinone or bisphenol A. The thermal stabilities of these polymers have been investigated briefll to gain some indication of the potential value of a more detailed study in this field. Although their thermal stability to weight loss is less than the silphenylene-siloxane polymers, it is comparable with that of DMS. Thus a more thorough investigation of the thermal degradation behaviour of the aryloxysilane polymers has been recommended.