The thermal decomposition of some silphenylene-siloxane polymers

Beattie, Sheila R. (1981) The thermal decomposition of some silphenylene-siloxane polymers. PhD thesis, University of Glasgow.

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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.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Chemistry
Supervisor's Name: Grassie, Professor N.
Date of Award: 1981
Depositing User: Ms Dawn Pike
Unique ID: glathesis:1981-5402
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 17 Jul 2014 11:12
Last Modified: 17 Jul 2014 11:12

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