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Scheme 2. Fused ring hydroxy-6F-benzoxazole structures for
T
g
enhancement.
Although they have met the microelectronic packaging material requirements
with regard to glass-transition temperatures, dielectric properties and solution-
processability, these materials need to be solvent-resistant after the fabrication
process in order to be device qualified. The following effort was directed toward
designing chemical routes to impart insolubility to the hydroxyl-pendant homo-
and copolymer systems after processing by incorporating a crosslinking site, viz.,
an O-allyl moiety, directly onto the polymer backbone [7]. A unique concept that
entails the sequential utilization of thermally-driven Claisen rearrangement and
allyl-mediated crosslinking reactions has been investigated. Briefly described, the
polymers undergo an intramolecular Claisen rearrangement (typically occurring at
~ 200°C), thus allowing further T
g
enhancement, by increasing the number of
fused rings via intramolecular hydrogen bonding between the in-situ formed OH
group and the nitrogen atom of the nearby benzoxazole group (Scheme 3). At ele-
vated temperatures (250-300°C), crosslinking of the allyl groups should occur,
thus providing a mechanism for promoting insolubility as well as the dimensional
stability to the polymer system. To demonstrate this concept, three model com-
pounds containing pendant allylether and bis(allylether) groups were synthesized.
They were analyzed via differential scanning calorimetry (DSC) to determine
whether the expected Claisen rearrangement had taken place and to observe what
effects this had upon the thermal properties of the compounds. The initial DSC
trace of the diallyloxybenzoxazole model compound did indeed demonstrate that
a Claisen rearrangement occurred first, followed by the thermal cure of the allyl
group in the 300-400
°
C range.
Scheme 3. Intramolecular Claisen rearrangement for the O-allyl-6F-benzoxazole unit.
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