The thermal aging of silicone rubber mainly occurs through the oxidation of side chains and the breaking of siloxane bonds in the main chain. So, in one aspect, the heat resistance of silicone rubber depends on its side chains.
On the other hand, the breakage of the main chain can also be called sealing heat resistance, which is different from oxidation aging. It is related to the flexibility of the main chain and the presence of end groups. Moreover, the presence of water, acids, bases, etc. has a significant promoting effect on the breakage of the chain.