This technical report is a review of the literature that pertains to two types of thermally stable elastomers: silarylene-siloxane polymers and polysilmethylenes. The silarylene-siloxane copolymers were generally prepared from the reaction of a bis(hydroxydialkylsilyl)arylene with a dialkylsilane containing two active groups, such as dichloro or bis(dimethylamino). These copolymers are thermally stable, decomposing at temperatures 50-100 C higher than that of the silicone polymers. The preparation and properties of the polydialkylsilmethylenes are discussed, with emphasis placed on polydimethylsilmethylene. These polymers are typically prepared from the ring-opening polymerization of 1,3-disilabutadienes; the preparation and physical properties of the polymers and the monomers are presented. Originator supplied keywords include: Carbosilane; Elastomers; Polymers; Silicon; Silmethylene; Siloxane; Silphenylene; Thermal stability.