Abstract: According to the invention, the polymer composition comprises 8 to 20 parts by weight of polyborsiloxane having a molecular weight of 2,000 to 5,000 and including structural units --BOSi-- --(CH.sub.3).sub.2 SiO-- with a B:Si molar ratio of 1:4 to 5, 40 to 90 parts by weight of polysiloxane rubber, 10 to 60 parts by weight of an organic polymer which may be polyolefins, copolymers of olefins and dienes, halogenated polyolefins, and halogenated copolymers of olefins and dienes, taken separately or in combination with one another, 2 to 10 parts by weight of branched polyorganosiloxane having a molecular weight of 2,000 to 5,000, of the general formula: ##STR1## wherein: R=CH.sub.3, C.sub.6 H.sub.5, a=43 to 50 mole %, b=0 to 2 mole %, c=50 to 55 mole %, as well as 10 to 25 parts by weight of silicon dioxide, and 5 to 10 parts by weight of a variable valency metal oxide.

Abstract: According to the invention, the polymer composition comprises 8 to 20 parts by weight of polyborsiloxane having a molecular weight of 2,000 to 5,000 and including structural units [BOSi] [(CH.sub.3).sub.2 SiO] with a B:Si molar ratio of 1:4 to 5, 40 to 90 parts by weight of polysiloxane rubber, 10 to 60 parts by weight of an organic polymer which may be polyolefins, copolymers of olefins and dienes, halogenated polyolefins, and halogenated copolymers of olefins and dienes, taken separately or in combination with one another, 2 to 10 parts by weight of branched polyorganosiloxane having a molecular weight of 2,000 to 5,000, of the general formula: ##STR1## WHEREIN: R=CH.sub.3, C.sub.6 H.sub.5, a=43 to 50 mole %, b=0 to 2 mole %, c=50 to 55 mole %, as well as 10 to 25 parts by weight of silicon dioxide, and 5 to 10 parts by weight of a variable valency metal oxide.