(JP2581880-B2)      Prepn. comprises (a) providing carborane bisdimethyl silanol in a dried chlorobenzene solvent to form a slurry and cooling the slurry to -10 +/-5 deg.C: (b) adding to the slurry a mixt. of dimethylbisureido silane and methylphenylbisureido silane to form a reaction mixt. at -10 +/- 15 deg.C; (c) sepg. from the reaction mixt. a silanol end-capped prepolymer of the polycarborane siloxane polymer; (d) dissolving the prepolymer in chlorobenzene to form a soln.; and (e) adding to the prepolymer soln. an excess of dimethylbisureido silane and/or methylphenylbisureido silane to form the very high MW polycarbonate siloxane polymer.    The improvements comprise (i) providing the carborane bisdimethyl silanol in an ultrapure state by dissolving in a dry, oxygen-free mixt. of hexane and toluene by heating to 70 deg.C in an inert atmosphere to form a first soln., filtering, and cooling to form a first crystalline prod., and repeating the dissolving, filtering and cooling for 2-5 additional times; (ii) providing the dimethylbisureido silane in an ultra-pure state by dissolving in a dry oxygen-free mixt. of diisopropyl ether and THF by heating to 60 deg.C in an inert atmosphere to form a second soln., filtering and cooling to form a second crystalline prod., and repeating the dissolving, filtering and cooling once; and (iii) providing methylphenylbisureido silane in an ultrapure state by dissolving in dry oxygen-free diisopropyl ether by heating to form a third soln., filtering and cooling to room temp. to form a third crystalline prod., and repeating the dissolving, filtering and cooling two additional times. The polycarborane siloxane polymer has a MW in excess of 120,000.    USE/ADVANTAGE -   Useful in the space environment e.g. for bonding solar cell covers to solar cells and solar cells to an underlying substrate, as hardened ablative heat shields, thermally stable optical coatings, spacecraft thermal control coatings and other components; the polymers are also useful for forming O-rings and other elastomer applications.