NOVELTY - Making a pure higher molecular weight (MW) phenylalkylsiloxane from a lower MW phenylalkylsiloxane by polymerization of the lower MW phenylalkylsiloxane under vacuum in the presence of an aqueous alkaline solution containing at least one alkali ( greater than or equal to 50 ppm) selected from sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, rubidium hydroxide, ammonium hydroxide, tetraalkylammonium hydroxide, tetraalkyl ammonium alkoxide or phosphonium hydroxides.    USE - For making a pure higher molecular weight (MW) phenylalkylsiloxane useful in sealant composition which is useful in an insulating glass unit; useful in optical adhesive, sealant, coating and/or encapsulant, for opto-electronic equipment, light emitting diodes (LEDs), lenses, solar cells and arrays; and in personal care products for hair and skin and for coatings to treat textiles and paper (claimed); also useful in other applications, such as conformal coatings and corrosion protection; in applications where their resistance to high temperature is beneficial such as in hydraulic fluids, transformer fluids, fluids exposed to radiation, and lubricants; and in solar cells and arrays.    ADVANTAGE - The method provides higher MW phenylalkylsiloxane having average MW in excess of 10000 (preferably 30000). The bulk polymerization method requires no co-monomers and no solvents and enables the production of high molecular weight polymers at significantly higher speeds than previously described. Contrary to conventional process in the method dramatically larger polymers are prepared over short reaction times. Particularly the use of potassium hydroxide in a process has provided an added advantage of an unexpected apparent two stage polymerization reaction whereby initial polymerization is slower than that of an equivalent monomer polymerized with sodium hydroxide but after a certain period of time there is a step change and the rate of increase in chain length increases significantly. The vacuum is used to remove water or other condensation by-products during the reaction. Extraction of the water or other condensation by-products by vacuum generally causes the polymerization rate to increase. However, once the polymerization process is fully functioning e.g. after 15 to 20 minutes removal of the vacuum results in a slow down of the reaction and in turn enables better control of the molecular weight (and viscosity) of the end product. The method prevents polymerization process from going out of control and enables the production of a industrially applicable polymer in a timely manner, contrary to the teaching of the prior art. The higher MW phenylalkylsiloxane which is made by the method is pure, i.e. all of the silicon atoms are bonded to a phenyl group. The polymerization of pure lower MW phenylalkylsiloxanes (oligomers) in total absence of dimethylsiloxane oligomers prevents the formation of traces of 2,6-cis-diphenylhexaalkylcyclotetrasiloxane, which can impair human fertility. As a result, polymers produced have no measurable trace of this intermediate, allowing its use in applications in which the dialkyl/phenylalkyl siloxane copolymer would not be usable.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a sealant composition comprising (wt.%) a higher MW phenylalkylsiloxane (10-60), plasticizer (0-40), a rheological additive (0-10), an inorganic filler (0-85) or a mixture of inorganic fillers, a crosslinker (0.1-10), an adhesion promoter (0.01-5), and a catalyst (0.01-5);    (2) a two part sealant comprising the sealant composition.