The synthesis of new polysiloxanes was performed via hydrosilylation reactions of polymethylhydrosiloxane with 4-allyl-1methoxybenzene and 4-allyl-1,2-imethoxybenzene in the presence of Karstedt's catalyst {Pt-2[(VinSiMe(2))(2)O](3)} and platinum hydrochloric acid (0.1M solution in tetrahydrofuran). The hydrosilylation reactions were carried out at 60 degrees C. The molar ratio of Si-H groups to the allylic compound was 1 : 1.2. The synthesized oligomers were characterized with H-1-NMR, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetry, and gel permeation chromatography. In the presence of platinum hydrochloric acid, not all active Si-H bonds took part in hydrosilylation, and because of this, gelation took place and the molecular masses of the extracted parts increased 7-8 times; in contrast to this, in the presence of Karstedt's catalyst, all active Si-H bonds participated in hydrosilylation, and an increase in the molecular masses did not occurs. The influence of substituted methoxy groups on the glass-transition temperatures was studied. (c) 2007 Wiley Periodicals, Inc.