To prepare new polycarbosilane polymer precursors with high solubility and the capability of UV cross-linking, commercial polycarbosilane was modified by a chemical route. These modifications involved AlCl3 catalyzed chlorination reactions of polycarbosilane's Si-H bonds. The resultant Si-Cl bonds were substituted by a reaction with sodium acetylyde to form Si-C=CH ligands. These ligands are suitable for controlled, free radical initiated cross-linking of the polycarbosilane polymers. The increase in molecular weight should allow for increased Tg's and the retention of polymer pre-forms. In this report, the chlorination of the polycarbosilane polymer and the substitution reactions of polycarbosilane were examined with IR, 29Si and 13C NMR spectroscopy. In addition, the retention of polymer pre-forms were analyzed after UV exposure and inert atmosphere pyrolysis.