The control of structure formation of polycarbosilane (PCS) synthesized from polydimethylsilane (PDMS) was studied. It was found that the molecular structure of PCS was strongly dependent on the reaction time and reaction temperature. Shorter reaction time or lower reaction temperature is preferred to produce higher concentration of HSiC3 group, and less SiC4 group in PCS skeleton, which was confirmed by Si-29 NMR analysis. Higher reaction temperature or longer reaction time tended to dissociate more Si-Si chains to give PCS with higher molecular weight and broader distribution, as verified by GPC characterization. By interrupting the structure evolution of PCS from PDMS at low reaction temperature with short reaction time, solid PCS with controllable molecular structure can be obtained after the removal of the low molecular fraction by precipitation with ethanol. A scissor-couple rearrangement model was proposed for the formation of the PCS skeleton. (C) 2008 Wiley Periodicals, Inc.