This paper reports the spontaneous formation of a high-molecular weight multiblock PSi copolymer made up of i-butyl-n-decyldichlorosilane (M1) and methyl-3,3,3-trifluoropropyldichlorosilane (M2). The copolymer was formed under conditions of simultaneous addition via a Wurtz-type condensation reaction with sodium in refluxing toluene, without the help of a cocatalyst. The multiblock structure of poly[(i-butyl-n-decylsilane)(x)-b-(methyl-3,3,3-trifluoropropylsilane)(1 -x)] (PSi1) was characterized by means of Si-29{H-1} NMR, UV absorption, photoluminescence (PL), and photoluminescence excitation (PLE) measurements. The average block lengths (L) of the i-butyl-n-decylsilane (Si3) block and the methyl-3,3,3-trifluoropropylsilane (Si4) block in PSi1 were successfully evaluated by a block-selective scission technique using tetra-n-butylammonium fluoride (TBAF). This technique allowed for the selective decomposition of Si4 in PSi1 under reaction conditions when the concentration of TBAF and reaction time were optimal. The L values of Si3 and Si4 were, respectively, similar to 4.8 nm (corresponding to an averaged repeating unit of n similar to 26) and similar to 3.5 nm (n similar to 19). The copolymerization mechanism of PSi1 was further studied using the time-conversion curves of M1 and M2 by gas chromatography. Gel permeation chromatography using a photodiode array UV detector and H-1 NMR measurements were used to analyze the PSil product. Upon simultaneous addition of M1 and M2 comonomers, only M2 initially formed the Si4 oligomer (M-n similar to 3 000), and after a certain period of polymerization time, M1 abruptly copolymerized with the Si4 oligomer, followed by formation of the multiblock PSi1 copolymer.