The reaction of bis(chloromethyl)diorganosilanes (R2Si(CH2Cl)2: 1a, R = Me; 1b, R = allyl; 1c, R = benzyl; 1d, R = Ph) with magnesium gives 2a−d (65−90%) of formula MeR2SiCH2(SiR2CH2CH2)nH (n = 1, 2, 3, ...) and dimethyldiorganosilanes 3a−d (17−9%). The trapping reaction of the intermediates generated in the reaction of 1d with magnesium using CD3OD gives the corresponding CD3OD-trapped product of the 1,1-diphenylsilacyclopropane intermediate, Ph2Si(OCD3)(CH2CH2D) (6‘), along with the deuterated products of Ph2Si(CH2D)(CH2Cl) (5‘), Ph2Si(CH2D)2 (3d‘), and 2d‘. The results are consistent with a reaction course in which silacyclopropane intermediates are generated in the reaction of 1a−d with magnesium and then polymerized to 2a−d.