Three substituted unsaturated polycarbosilanes were prepared from the same parent using a two-step, one-pot ADMET polycondensation-nucleophilic substitution route. Condensation metathesis of di(4-pentenyl)dichlorosilane using Schrock's [Mo] catalyst produces a polymer backbone containing two reactive silicon-chlorine bonds per unit. Replacement of these bonds with stable alkyl moieties was performed using an excess of alkyllithium reagent without undesirable main chain alkylation or cross-linking. Upon functionalization, moisture-stable polymers were synthesized, indicating quantitative substitution was achieved. All of the resulting polymers are amorphous, elastomeric materials. Variation of the two pendant groups from phenyl to methyl resulted in a T-g change of over 85 degreesC.