Electrophilic substitution of cyclopropenium ions on aromatic polymers offers a unique opportunity to introduce polar functionality in a controlled manner to conventional, nonpolar polymers. Phenylcyclopropenone substituted polystyrene with predictable chemical composition and narrow molecular weight distribution were prepared. Size exclusion chromatography (SEC) analysis demonstrated the absence of branching or crosslinking in these functionalized polystyrenes during electrophilic substitution of the parent homopolymer. C-13-NMR confirmed that the degree of phenylcyclopropenone substitution was both highly efficient and predictable over a broad compositional range. The glass transition temperature (T(g)) of the polymers was found to vary linearly with mole % phenylcyclopropenone substitution of the polystyrene. Thermal gravimetric analysis (TGA) indicated that thermal decarbonylation of the appended cyclopropenones occurred at approximately 180-degrees-C. Weight loss vs. temperature profiles correlated reasonably well with levels of substitution based on C-13-NMR analysis, confirming that decarbonylation of the calculated cyclopropenone substituents was the predominant thermal decomposition pathway.