The free radical synthesis of hyperbranched polymer architectures using a thermally and photochemically labile styrenic Barton ester monomer of the AB type, designed to combine the functions of initiator, monomer, and branch unit, is described. Highly branched poly(styrene)s were obtained in one step by copolymerizing this monomer with styrene. Residual Barton ester functionality in the hyperbranched products enabled their use as macroinitiators in subsequent polymerizations. Linear PMMA was thus successfully grafted from the hyperbranched poly(styrene) cores. This approach is a versatile route for the synthesis of hyperbranched homopolymers and copolymers via free radical polymerization chemistry.