This paper describes a method for the synthesis of well-defined AB block copolymers, where one block is synthesized via anionic polymerization initiated with alkyllithium compounds, and one by ring opening metathesis polymerization ( ROMP) using well-defined ruthenium macroinitiators. This methodology was demonstrated by copolymerizing styrene with norbornene derivatives. Polystyrene was synthesized via living anionic polymerization initiated by sec-butyllithium, and functionalized to form macromonomers. These were used as precursors to well-defined ruthenium macroinitiators, the macromonomers being converted by an alkylidene exchange reaction with ruthenium propylidene initiator RuCl2(=CHEt)(PCy3)(2). These macroinitiators were used to initiate the ROMP of various norbornene derivatives in order to synthesize well-defined block copolymers with narrow molecular weight distributions.