A method for preparing a graft copolymer having a polyolefin backbone and polyorganosiloxane pendant groups can be catalyzed by a Lewis Acid catalyst, Piers-Rubenstzjan catalyst, or hydrosilylation reaction catalyst. The graft copolymer prepared by the method is useful as a thermoplastic elastomer or as a compatibilizer in compositions containing both a polyolefin and a polyorganosiloxane.