Abstract A novel reactive extrusion process was developed to obtain compatibilized A/B immiscible polymer blends. It consisted of polymerizing a monomer of polymer A in the presence of polymer B. A fraction of polymer B chains bore initiating sites either at the chain end(s) or along the chain backbones, from which polymer A chains could grow. In the process, polymer A and a graft or block copolymer of A and B were formed simultaneously leading to in situ polymerized and in situ compatibilized A/B polymer blends. This paper shows the feasibility of this novel process in a batch mixer using polypropylene (PP) and polyamide 6 (PA 6) blends as an example. Specifically, ε-caprolactam (ε-CL) was polymerized anionically in the presence of PP. A fraction of the PP chains bore 3-isopropenyl-α,α-dimethylbenzene isocyanate (TMI) which acted as growing centers to initiate PA 6 chain growth from the PP chains and to form a graft copolymer of PP and PA 6.