This work deals with the synthesis of high molecular weight, thermoplastic phenol formaldehyde (PF) resins for the application in the synthesis of polypropylene-PF compatibilizers by reactive extrusion. Phenol formaldehyde of high molecular weight is required in reactive extrusion with most polypropylene grades to meet the viscosity ratio (lambda) requirement for best mixing. Special lab-grades of PF were therefore developed. The polymers synthesized were highly linear and in the M-w range of 10-30 000 g mole(-1).