We present a novel process for the manufacture of ultra high molecular weight polyethylene precursors which can be subsequently made into high-stiffness, high-strength fibers or films. This process combines solid-state extrusion and an in situ constrained solvent swelling process into a single stage for precursor preparation. The precursors produced by this process are shown to be highly ductile and porous. The inclusion of a small quantity of nontoxic solvent (10 wt% mineral oil) in the process has also been shown, using DSC and SEM characterization techniques, to provide the following process advantages: (i) better interfacial cohesion, because of the enhanced chain mobility at the particle surface due to the solvating effects; (ii) improved processability; and (iii) more uniform thermal properties and morphology across the extrudates due to enhanced heat transfer.