The purpose of this study is to reinforce the flexible nylon-6 by using Poly (p-diphenyl methyl terephthalamide) (PMA) using MDI as a chain extender. Observation by scanning election microscopy showed that the multiblock copolymer had a homogeneous texture. From the differential scanning calorimetry measurements, the multiblock copolymers were shown to have only one melting point, Tm, which is higher than that of nylon-6. The wide-angle X-ray diffraction pattern shows that nylon-6 has two diffraction peaks at 2theta = 20-degrees and 23.8-degrees. However, the multiblock copolymer has only one peak at 2theta = 20-degrees, indicating a different crystal structure of multiblock copolymers. The copolymers appears to have a significant reinforcing effect on the mechanical properties. 1994 John Wiley & Sons, Inc.