Traditionally, the upper application temperature of semi-crystalline polymers is improved by increasing the modulus at temperature by simple addition of glass fiber or manipulation of the crystalline behavior. In the current study, the heat distortion temperature (HDT) of LDPE was increased by as much as 11^oC by blending with high T_g thermally stable amorphous styrene maleic anhydride (SMA) copolymers. Microdomains of SMA prevented LDPE chains from conforming to the applied load at temperature. The HDT of PBT was shown to increase by up to 13^oC by a proposed reaction with the end groups yielding a homogenized single phase. In PA6/6 the HDT was found to increase by as much as 18^oC by reaction with the amine groups yielding a crosslinked structure. Although the mechanism was different for each polymeric system, the net effect on the crystallization behavior was negligible.