Vinyl chloride／N-phenylmaleimide copolymer (VC/PMI) and vinyl chloride／N-phenylmaleimide／acrylonitrile terpolymer (VC／PMI／AN) were synthesized in the suspension copolymerization process for improving the heat resistance of poly(vinyl chloride) (PVC)．This paper emphasized on the studies on the characterization of the heat resistant properties of these three (CO)polymers mainly by means of dynamic mechanical experiment with the change of storage modulus(E′)

change rate of E′with temperature and mechanical loss factor (tanδ) near the glass transition．The Vicat softening point (VSP) was also tested supplementaLly for comparison．The lower the change rate of E′with temperature is

the higher the heat resistance of VC／PMI and VC／PMI／AN will be．The effects of the amount of PMI and AN in the recipes of copolymerization on the change rate of E′with temperature

tanδ and VSP were also investigated．The results show that the increase of the amount of PMI will increase the rigidity of the backbone of the copolymer chain so as to improve the heat resistance．On the other hand

the introduction of AN into the copolymer will give more uniform distribution of PMI unit or segment in the backbone

and thus improves the heat resistance of the copolymer further．It is discovered that there are two peaks of mechanical loss or two glass transition temperatures of VC／PMI／AN copolymerization product．It may be interpreted from the higher reactivities of PMI and AN than VC．During the copolymerization process of VC

PMI and AN

the more active PMI and AN will copolymerize more rapidly and will consume entirely at conversion of about 45％～50％．Thereafter

VC continues to homopolymerize to form PVC homopolymer until a final conversion of 85％．Actually

the final product is a mixture of VC/PMI/AN terpolymer and homo-PVC

showing two TgS．