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The polycrystalline powder of para- and meta-dimethyl ester of pyromellitic acid （PMDE）have been prepared by fractional crystallization

and their crystal structures have been de-termined by Wide-Angle X-ray Diffraction （WAXD）. Both p-PMDE and m-PMDE wasfound to be orthorhombic crystal system

and their unit cell parameters a = 0.840nm

b = 0.707 nm

c = 1.136 nm and a = 1.032 nm

b = 0.835 nm

c = 0.714 nm

respectively.Space group all belongs to P

mmm

. p-PMDE has two molecules per unit cell with crystaldensity 1.388g.cm

-3

while m-PMDE has two molecules per unit cell with crystal density1.522 g.cm

-3

. Indices of crystal diffraction peaks are also detailed in the present work.The difference in crystal structures between p-PMDE and m-PMDE has thus been usedto explain the curing behavior of isomerically pyromellitic dianhydride-based poly（amicester）s.