Polydisperse melts of nematic polymer liquid crystals (PLC's) display
a biphase region at the nematic-isotropic transition. We describe this
phase separation for a melt of transesterifying chains. Transesterifi
cation, a process where chains exchange material at fixed overall chai
n number, results in an annealed rather than quenched polydispersity.
The PLC is modeled as a wormlike chain with a Maier-Saupe nematic mean
field. The phase diagram and length distribution are calculated. The
biphasic gaps reported in experiments on transesterifying melts are di
scussed.