By preparing homogenous blend samples with different degrees of chain entanglement

we report an anomalous contribution of chain entanglement to phase separation temperature and rate of poly(methyl methacrylate)/poly(styrene-

co

-maleic anhydride) (PMMA/SMA) blends presenting a typical lower critical solution temperature (LCST) behavior. The meltmixed PMMA/SMA blends with a higher chain entanglement density present a lower cloud point (

T

c

) and shorter delay time

but lower phase separation rate at the given temperature than solution-cast ones

suggesting that for the polymer blends with different condensed state structure

thermodynamically more facilitation to phase separation (lower

T

c

) is not necessarily equivalent to faster kinetics (decomposition rate). The experimental results indicate that the lower

T

c

of melt-mixed sample is ascribed to smaller concentration fluctuation wavelength (

Λ

m

) induced by higher entanglement degree

while higher entanglement degree in melt-mixed sample leads to a confined segmental dynamics and consequently a slower kinetics (decomposition rate) dominated by macromolecular diffusion at a comparable quench depth. These results reveal that the chain packing in polymer blends can remarkably influence the liquid-liquid phase separation behavior

which is a significant difference from decomposition of small molecular mixtures.