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An improved configurational-confomational statistical method is developed and the mean-square radius of gyration for atactic poly（α-methylstyrene）（PαMS）chains is studied

in which the effect of large side groups is considered. The deduced formulas

based on the rotational isomer state theory

are used to investigate the configuration-dependent properties of the atactic polymer chain

and the statistical correlation of the unperturbed polymer chain dimension and structure parameters are calculated.For the fraction of meso dyads w

m

=0.4

the dependence of the radius of gyration R

g

and the intrinsic viscosity[η]on the molecule mass M are R

g

=2.63×10

-2

M

0.50

nm and[η]=7.36×10

-2

M

0.497

respectively

which are in agreement with the previous experimental data for the PαMS samples.A small hump is detected in the curve of the characteristic ratio of the unperturbed mean-square radius of gyration versus the chain length for short PαMS chains.The R

g

increases linearly with the temperature T

and the effects of the chain length and the tacticity on the temperature coefficient are remarkable.These are quite different from the results for PαMS chains not considering side groups or for the monosubstituted polystyrene chain.