The mean-square radius of gyration,the mean-square dipole moment,the mean-square end-to-end distance and their temperature coefficients of unsymmetrical disubstituted poly(methylphenylsiloxane)(PMPS) chains,as a function of stereochemical structure,confomational energies and length of polymers,were studied by using an improved configurational-confomational statistical method based on the rotational-isomeric-state theory.It is found that the increase in isotacticity of PMPS chains causes a significant decrease in the unperturbed dimensions and increase in their temperature coefficients.This correlation of the properties of PMPS and its stereochemical structure is different from that of vinyl monosubstituted polystyrene and disubstituted poly(α-methylstyrene) chains.Dependence of the characteristic ratio ofon conformational energy Ew,of C6H5...C6H5 four-bond interactions increases markedly with the degree of isotacticity increasing,the dependence on the conformational energy E8 and dE6 decreases with increasing the degree of isotacticity.It is also found that the characteristic ratios of and its temperature coefficients are in agreement with corresponding experimental results in the degree of isotacticity of about 0-40%.Moreover,the C6H5...C6H5 four-bond interactions play a dominant role in regard to the dimensions of high syndiotactic chains.In addition,the effect of large side groups on unperturbed dimensions is investigated further by the improved expressions of .