A study of the mean-square radius of gyration and mean-square dipole moment of the symmetric di-substituted polymers is presented basing on the improved rotational-isomeric-state method.The deduced formulas are used to investigate the conformation-dependent properties of poly(tetrafluoroethylene) (PTFE)

poly(dimethylsilylene) (PDMS) and polyethylene (PE) chains.Here

the configuration parameters of the PTFE4 (PTFE of the 4state model) chain and PDMS chain are calculated to be about 0.44 .mol0.5 . g-0.5 and 0.43 . mol 0.5. g-0.5 respectively for large polymerization degree x.However

the temperature coefficient of PDMS chain -7.32×10-3 K-1 is different obviously from that of PTFE and PE chains (about -1.00×10-3 K-1).The calculated characteristic ratios of meansquare dipole moment for long PTFE and PDMS chains are rather small

such as the ratio of PDMS is 0.27.The results show that PTFE and PDMS are non-polar polymers，good dielectric and poor in induction

even though the dipole moments of C—F and Si—C are large in PTFE and PDMS chains.Therefore

the polar and dielectric properties of polymers are also affected by the conformation and structure of chains.In addition

the chain end effect of short PTFE and PDMS chains (repeat unit x 10) results in obvious change of the ratios with x

which is in agreement with the results of Flory et al.