The local motion of polystyrenes in dilute solutions was examined by the fluorescence depolarization technique. The samples, polystyrene (PS), poly(a-methylstyrene) (PaMS), and poly(pmethylstyrene) (PpMS), were labeled with the fluorescent probe anthracene in the middle of the main chain. The relaxation time of their local motion in dilute solutions was examined by fluorescence anisotropy measurement. The activation energy of the relaxation time of the polymer chain, E*, was also evaluated by the theory of Kramers' diffusion limit. There was a close correlation between the reduced relaxation time, TJ , or its activation energy and the chain expansion factor; i.e., both the reduced relaxation time and the activation energy decrease as the solvent quality becomes better. The reduced relaxation time and the activation energy depended on the local segment density of the polymer chain in the solution. The local motion for each polymer was compared in a solvent. The relaxation time and the activation energy of PaMS were larger than those of PS. This indicated that the barrier height of the local motion for a disubstituted polymer chain is higher than that for a monosubstituted one. Furthermore, the relaxation time and the activation energy of the PpMS chain were larger than those of PS.

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The local chain motion of syndiotactic and isotactic poly(methyl methacrylatejs (s-PMMA and i-PMMA) was examined by the time-resolved fluorescence depolarization method. The reduced relaxation time, Tm/p, and its activation energy, E*, were measured in various solvents. Both the TJr¡ and E* for s-and ¿-PMMAs decreased with a decrease in the local segment density. As for the effect of the stereoregularity, the activation energy of both PMMAs showed similar values, but the relaxation time of s-PMMA was ca. 1.5-2.0 times larger than that of ¿-PMMA. Such a difference in relaxation time is closely related to the static chain stiffness predicted by the helical wormlike (HW) chain model.

syndiotactic Poly(methyl methacrylate)s (s-PMMAs) with anthracene in the middle of a main chain were synthesized, and their local motions in dilute solutions were examined by the fluorescence depolarization method. The local motion of two kinds of s-PMMAs with different molecular weights was examined in several low viscosity solvents. The mean relaxation time was measured and the activation energy was evaluated for the local motion of each s-PMMA. First, the•effect of the solvent quality on the local chain dynamics was observed. The value of the relaxation time and its activation energy became smaller as the solvent became better one. Secondly the molecular weight dependence of the local motion in poor solvents differs from that in good solvents. In poor solvents, the relaxation time of the local chain became longer as the molecular weight increased, while in a good solvent, the local motion was not influenced by the change of molecular weight. These observations could be explained by the concept of the segment density.

The mean relaxation times, T m's, of four different polymers measured by the fluorescence depolarization method were compared at their ϑ temperatures. These polymers, i.e., cis-polyisoprene (cis-PI), polystyrene (PS), poly(α-methylstyrene) (PαMS), and syndiotactic poly(methyl methacrylate) (s-PMMA), were labeled with anthracene in the middle of the main chain by living anionic polymerization. The ϑ temperature is a standard point without an excluded volume effect, and the ϑ temperature of these four polymers are almost the same, 34−35 °C. Therefore, the comparison of the T m's at the ϑ temperature enables us to clarify the relationship between the chain local motion and the chemical structure. The order of the reduced relaxation times, T m/η, was T m/η(cis-PI) < T m/η(PS) < T m/η(PαMS) < T m/η(s-PMMA), and the order of activation energies, E*, was E*(cis-PI) < E*(PS) < E*(PαMS) < E*(s-PMMA). From a molecular standpoint, the relaxation times depend on both the substituents attached to the main chain and the stereoregularity of the polymer chains. In consideration of the stereoregularity of the polymer chains, the relaxation time of atactic PαMS is estimated to be not much different from the relaxation time of atactic PMMA.