## 3 Results and Discussion
This point is an indicator of chain flexibility because a more flexible polymer will allow all pyrene labels to form an excimer more readily than a stiffer one. A comparison of the _xf_(free = 0) value of 0.03 for the Py-PDMS series in THF, DMF, and dioxane to that of 0.25 (10 times larger) for Py-PIMA in THF [11] clearly underscores the difference in flexibility of the two backbones. PIMA, with its succinin anhydride and succinimide rings being parts of the polymer backbone, is much stiffer than PDMS. Only at very high pyrene contents in Py-PIMA, with _x(f_free = 0)_ = 0.25, were all pyrene labels capable of forming an excimer. Information about the LRPCD of PDMS was obtained by calculating <kMF>blob for each Py-PDMS sample. The plots of <kMF>blob as a function of pyrene content indicate that <kMF>blob does not depend on pyrene content (Figure 6). The <kMF>blob values at 22 \({}^{\circ}\)C were 1.16 (\(\pm\)0.13) ns\({}^{-1}\) in THF, 1.14 (\(\pm\)0.12) ns\({}^{-1}\) in DMF, and 0.99 (\(\pm\)0.10) ns\({}^{-1}\) in dioxane. At least within the limited range explored, <kMF>blob does not depend on viscosity either (0.47, 0.79, and 1.37 mPa-s for THF, DMF, and dioxane at 25 \({}^{\circ}\)C, respectively) [17]. <kMF>blob for Py-PDMS in THF was then compared to the values found with other polymeric backbones from previous studies [9; 11]. The data are plotted in Figure 7 as a function of the molar mass of the structural units (\(M_{\rm{SU}}\)) of the different polymers. In general, <kMF>blob was found to decrease quasi-exponentially with increasing \(M_{\text{SU}}\). However, an obvious exception is the comportment of PIMA whose rigid backbone yielded the lowest \(<\)kMF\(>\)blob value despite PIMA having one of the smallest \(M_{\text{SU}}\). With its low \(M_{\text{SU}}\), PDMS resulted in the largest \(<\)kMF\(>\)blob value obtained reported for any polymer thus far (1.26 ns\({}^{-1}\)), being 2.9 and 27 times larger than for polystyrene (0.43 ns\({}^{-1}\)) and PIMA (0.047 ns\({}^{-1}\)) in THF, respectively. Nevertheless, the overall trend is reasonable in that bulkier and less flexible structural units slow LRPCD. Another interesting feature of Figure 7 is that, except for PIMA, the polymer backbones yield \(<\)kMF\(>\)blob values that seem to cluster around a master curve. If this preliminary trend proves correct, the LRPCD of any polymer in THF should be quantitatively predictable from knowledge of its \(M_{\text{SU}}\) using Figure 7 as a calibration curve. \[<\)kMF\(>\)blob = 21.5 \(\times\) (\(M_{\text{SU}}-65\))\({}^{-1.21}\)  
While it is reasonable and expected that \(<\)kMF\(>\)blob for PDMS, a polymer well-known for its flexibility [10; 18], is greater than for PS in THF, the 2.9 fold difference is much smaller than that reported for the dynamics of pyrene end-labeled monodisperse PDMS and PS polymers in toluene [19]. Thus, the fact that the LRPCD probed by PDMS constructs that are monodisperse and end-labeled are thrice the value for polydisperse and randomly labeled PDMS with pyrene may be a consequence of the different bond angles of the PDMS backbone: the Si-O-Si bond angle is 146\({}^{\circ}\) whereas that of O-Si-O is 108\({}^{\circ}\)[18]. This difference implies that a PDMS chain in its most energetically stable _all trans_-conformation would be coiled, whereas the backbone of a PS chain in the same _all trans_-conformation would be fully extended because its C-C-C bond angles are near 109\({}^{\circ}\). Consequently, the ends of a PDMS chain in solution are much closer to each other than those of a PS chain. For experiments in which the dynamics of end-to-end cyclizations are being measured, PDMS allows faster rates of encounter than does PS, and, thus, an apparently larger difference in LRPCD. However, this effect is expected to be much less pronounced for pyrene labels that are randomly distributed along the PDMS chains, and the difference in LRPCD is expected (and found) to be smaller.