Transient effects in diffusion‐controlled polymer cyclization

Abstract

hnsient effects in diffusion-controllexI polymer cyclization a)Dedicated to Paul Rempp on the occasion of his 60th birthday with best wishes and thanks for many years of scientific interaction SUMMARY:Fluorescence decay measurements were carried out to study the kinetics of intramolecular exciplex formation in two low-molecular-weight samples of polystyrene (1) with narrow molecular weight distribution capped on one end with a pyrene moiety and on the other with an Nfl-dimethylaminophenyl group. Monomer and excimer decay profiles, I , ( t ) and I&), resp., were inconsistent with the predictions of the simple two-state (Birks) model, but could be well-fitted by assuming that the diffusion-controlled rate coefficient for exciplex formation, k, (t), is timedependent. Using a Smoluchowski formulation for this rate coefficient k, (?) = a(1 + bt-'"), we could use the a and b parameters to calculate values for the chain-end diffusion coefficient characterizing the reaction for samples of the polymers in cyclopentane and cyclohexane solutions.

Studying Pyrene-Labeled Macromolecules with the Model-Free Analysis

“…Applying Birks’ scheme to analyze the decays, they obtained for the first time a direct measure of the rate constant of pyrene excimer formation or end-to-end cyclization ( k cy ) which decreased strongly with polymer chain length as N –1.6 and showed that k cy was proportional to ( I E / I M ) SS . This study led to the rapid development of the use of TRF to obtain k cy for pyrene end-labeled polymers. ,− ,,− Unfortunately, as the study of PLMs was extended to polymeric architectures more representative of those encountered in polymer science that differ from pyrene end-labeled monodisperse polymers, it became ever clearer that handling the kinetics of pyrene excimer formation with a single pyrene excimer formation rate constant as done in Birks’ scheme was unsatisfactory. , Macromolecules bearing more than two pyrene pendants generated a distribution of rate constants corresponding to the distribution of chain lengths separating every two pyrene labels and the fluorescence decays needed to be fitted with more than the two exponentials required by Birks’ scheme. , Considering the well-known uncertainties associated with the analysis of fluorescence decays with sums of exponentials, , the scientific community held the view that only qualitative information could be retrieved about the internal dynamics of a macromolecule labeled with more than two pyrenes. Although not explicitly stated, this point of view was reflected in the 1993 review by Winnik, which described a large number of qualitative results obtained with a wide variety of pyrene-labeled macromolecules.…”

Effect of Viscosity on Long-Range Polymer Chain Dynamics in Solution Studied with a Fluorescence Blob Model

“…By and large, the bulk of FDQ studies aiming at determining 〈k Q 〉 uses the chromophore pyrene for both C and Q as the excited pyrene can undergo self-quenching by forming an excimer upon an encounter with a ground-state pyrene. 4,5 To this end, pyrene has been attached at the end of small oligomethylene chains, 9,14-22 short monodisperse polymers, 4,6,10,[23][24][25][26][27][28][29][30][31] or dendrimers 32,33 and randomly along longer polymers. 5,7,[34][35][36][37][38][39][40][41][42] As predicted by theory, [1][2][3] FDQ experiments conducted on end-labeled chains yield a single 〈k Q 〉 value, an important simplification when compared to studies dealing with randomly labeled polymers which yield a distribution of 〈k Q 〉 values.…”

“…5,7,[34][35][36][37][38][39][40][41][42] As predicted by theory, [1][2][3] FDQ experiments conducted on end-labeled chains yield a single 〈k Q 〉 value, an important simplification when compared to studies dealing with randomly labeled polymers which yield a distribution of 〈k Q 〉 values. 5,34 The simpler analysis associated with the fluorescence data obtained with end-labeled polymers has led to their overwhelming use in the literature with 4,6,10,[23][24][25][26][27][28][29][30][31] or without pyrene. [11][12][13][43][44][45][46][47][48][49][50][51][52][53] Experiments aiming at probing the effect of viscosity on LRPCD monitor the kinetics of excimer formation for pyrene end-labeled chains by either using solvents having different viscosity 7,10 or changing the temperature of the solvent.…”

“…By and large, the bulk of FDQ studies aiming at determining ⟨ k Q ⟩ uses the chromophore pyrene for both C and Q as the excited pyrene can undergo self-quenching by forming an excimer upon an encounter with a ground-state pyrene. , To this end, pyrene has been attached at the end of small oligomethylene chains, ,− short monodisperse polymers, ,,,− or dendrimers , and randomly along longer polymers. ,,− As predicted by theory, − FDQ experiments conducted on end-labeled chains yield a single ⟨ k Q ⟩ value, an important simplification when compared to studies dealing with randomly labeled polymers which yield a distribution of ⟨ k Q ⟩ values. , The simpler analysis associated with the fluorescence data obtained with end-labeled polymers has led to their overwhelming use in the literature with ,,,− or without pyrene. − ,− Experiments aiming at probing the effect of viscosity on LRPCD monitor the kinetics of excimer formation for pyrene end-labeled chains by either using solvents having different viscosity , or changing the temperature of the solvent. ,− …”

“…However, the relationship between [Py] loc and viscosity might not be best probed with short monodisperse end-labeled polymers, since the small coil dimension of a short chain should enable an excited pyrene to probe the entire polymer coil regardless of solvent viscosity. Consequently, [Py] loc is not expected to change much when using the short monodisperse chains most commonly used in the literature. ,,− ,− The drastic reduction in ⟨ k Q ⟩ with increasing chain length for end-labeled polymers prevents the use of long polymers for which too few end-to-end encounters occur. , Furthermore, the inherently weak excimer emission obtained with long end-labeled polymers would be further reduced in high-viscosity solvents . Considering the drastic enhancement in excimer formation observed when dealing with randomly labeled polymers, we set out to monitor the effect of viscosity on ⟨ k Q ⟩ for two series of polystyrene randomly labeled with pyrene.…”