Direct Observation of Chain Lengths and Conformations in Oligofluorene Distributions from Controlled Polymerization by Double Electron–Electron Resonance

Abstract

Synthetic polymers are mixtures of chains with different lengths, and their chain length and chain conformation are often experimentally characterized by ensemble averages. We demonstrate that double electron–electron resonance (DEER) spectroscopy can reveal the chain length distribution and the chain conformation and flexibility of the individual n-mers in oligo-(9,9-dioctylfluorene) from controlled Suzuki–Miyaura coupling polymerization. The required spin-labeled chain ends were introduced efficiently via a TEMPO-substituted initiator and chain-terminating agent, respectively, with an in situ catalyst system. Individual precise chain length oligomers as reference materials were obtained by a stepwise approach. Chain length distribution, chain conformation, and flexibility can also be accessed within poly­(fluorene) nanoparticles.

Origin of the Anisotropy and Structure of Ellipsoidal Poly(fluorene) Nanoparticles

“…More detailed elucidation of model based and model free evaluation of DEER data on oligomer mixtures obtained from controlled Suzuki-Miyaura coupling polymerization can be found in the cited litertaure. 18 In conclusion, the successful implementation of DEER measurements on a real-life synthetic polymer, containing a multitude of different chain length species, was demonstrated. The analysis technique allows for the quantification of individual oligomer species and the respective conformation and flexibility directly on mixtures.…”

“…191 More details on the measurements of the individual oligomer species can be found in recent literature. 18 As apparent from the contour lengths Lc, determined using either free or fixed persistence length, the two data sets only showed marginal differences, which indicated accessibility of the form factors using fixed persistence lengths. Conclusively, the model WLC_rigid_gauss, a Gaussian broadening of σ = 0.06 nm and a global persistence length of Lp = 14 nm were suitable conditions to obtain a good accordance of experimental data and fit.…”

“…Conjugated polymer chains are considered semi-flexible, with a rigid character at short length scales and flexibility at larger dimensions. 17,18 The main proportion of electronic coupling within conjugated polymers was found to occur along the polymer backbone. But, in order to allow for macroscopic charge transport as necessary in applications, intermolecular aggregates, for example formed by π-stacking, are required.…”

“…Details on the synthesis and labeling of oligomers are more thoroughly elucidated in the corresponding doctoral thesis and in cited literature. 18,186 DEER measurements and the processing of the acquired data were performed by D.…”

“…Extensive protocols and parameters of the DEER measurements can be found in published literature. 18…”