Fragment Parameters for the Calibration of Molecular Weights of Rodlike Oligomers/Polymers by Gel Permeation Chromatography

Molecular weights for rodlike polymers determined by the gel permeation chromatography (GPC) method are in general inflated when polystyrene references are used. In this paper, we have established a set of new empirical parameters, χⁱ, of commonly used fragments including a range of different substituted thiophenes, benzenes, pyridines, ethylenes, ethynylenes as well as butadienes for the calibration of the inflated <i>M</i>_n data obtained by GPC measurements for rodlike oligomers/polymers. The χⁱ parameters are accessed by adjusting the slopes of the plots of <i>M</i>_n relative to the real molecular weights, <i>M</i>_r, for a series of oligomers/polymers of different structural varieties to approximately 1. The calibrated molecular weights, <i>M</i>_c, are calculated based on the equation, <i>M</i>_c = [Σ χⁱFⁱ]<i>M</i>_n, where Fⁱ stands for the percent of the total number of fragment <b>Φ</b>^<b>i</b> in an oligomer/polymer, which matches nicely with the <i>M</i>_r of oligomers/polymers. More than 84% of the calibrated <i>M</i>_c values are within 12% deviation from the real molecular weights <i>M</i>_r

On the Stereoselectivity of Ring-Opening Metathesis Polymerization (ROMP) of <i>N</i>‑Arylpyrrolidine-Fused Cyclobutenes with Molybdenum– and Ruthenium–Alkylidene Catalyst

Ring-opening metathesis polymerization (ROMP) of cyclobutenes fused with <i>N</i>-arylpyrrolidene with Schrock–Hoveyda catalyst containing a racemic biphenolate ligand [Mo­(N-2,6-<i>i</i>-Pr₂C₆H₃)­(CHCMe₂Ph)­(biphenolate)] gives polycyclobutenes with homogeneous tacticity and predominantly double bonds in <i>Z</i>-configuration. Reactions of the same substrates with the first-generation Grubbs catalyst [(Cy₃P)₂Cl₂RuCHPh] or Schrock molybdenum carbene with monodendate alkoxy ligands [Mo­(N-2,6-<i>i</i>-Pr₂C₆H₃)­(CHCMe₂Ph)­(OCMe­(CF₃)₂] yield the corresponding polycyclobutene containing a mixture of <i>Z</i>- and <i>E</i>-double bonds. Upon diimide reduction, all these polycyclobutenes give the same tactic hydrogenated polymers, indicating that the stereochemistry at the asymmetric carbons remains the same in all these reactions. The stereoselectivities of ROMP with cyclobutenes and with norbornenes are compared, and the plausible mechanisms are proposed

Controlling the Orientation of Pendants in Two-Dimensional Comb-Like Polymers by Varying Stiffness of Polymeric Backbones

The electronic communications between chromophores are closely related to distances and orientation of these π-conjugated systems. Reported herein is a collection of well-defined two-dimensional comb-like polymers containing porphyrin pendants obtained by ring-opening metathesis polymerizations of norbornene and cyclobutene derivatives using ruthenium or molybdenum catalysts. The spacing separating the adjacent pendants are defined by ring sizes of cycloalkenes and the orientations are determined by the stiffness of the polymeric backbone, which is, in turn, discerned by the percentage of <i>Z</i>-double bonds. Both peak widths of the porphyrin radical cation of the EPR spectra and the absorption profiles in the Soret band region reflect the degree of the spin delocalization and exciton coupling between porphyrin pendants in these polymers and, hence, the stiffness of the polymeric backbone. Our approach offers a useful protocol to align an array of chromophores appended onto a rigid polymeric backbone so that the optoelectronic properties can be tuned