Eric S. Tillman

Accurately Determining the Extent of Coupling in Post Polymerization Reactions of Polystyrene

Polymers prepared by controlled radical polymerization (CRP) can be employed in subsequent chain-end joining reactions, yet accurately assessing the extent of coupling in mechanistically unique paths is not straightforward. Precisely known mixtures of polystyrene standards were prepared and analyzed by gel permeation chromatography (GPC), mimicking the coupled product and precursor that could be present after a post-polymerization, chain-end joining reaction. The exactly known percentages of each polymer in the mixture allowed for comparison of the true “extent of coupling” (Xc) to that determined by a commonly used equation, which is based on number average molecular weights (Mn) of the precursor and coupled product. The results indicated that an improvement in accuracy could be achieved by instead using refractive index (RI) signal height ratios under the peak molecular weight (Mp) of each component, with all calculations being within 0.05 of the true Xc of the fabricated “product” mixture (compared to greater than 0.10 average error using the more established method) when the sample mixture had nominal molecular weights of 2500 and 5000 Da. Moreover, when “precursor” and “coupled” pairs mixed were not related as a simple doubling of molecular weight, the calculation method presented here remained effective at determining the content of the mixture, especially at higher Xc values (>0.45). This second case is important for experiments that may link polymer chains together with a spacer, such as a radical trap, a triazole, or even larger structure such as an oligomer.

Identity of Low-Molecular-Weight Species Formed in End-To-End Cyclization Reactions Performed in THF

Cyclic polymers were produced by end-to-end coupling of telechelic linear polymers under dilute conditions in THF, using intramolecular atom transfer radical coupling or click chemistry. In addition to the expected shift to longer elution times on gel permeation chromatography (GPC) indicative of the formation of cyclic product, lower molecular weight species were consistently observed upon analysis of the unpurified cyclization reaction mixture. By systematically removing or altering single reaction components in the highly dilute cyclization reaction, it was found that THF itself was responsible for the low-molecular-weight material, forming oligomeric chains of poly(THF) regardless of the other reaction components. When the reactions were performed at higher concentrations and for shorter time intervals, conducive to intermolecular chain-end-joining reactions, the low-molecular-weight peaks were absent. Isolation of the material and analysis by 1H NMR confirmed that poly(THF) was being formed in the highly dilute conditions required for cyclization by end-to-end coupling. When a series of mock cyclization reactions were performed with molar ratios of the reactants held constant, but concentrations changed, it was found that lower concentrations of reactants led to higher amounts of poly(THF) side product.