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" (X_c) to that determined by a commonly used equation, which is based on number average molecular weights (M_n) 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 (M_p) of each component, with all calculations being within 0.05 of the true X_c 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 X_c 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.