Poly(alkyl acrylate)s are of industrial importance, due to their wide use in pressure-sensitive adhesives, paints, or coatings.1 Most of those products are currently developed by trial-and-error procedures, necessitating synthesis and tests of functional properties for numerous samples. Therefore, it is important to be able to characterize and understand the polyacrylate properties on a molecular level. Unwanted side reactions during the polyacrylates synthesis leads to the known, although often ignored, formation of branched polyacrylates chains. NMR is the method of choice to quantify degrees of branching, while chromatography is sensitive to the type of branches.2
Chain branching has been investigated in a homologous series of poly(n-alkyl acrylate)s (methyl, ethyl, n-butyl, n-hexyl). The total amount of chain branching was quantified using 13C nuclear magnetic resonance (NMR) spectroscopy.3 Solution-state, solid-state, swollen-state and melt-state 13C NMR spectroscopy were compared for the quantification of the quaternary carbon at the branching point. Solution-state NMR yields inaccurate results for incompletely soluble samples. Solid-state NMR exhibits insufficient resolution of the quaternary carbon signals. By comparing the signal-to-noise ratio of the weak quaternary carbon signal, melt-state 13C NMR spectroscopy was found to give the highest sensitivity. This method gave access to low degrees of branching in both soluble and insoluble polyacrylates, homopolymers and copolymers. The lowest degree of branching was found for the ethyl member of the series with quantification by conventional solution-state NMR found to take a prohibitively long time.
Quantitative melt-state NMR was then used for the first time in a kinetic study quantifying the degree of branching in polyacrylates,4 revealing the influence of the synthesis temperature as well as the controversial reduction of degree of branching by the use of chain-transfer agents during the synthesis. The essence of the controversy lies in the precision of the quantification of the degree of branching.