Global Trends for k_p? The Influence of Ester Side Chain Topography in Alkyl (Meth)Acrylates − Completing the Data Base

The Arrhenius parameters of the propagation rate coefficient, k_p, are determined via the IUPAC recommended pulsed laser polymerization–size exclusion chromatography (PLP-SEC) method for two linear alkyl acrylates (stearyl and behenyl acrylate), four branched alkyl acrylates (isononyl (INA-A), tridecyl (TDN-A and TDA-A), and henicosyl acrylate (C21A)), and two branched alkyl methacrylates (tridecyl methacrylates (TDN-MA and TDA-MA)) in bulk. Furthermore, the above stated acrylates and heptadecyl acrylate (C17A) were studied in 1 M solution in butyl acetate (BuAc). On the basis of such a wide data basis in combination with the already literature known data of relatives of the herein investigated monomers, we are able to identify and extend global trends and family type behavior for the propagation rate coefficients of a wide array of alkyl (meth)­acrylates. In order to ensure a valid SEC evaluation, the polymer specific Mark–Houwnik–Kuhn–Sakurada (MHKS) parameters are determined for each of the polymers, via multidetector SEC analysis (multi angle laser light scattering (MALLS) in combination with differential viscosimetry (Visco) and refractive index (RI)) of narrowly distributed polymer samples obtained via fraction with a preparative SEC column. By employing further physicochemical polymer specific data (e.g., glass transition temperatures (T_g)), we provide a hypothesis for the reported trends and family type behaviors: (i) the steady increase of k_p with increasing ester side chain length for linear alkyl (meth)­acrylates may be explained by a decreasing concentration of the polar ester moieties, resulting in a decreasing stabilization of the attacking radical in the transition state of the propagation reaction, and (ii) the family type behavior of the branched alkyl methacrylates can be understood by considering steric and entropic influences. For the branched alkyl acrylates, no clear trend is detectable, and a family type behavior is clearly not observed in contrast to the corresponding methacrylates.