|520. Grune, E.; Johann, T.; Appold, M.; Wahlen, Ch.; Blankenburg, J.; Leibig, D.; Müller, A.H.E.; Gallei, M.; Frey, H.: One-Step Block Copolymer Synthesis versus Sequential Monomer Addition: A Fundamental Study Reveals that one Methyl Group Makes a Difference, Macromolecules 51, 3527 (2018) -- DOI: 10.1021/acs.macromol.8b00404|
Block copolymers of polyisoprene and polystyrene are key materials for polymer nanostructures as well as for several commercially established thermoplastic elastomers. In a combined experimental and kinetic Monte Carlo simulation study, the direct (i.e., statistical) living anionic copolymerization of a mixture of isoprene (I) and 4-methylstyrene (4MS) in nonpolar media was investigated on a fundamental level. In situ1H NMR spectroscopy enabled to directly monitor gradient formation during the copolymerization and to determine the nature of the gradient. In addition, a precise comparison with the established copolymerization of isoprene and styrene (I/S) was possible. Statistical copolymerization in both systems leads to tapered block copolymers due to an extremely slow crossover from isoprene to the styrenic monomer. For the system I/4MS the determination of the reactivity ratios shows highly disparate values with rI = 25.4 and r4MS = 0.007, resulting in a steep gradient of the comonomer composition. The rate constants determined from online NMR studies were used for a kinetic Monte Carlo simulation, revealing structural details, such as the distribution of the homopolymer sequences for both blocks, which are a consequence of the peculiar kinetics of the diene/styrene systems. DFT calculations were used to compare the established copolymerization of isoprene and styrene with the isoprene/4-methylstyrene system. A variety of gradient copolymers differing in molecular weight and monomer feed composition were synthesized, confirming strong microphase segregation as a consequence of the blocklike structure. The one-pot synthesis of such tapered block copolymers, avoiding high vacuum or break-seal techniques, is a key advantage for the preparation of ultrahigh molecular weight block copolymers (Mn > 1.2 × 106 g/mol) in one synthetic step. These materials show microphase-segregated bulk structures like diblock copolymers prepared by sequential block copolymer synthesis. Because of the living nature of the tapered block copolymer structures, a vast variety of complex structures are accessible by the addition of further monomers or monomer mixtures in subsequent steps.