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 situ 1 H 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 r I = 25.4 and r 4MS = 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 (M n > 1.2 × 10 6 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.

The monoterpene myrcene is a bio-based diene monomer. The statistical, living anionic copolymerization with isoprene, styrene and 4-methylstyrene leads to gradient or tapered block copolymers, studied by in-situ NMR, SAXS and TEM.

The synthesis of tapered multiblock copolymers by statistical living anionic copolymerization of a mixture of isoprene (I) and 4-methylstyrene (4MS) in cyclohexane is based on vastly different reactivity ratios of I and 4MS (r I = 25.4 and r 4MS = 0.007). A library of tapered multiblock copolymers was prepared with different molecular weights (approximate molecular weights of 80, 240, and 400 kg/mol) and number of blocks (P(I-co-4MS) n with 1 ≤ n ≤ 5), and their thermomechanical properties were investigated by differential scanning calorimetry, rheology, and tensile testing in relation to their nanodomain structure, the latter investigated by small-angle X-ray scattering. The interaction parameter between I and 4MS segments was obtained based on the order-to-disorder transition temperatures of a series of PI-b-P4MS diblock copolymers prepared by sequential addition of monomers. The obtained χ(T) dependencies (χ MFT = 23.2/T − 0.024 and χ FH = 36.0/T − 0.041) are weaker than in the corresponding PI-b-PS system, revealing that the different reactivity ratios of the monomers is not the sole factor that controls the miscibility of the segments in the tapered multiblock copolymers. The latter is controlled by the value of the interaction parameter, the width of the tapered interfaces, and the number of blocks and total molecular weight. Tapered multiblock copolymers undergo a fluctuation-induced first-order transition from the ordered to the disordered state. The domain spacing scales as d ∼ n −0.83±0.02 when compared under a fixed total molecular weight, reflecting the conformational properties of the middle blocks. In addition, the domain spacing depends on molecular weight, as d ∼ N 0.55 , revealing stretching of chains and nonideal configurations. These structural features of the tapered multiblock copolymer affected their mechanical properties. Tensile tests showed a dramatic enhancement of the strain at break with a concomitant increase in toughness. These mechanical properties can be fine-tuned by the judicious selection of molecular weight and number of blocks. The state of order (ordered, weakly ordered vs disordered) and proximity to the glass temperature of the hard phase are additional parameters that affect the mechanical response. The improved mechanical properties reflect the enhanced interfacial strength, the latter provided by the configurations of the middle blocks in the copolymers. The influence of methyl group substitution in the para position of styrene is discussed by comparing the self-assembly and thermomechanical properties of the current P(I-co-4MS) n with the P(Ico-PS) n system. We found that the shorter tapered interface in the former is counterbalanced by its lower effective interaction parameter resulting in similar domain spacings.