One-Step Block Copolymer Synthesis versus Sequential Monomer Addition: A Fundamental Study Reveals That One Methyl Group Makes a Difference

Abstract

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.

Kinetics of the anionic homopolymerizations of ß–myrcene and 4–methylstyrene in cyclohexane initiated by n–butyllithium

“…The information present in the literature about kinetic parameters of the anionic polymerization of 4MS in nonpolar solvents is almost null. Grune et al reported a point value of the apparent propagation coefficient for the polymerization of 4MS in cyclohexane and initiated by sec –butyllithium of = 1.26×10 −3 (L/mol) 1/2 s −1 at 40 °C and another value is reported by Pakuro et al of = (3.58 ± 0.5) × 10 −3 (L/mol) 1/2 s −1 at 26 °C; but this last value was obtained in benzene. However, it would be desirable to have enough kinetic information to calculate the value of at different temperatures, and thus be able to predict the polymerization rate of the 4MS in nonisothermal processes as these are common in industry.…”

“…On the other hand, another monomer that is currently gaining renewed interest in the field of anionic polymerization carried out in nonpolar solvents, is 4–methylstyrene (4MS), see structure in Scheme a. There are some reports on its anionic homopolymerization, and its anionic copolymerization with 1,3‐butadiene and isoprene .…”

Pyrene label used as a scale for sequence-controlled functionalized polymers

“…Controlled chain-growth polymerization, by contrast, exhibits significant advantages in gram-scale syntheses of functionalized polymers with high molecular weights and controlled topologies. [10][11][12] Unfortunately, the sequence structures prepared via this strategies generally exhibit statistical features and poor precision, i.e., inhomogeneities in the degree of polymerization (DP) distributions of monomer units. 13 Therefore, evaluating and improving sequence precision has become a major priority.…”

One‐Step Sequence‐Selective Synthesis of Block Copolyester from Mixed Phthalic Anhydride, Cyclohexene Oxide, and δ‐Valerolactone

“…For most comonomer combinations, one‐step kinetically controlled (sequence‐selective) block copolymerization is impeded by the mismatched monomer activities. So, only a few successful cases have been reported for one‐step synthesis of classic A n B m type block copolymers from mixed vinyl monomers [ 14 ] or heterocyclic monomers. [ 15 ] Such limitations have been frequently broken in the realm of recently revived epoxide‐based copolymerization.…”