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.
Biomass-derived materials possess vast potential for material science and industry in the next decades. Dwindling fossil resources and an increasing environmental awareness increase the demand for sustainable feedstock-based alternatives. In addition to natural rubber (cis-1,4-polyisoprene), the class of terpenes offers a large variety of renewable monomers, like the 1,3-diene monomers β-myrcene and β-farnesene. Living anionic polymerization of biobased 1,3-diene monomers enables the synthesis of well-defined, high molecular weight block-and statistical copolymers with unique control over molecular weights, polymer architecture, and polydiene microstructure. The resulting materials can be used for a variety of applications. For instance, polyfarnesene has been introduced as an additive in tire mixtures and replaces fossil resource-based rubbery building blocks in styrenic thermoplastic elastomers. In addition, the unsaturated nature of polymyrcene and polyfarnesene renders them accessible for functionalization by a variety of postmodification reactions, which results, for example, in improved interaction with functional fillers. (End-)functionalized polyterpenes are promising candidates as precursors for the synthesis of fully bio-based thermoplastic elastomers. In this Perspective we provide an overview of recent developments regarding the anionic polymerization of terpenes and the considerable potential the resulting polymer architectures offer for material science and a more sustainable future.
The reactivity of the biobased monomer β-farnesene in the statistical anionic copolymerization with styrene and the effect of the bottlebrush-like polyfarnesene structure on the phase separation behavior were investigated. Furthermore, thermal and material properties of β-farnesene-based thermoplastic elastomers, based on tri- and pentablock copolymers with styrene, and their processing behavior were investigated. As shown by 1H NMR online kinetics, in analogy to both isoprene and β-myrcene, the direct (i.e., statistical) anionic copolymerization of β-farnesene and styrene in cyclohexane affords block-like, tapered copolymers because of the highly diverging reactivity ratios (r Far = 27; r S = 0.037). Algebraic expressions for both the molar and volume composition profiles were derived, which provide a mathematically accurate picture of the tapered copolymer structure. The one-pot, tapered copolymer approach was used to synthesize series of tri- (ABA) and pentablock (ABABA) copolymers of styrene (A) and β-farnesene (B), varying the polydiene volume fraction between 0.50 and 0.68, respectively. Depending on the polydiene volume fraction, the tapered multiblock copolymers showed phase separation in lamellar or hexagonally packed cylindrical structures, as determined by small-angle X-ray scattering. Well-defined tapered tri- and pentablock copolymers with molecular weights of 120 kg mol–1 and low dispersity (Đ = 1.05–1.16) were obtained. The order of the tapered poly(farnesene-co-styrene) copolymers bears many similarities (same morphology, practically the same domain spacing, and a similar degree of segregation) to the corresponding polyisoprene copolymers with the same polydiene volume fraction. The similar domain spacing is suggestive of looped configurations mainly in the polyisoprene copolymers that are reduced in the polyterpene copolymers. The influence of the long alkenyl side chains of the polyfarnesene middle blocks on the mechanical properties of the multiblock copolymers was investigated by tensile testing. For this purpose, the respective tri- and pentablock copolymers of isoprene (C5 unit) and β-myrcene (C10) with styrene were synthesized as well, containing equal polydiene volume fractions as their β-farnesene-based (C15) analogs. The mechanical toughness of the polymers increased with decreasing length of the alkenyl side chains (from β-farnesene to isoprene). Furthermore, tapered polyfarnesene tri- and pentablock copolymers with styrene exhibit reduced solution viscosity in comparison to, for example, polyisoprene-based tapered PS-b-P(I-co-S) triblock copolymers, resulting in improved processability by electrospinning. These properties are discussed in terms of the different configurations of the polyterpene blocks and the pronounced differences of the entanglement molecular weights.
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.
Olefin metathesis step-growth (acyclic diene metathesis (ADMET)) and chain-growth (ring-opening metathesis) polymerization was used to prepare linear poly-(phosphonate)s with variable hydrophilicity. The first phosphonate monomer, i.e., di(undec-10-en-1-yl) methylphosphonate, for ADMET polymerization was developed, and potentially degradable and biocompatible, unsaturated poly(phosphonate)s were prepared with molecular weights up to 23 000 g mol −1 with molecular weight dispersities Đ < 2. These polymers were studied with respect to their interaction with a calcium phosphate bone substitute material from an aqueous nanoparticle dispersion that was prepared by a solvent evaporation miniemulsion protocol. Ring-opening metathesis polymerization (ROMP) was employed to synthesize more hydrophilic amorphous polyphosphonates from a novel seven-membered cyclic phosphonate monomer, i.e., 2-methyl-4,7dihydro-1,3,2-dioxaphosphepine 2-oxide, as well as hydrophobic crystalline copolymers with cis-cyclooctene. ROMP yielded polymers with molecular weights up to 6000 g mol −1 (homopolymer) and 47 000 g mol −1 (copolymers). Poly(phosphonate)s are potentially hydrolytically degradable materials and therefore promising materials for biomedical applications.
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