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Abstract:
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 H 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 (rFar = 27; rS = 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.

Abstract:
Highly efficient stoichiometric coupling of sterically hindered polystyryllithium (PS-Li) chain ends was achieved using tetra[3-(chloro-dimethylsilyl)propyl]silane (TCDMSPS) as the linking agent. Based on the disparate reactivities of isoprene (I, rI = 11.0) and styrene (S, rS = 0.049) in the anionic copolymerization in nonpolar media, poly(isoprene0.5-grad-styrene0.5) (P(I0.5-grad-S0.5)) tapered 4-arm star copolymers were synthesized in only two steps. The tapered 4-arm star copolymers (Mwtargeted = 40 to 160 kg mol^-1) were synthesized with high star functionalities f (Mw,star/Mw,arm = 3.68 - 3.98), low dispersity (Ð = 1.06 - 1.15) and minimal residual precursor content (2-8 wt%), avoiding fractionation or other purification methods. Coupling kinetics measurements revealed that for the synthesis of polystyrene (PS) 4-arm stars (12 kg mol^-1) a coupling efficiency of 98% was already achieved within 2 minutes. All star polymers were analyzed by size exclusion chromatography (SEC) viscometry with universal calibration (UC) as well as NMR spectroscopy. Well-defined nanofibers from the tapered copolymer stars were obtained via electrospinning.