Abstract
The anionic polymerization of styrene and 1,3-butadiene in the presence of phosphazene bases (t-BuP4, t-BuP2 and t-BuP1), in benzene at room temperature, was studied. When t-BuP1 was used, the polymerization proceeded in a controlled manner, whereas the obtained homopolymers exhibited the desired molecular weights and narrow polydispersity (Ð < 1.05). In the case of t-BuP2, homopolymers with higher than the theoretical molecular weights and relatively low polydispersity were obtained. On the other hand, in the presence of t-BuP4, the polymerization of styrene was uncontrolled due to the high reactivity of the formed carbanion. The kinetic studies from the polymerization of both monomers showed that the reaction rate follows the order of [t-BuP4]/[sec-BuLi] >>> [t-BuP2]/[sec-BuLi] >> [t-BuP1]/[sec-BuLi] > sec-BuLi. Furthermore, the addition of t-BuP2 and t-BuP1 prior the polymerization of 1,3-butadiene allowed the synthesis of polybutadiene with a high 1,2-microstructure (~45 wt %), due to the delocalization of the negative charge. Finally, the one pot synthesis of well-defined polyester-based copolymers [PS-b-PCL and PS-b-PLLA, PS: Polystyrene, PCL: Poly(ε-caprolactone) and PLLA: Poly(L-lactide)], with predictable molecular weights and a narrow molecular weight distribution (Ð < 1.2), was achieved by sequential copolymerization in the presence of t-BuP2 and t-BuP1.
Highlights
Anionic polymerization has been known for more than sixty years
Möller et al described the synthesis of poly(ethylene oxide) (PEO) utilizing sec-BuLi and t-BuP4 [41], while Förster et al reported the in situ formation of sec-Bu− [(t-BuP4 )Li+ ] complex in polar solvent, which served as the initiator for the synthesis of phosphazene superbases (PBs)-b-PEO and PI-b-PEO [PB: polybutadiene and PI: polyisoprene] block copolymers [42]
The polymerization of styrene and 1,3-butadiene was performed in benzene, at a constant temperature of 25 ◦ C, using one or 0.5 equivalent of the corresponding phosphazene superbases (t-BuP4, t-BuP2 and t-BuP1 ) with respect to the alkyllithium initiator
Summary
Anionic polymerization has been known for more than sixty years. An early report on anionic polymerization is traced back in the late 1920s by Ziegler and Bähr [1], but the first clear evidence for “living” anionic polymerization was reported by Michael Szwarc in the 1950s [2]. Anionic polymerization of styrene, butadiene, and methyl of the initiator/chain-end significantly complexation with thecation counterion These compoundsby form exceptionally stable inclusion complexes, which cation), resulting in a rapid and controlled anionic polymerization [36]. Phosphazene the cation is completely surrounded by the ligand and hidden inside the molecular cavity This leads bases are commercially andin soluble non-polar and polar to a high increase of theavailable, interionicchemically distance inand thethermally ion pairs,stable resulting higherinpolymerization rates, solvents [hexane, toluene and tetrahydrofuran (THF)]. In this work, we aim at expanding the scope of using phosphazene bases as organic catalysts of the initiator/chain-end significantly by complexation with the counterion (e.g., proton or lithium through the investigation of and the anionic polymerization of styrene and in non-polar cation), resulting in a rapid controlled anionic polymerization [36].1,3-butadiene.
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