Synthesis of polyolefin (PO)-based block copolymers is of immense research interest. In this work, we report a strategy for the construction of polystyrene (PS)-block-PO-block-PS, a useful thermoplastic elastomer, directly from olefin and styrene monomers. Multinuclear zinc species Et[Zn(CH2)6] a ZnEt were prepared through successive additions of BH3 and Et2Zn to 1,5-hexadiene. Poly(ethylene-co-propylene) chains were biaxially grown from the −(CH2)6– units in Et[Zn(CH2)6] a ZnEt via “coordinative chain transfer polymerization (CCTP)” using the pyridylaminohafnium catalyst. PS chains were subsequently grown in one pot from the generated polymeryl–Zn sites by subsequent introduction of the anionic initiator Me3SiCH2Li·(pmdeta) (pmdeta, pentamethyldiethylenetriamine) and styrene monomers. The fraction of the extracted PS homopolymer grown from the Me3SiCH2 sites was low (homo-PS (g)/total PS (g), 15–22%). The gel permeation chromatography (GPC) curves shifted evidently after styrene polymerization, and change in the molecular weight (ΔM n, 39–56 kDa) was approximately twice the homo-PS M n (20–23 kDa), in accordance with attachment of the PS chains at both ends of the PO chains. Transmission electron microscopy analysis of the thin films showed segregation of the PS domains in the PO matrix to form spherical or wormlike rippled structures depending on the PS content. The prepared triblock copolymers exhibited elastomeric properties in the cyclic tensile test, similar to the commercial PS-block-poly(ethylene-co-1-butene)-block-PS.
Two representative bulk polymers, polyolefin (PO) and polystyrene (PS), were molecularly connected to form POblock-PS by sequentially performing coordination and anionic polymerizations in one pot. Ethylene/1-octene copolymerization was performed using a typical ansa-metallocene catalyst, rac-[Me 2 Si(2-methylindenyl)] 2 ZrCl 2 , in the presence of (benzyl) 2 zinc to grow PO-chains at the Zn site. Anionic styrene polymerization was subsequently performed using nBuLiÁ(tmeda) (tmeda, N,N,N 0 ,N 0 -tetramethylethylenediamine) initiator to consecutively grow PS-chains at the Zn site. The composition and the molecular weight of the PO-blocks were controllable depend-ing on the feed amounts of 1-octene and (benzyl) 2 zinc (1octene fraction: $20, $40 wt %; PO-M w , 77,000-174,000) and the PS-block size was also controlled (PS-M n , $21,000) with the complete conversion of the styrene monomer. Formation of block copolymers was evident in the GPC curves, TEM images, and strain-stress curves.
The polyolefin and polystyrene chains were successively grown from [4-(isopropenyl)benzyl]2Zn to form polystyrene-b-polyolefin-b-polystyrene units.
Triblock copolymers of polystyrene (PS) and a polyolefin (PO), e.g., PS-block-poly(ethyleneco-1-butene)-block-PS (SEBS), are attractive materials for use as thermoplastic elastomers and are produced commercially by a two-step process that involves the costly hydrogenation of PS-block-polybutadiene-block-PS. We herein report a one-pot strategy for attaching PS chains to both ends of PO chains to construct PS-block-PO-block-PS directly from olefin and styrene monomers. Dialkylzinc compound containing styrene moieties ((CH 2 =CHC 6 H 4 CH 2 CH 2 ) 2 Zn) was prepared, from which poly(ethylene-co-propylene) chains were grown via "coordinative chain transfer polymerization" using the pyridylaminohafnium catalyst to afford di-end functional PO chains functionalized with styrene and Zn moieties. Subsequently, PS chains were attached at both ends of the PO chains by introduction of styrene monomers in addition to the anionic initiator Me 3 SiCH 2 Li·(pmdeta) (pmdeta = pentamethyldiethylenetriamine). We found that the fraction of the extracted PS homopolymer was low (~20%) and that molecular weights were evidently increased after the styrene polymerization (∆M n = 27-54 kDa). Transmission electron microscopy showed spherical and wormlike PS domains measuring several tens of nm segregated within the PO matrix. Optimal tensile properties were observed for the sample containing a propylene mole fraction of 0.25 and a styrene content of 33%. Finally, in the cyclic tensile test, the prepared copolymers exhibited thermoplastic elastomeric properties with no breakage up over 10 cycles, which is comparable to the behavior of commercial-grade SEBS.
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