Synthesis of ethylene–styrene copolymer containing syndiotactic polystyrene sequence by trivalent titanium catalyst

Abstract

We describe the synthesis of a styrene-ethylene copolymer using a trivalent titanium-based polymerization catalyst system, tris(acetylacetonate)titanium (Ti(acac) 3 ), combined with triisobutylaluminum-modified methylaluminoxane. Gel-permeation chromatography measurement revealed that copolymerization using the above-mentioned catalyst system yielded a mixture of two different polymers. 13 C nuclear magnetic resonance (NMR) analysis of the Soxhlet-extracted fractions indicated that the insoluble part contained a long ethylene-ethylene sequence and an isolated styrene unit, whereas the soluble part contained a syndiotactic styrene-styrene sequence with ethylene units adjacent to continuous styrene units. The ratio of the styrene-styrene sequence to the styrene-ethylene joint part of the Soxhlet-soluble fraction, estimated from the NMR resonances, increased with the styrene content. The melting temperature of the Soxhlet-soluble fraction also increased with the (styrene-styrene)/ (styrene-ethylene) ratio of the polymers. These two results together indicate that a block-like copolymer was produced that contained long syndiotactic polystyrene portions separated by ethylene units.

Synthesis and Characterization of Syndiotactic Polystyrene-Polyethylene Block Copolymer

“…The key to this achievement is living polyinsertion, which can be obtained even with traditional catalysts by working at a low temperature. A similar result was reported by Chu et al by copolymerizing ethylene and styrene at 50 °C to obtain block copolymer with syndiotactic styrene sequences and short polyethylene segments, while other researchers succeeded in the synthesis of ethylene-styrene copolymers with syndiotactic styrene blocks and minor polyethylene segments with different catalysts [13,25,26].…”

“…The diagnostic signals for isolated styrene units (E-S-E; δ = 44.3, 35.0 and 25.7 ppm) [33,37] or isolated ethylene units (S-E-S; δ = 41.6, 35.5 and 25.1 ppm), as well as those for alternating styrene-ethylene units (44.3, 34.7, 23.4 ppm) [33,36], were not detected. 1 H NMR chemical shifts of S α and T β signals, for the S-E polymer junction, were identified from 1 H- 1 H COSY and 13 C- 1 H HSQC cross-correlations, evidenced, respectively, in Figures S2 and S3, starting from 13 C NMR signals previously reported [26]. The low molecular weight of the synthesized copolymer also allowed the identification of the NMR signals for the polymer end-groups.…”

“…The hexane insoluble-toluene soluble fraction, ascribed to the sPS- b -PE copolymer, was analyzed by 1 H, 13 C (Figure 1), two-dimensional 1 H- 1 H COSY (Figure S2), and 13 C- 1 H HSQC (Figure S3) NMR spectroscopy, who provided clear evidence for the formation of the diblock architecture. 13 C NMR analysis of this fraction revealed that the polymer consisted of styrene repeating units with predominant syndio tacticity (diagnostic signal for rrmrr , mrrrr , rrrrr, and rmrrr hexades, respectively, at 45.44, 44.61, 43.85, and 42.70 ppm) [32] and linear ethylene repeating units (29.47 ppm), connected by a single ( vide infra ) S-E hetero-sequence (S α at 37.29 ppm; T β at 43.49 ppm; see Figure 1 and Figures S2 and S3) [13,21,23,26,33,34,35]. Resonances centered at 31.68 and 33.43 ppm, associated with regioirregular insertions of S in the PS segment, are also observed to a minor extent [22,36].…”

“…The catalyst we chose was cyclopentadienyltitanium(IV) trichloride (CpTiCl 3 ) activated by modified methylalumoxane (MMAO), one of the more active catalysts for the syndiotactic poly-insertion of the styrene [30]. The same catalyst works in ethylene styrene copolymerization, producing a pseudorandom copolymer in mixture with syndiotactic styrene homopolymer [21] at 50 °C, and was also employed in references [25,26].…”

Engineering of Syndiotactic and Isotactic Polystyrene-Based Copolymers via Stereoselective Catalytic Polymerization

“…The origin of this selectivity was not clearly established but the simultaneous presence of both oxo- and pinacolo-bridges in the ligand is suspected by the authors to be part of the answer. Later on, Naga et al reported styrene-ethylene copolymerizations performed using trivalent Ti(acac) 3 18 in combination with TIBAL and MMAO [ 34 ]. The syndiotacticity level was also not quantified and activities of the catalytic system were quite low.…”