Cationic Tropidinyl Scandium Catalyst: A Perfectly
Acceptable Substitute for Cationic Half-Sandwich Scandium Catalysts
in <i>cis</i>-1,4-Polymerization of Isoprene and Copolymerization
with Norbornene
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Abstract
Different nonmetallocene rare earth
metal alkyl complexes such
as monotropidinyl (Trop) scandium dialkyl complex (Trop)Sc(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF) (<b>1</b>), ditropidinyl
yttrium alkyl complex (Trop)<sub>2</sub>Y(CH<sub>2</sub>SiMe<sub>3</sub>)(THF) (<b>3</b>) as well as binuclear lutetium alkyl complex
bearing one tetradentate dianionic 6-<i>N</i>-methyl-1,4-cycloheptadienyl
(NMCH) ligand [(NMCH)Lu(CH<sub>2</sub>SiMe<sub>3</sub>)(THF)]<sub>2</sub> (<b>2</b>) have been synthesized in high yields via
one-pot acid–base reaction by using of the tris(trimethylsilylmethyl)
rare earth metal complexes with the readily available natural product
tropidine. The polymerization experiments indicate that the monotropidinyl
scandium dialkyl complex <b>1</b> displays reactivity akin to
that of the analogous monocyclopentadienyl scandium dialkyl complexes.
In the presence of activator and a small amount of AlMe<sub>3</sub>, complex <b>1</b> exhibits similar activities (up to 1.6 ×
10<sup>6</sup> g mol<sub>Sc</sub><sup>–1</sup> h<sup>–1</sup>) but higher <i>cis</i>-1,4-selectivities (up to 100%)
than (C<sub>5</sub>H<sub>5</sub>)Sc(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF) (<i>cis</i>-1,4-selectivity as 95%)
in the isoprene polymerization, yielding the pure <i>cis</i>-1,4-PIPs with moderate molecular weights (<i>M</i><sub>n</sub> = 0.5–11.2 × 10<sup>4</sup> g/mol) and bimodal
molecular weight distributions (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> = 1.48–6.07). Moreover, the complex <b>1</b>/[Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>/Al<sup><i>i</i></sup>Bu<sub>3</sub> ternary system also
shows good comonomer incorporation ability in the copolymerization
of isoprene and norbornene similar to the [C<sub>5</sub>Me<sub>4</sub>(SiMe<sub>3</sub>)]Sc(η<sup>3</sup>-CH<sub>2</sub>CHCH<sub>2</sub>)<sub>2</sub>/activator binary system, affording the random
isoprene/norbornene copolymers with a wide range of isoprene contents
around 57–91 mol % containing <i>cis</i>-1,4 configuration
up to 88%
