.
Angewandte
Communications
ously reported the application of related iminopyridine-based
iron catalysts for 1,4-addition reactions across 1,3-dienes with
control of regio-, chemo-, and stereoselectivity through
appropriate substitution of the iminopyridine ligands.[19–21]
Iron-catalyzed polymerizations of isoprene have been
reported previously but typically do not afford the 1,4-
polyisoprene microstructure, and control over selectivity as
reported here has, to the best of our knowledge, not been
reported.[3,22–24]
The combination of the precatalyst 1 or 2, an alkylating
reagent (trialkylaluminum), a dealkylating reagent (Ph3C+
B(C6F5)4À, trityl BArF), and isoprene in an aprotic solvent
provided polyisoprene with molar masses greater than
105 gmolÀ1, and with high selectivity for either cis-1,4 or
trans-1,4 polyisoprene (Table 1; for kinetic profiles of the
polymerization, see the Supporting Information). Less than
5% conversion was observed in the absence of any of the
reaction components. The imine moiety of the iminopyridine
ligand controls the stereo- and regioselectivity of monomer
insertion on the active iron complex: The octyl-substituted
complex 1 yielded polymer with trans-1,4-polyisoprene con-
tent of up to 93% at 238C (Table 1). The supermesityl-
substituted complex 2 afforded polymer with cis-1,4-polyiso-
prene content up to 85% at À788C. While the catalysts
derived from both 1 and 2 can control the double-bond
geometry with > 99:1 selectivity, the selectivity for control of
1,4- versus 3,4- addition ranges from 2:1 to 12:1. However, the
3,4-addition motif results in the incorporation of pendant
terminal olefin side chains into the polymer, which can be
useful to modify the properties of the polymer (vide infra).
Treatment of iminopyridine ferrous chloride complexes
1 or 2 with triisobutylaluminum or triethylaluminum as the
alkylating reagent, presumably to replace the chloride
ligands, and trityl BArF20, to abstract one of the alkyl
groups, forms the active iron catalyst. Both 1 and 2 are
soluble in toluene and readily dispersed in heptane and
methylcyclohexane, which allows for a fast activation process.
The addition of trialkylaluminum to 1 or 2 in an apolar solvent
resulted in an immediate color change from bright orange (1)
and deep green (2) to brown-black. Subsequent addition of
trityl BArF20 as a solution (in toluene) or as a dispersion (in
alkanes) resulted in the formation of the active species,
a putative cationic FeII complex, to initiate polymerization.
The iron-catalyzed polymerizations proceed in alkanes with
boiling points that allow for both safe use on a large scale and
distillation from the polymer after polymerization.[25] The
molar masses of the resulting polymer of more than
105 gmolÀ1 and its controlled dispersity (D = 2–4) are appro-
priate for required tensile strength and elasticity.[26]
Polymers obtained from complexes 1 and 2 contain 7–8%
and 15% of the 3,4-insertion motif, respectively; the 1,2-
microstructure was not observed. The side-chain olefins
resulting from 3,4-insertion can increase the toughness of
synthetic rubber upon selective crosslinking, which can be
beneficial, for example to prevent abrasion of car tires.[26,27]
The crosslinked material could also find applications in high-
performance rubbers with wet-skid resistance and low-rolling
resistance tread.[26–28]
Table 1: Stereoselective isoprene polymerization.
Ferrous chloride complexes 1 and 2 are also suitable
precatalysts for the stereoselective polymerization of other
1,3-dienes, such as myrcene and farnesene (Scheme 2). In
these reactions catalysts 1 and 2 afford cis/trans and 1,4/3,4
ratios that are similar to those obtained for the polymeri-
zation of isoprene (Table 2). Both myrcene and farnesene are
available as mixtures of isomers (a and b isomers), but the
polymerization with 1 and 2 is chemoselective and isomer-
selective polymerization of the b isomers is possible. For
example, commercially available farnesene consists of a mix-
ture of different farnesene isomers (see 1H NMR spectrum in
Figure 1a) but polymerization selectively afforded poly-b-
farnesene (see Figure 1b). The other isomers could be
conveniently removed as monomers after polymerization.
The materials obtained from iron-catalyzed-polymerization
provide access to new elastomers, bearing pendant olefins,
from readily available materials.
The change in stereoselectivity from > 99:1 to < 1:99 for
the formation of trans- versus cis-polydiene is solely based on
the imine substituent of the otherwise identical catalysts and
not yet well understood. The complete lack of identified
intermediates in the catalytic cycle has complicated analysis
regarding the source of selectivity, and the selectivity-
determining step is currently unknown. Diene coordination
(s-cis or s-trans), migratory insertion into an h2- or h4-
coordinated diene, and s–p–s rearrangements of the iron–
allyl complexes may all be relevant for selectivity. We
discovered empirically that alkyl-substituted imines favor
Conditions
[Fe]/[Al]/
Polymer
Selectivity
1,4/3,4,
Yield
[%][c]
[Ph3C+]/[M] Mw/D[a]
trans/cis[b]
1/AliBu3
2 h, 238C
1:3:1:1000
1:3:1:5000
125000/2.0 12:1
>99
>99
>99:1
1/AliBu3
5 h, 238C
650000/3.9 12:1
>99:1
2/AlEt3
1 h, 238C 4 h, À788C 1:3:1:1000
150000/1.9 2:1; 1:>99 >99
140000/1.7 6:1; 1:>99 >99
1:3:1:1000
2/AlEt3
4 h, À788C
1:3:1:5000
800000/3.5 5:1
1:>99
>99
[a] Determined by size-exclusion chromatographic analysis in THF using
a refractive index detector and a UV detector (l=212 nm). [b] Deter-
mined by 1H and 13C NMR spectroscopy. [c] Determined gravimetrically.
Apolar solvent: toluene, heptane, or methylcyclohexane.
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 11805 –11808