Group 13 Chemistry - American Chemical Society

Chapter 10

Mono-and Dinuclear Olefin Polymerization at Aluminum Downloaded by COLUMBIA UNIV on September 18, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0822.ch010

1

2

Peter H. M. Budzelaar and Giovanni Talarico 1

Institute of Metal-Organic Chemistry, University of Nijmegen, The Netherlands (email: [email protected]) Dipartimento di Chimica, Università degli Studi di Napoli, Federico II, Italy (email: [email protected]) 2

Olefin polymerization is usually assumed to occur at a single metal atom, where the growing chain alternates between two adjacent coordination sites (modified Cossee mechanism). Using high-level ab initio and D F T calculations, we have compared this traditional mechanism with a dinuclear mechanism in which the chain alternates between neighbouring metal atoms. Surprisingly, insertion is not always more difficult than in a mononuclear system, suggesting that such a reaction might really occur. Moreover, the balance between chain transfer and propagation can be comparable to that for mononuclear systems, depending on the ligand environment. It appears that the possibility of such a dinuclear mechanism occurring in some of the known Al polymerization systems cannot be ruled out a priori.

© 2002 American Chemical Society

In Group 13 Chemistry; Shapiro, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

143

Introduction

Downloaded by COLUMBIA UNIV on September 18, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0822.ch010

General Coordination oligomerization and polymerization of olefins does not require a transition metal. Both reactions can also occur at e.g. aluminium. The Aufbau reaction ("living" oligomerization up to «100 monomers) has been known for a long time and is relatively well-understood (1). In contrast, the nature of the active species in most reported examples of polymerization at A l (2-7) is far less clear. This prompted us to consider the possibility that polymerization could occur between two metal centres of a dinuclear species, with switching of the chain at each insertion (8).

;AI

: Ά Ι ^ P

Τ ? ;A1^A1^

•AI

Surprisingly, calculations described here indicate that barriers for such a "new polymerization mechanism" (9) are not unduly high, suggesting that this possibility should also be taken into account when trying to interpret polymerization activity at A l .

Oligomerization at A l At high pressure and temperature, A l trialkyls undergo insertion of ethene to give longer-chain alkyls (up to «100 monomers). These can be converted into alcohols by air oxidation, or into long-chain olefins by chain transfer at low pressure and even higher temperature. Kinetic studies indicate a mechanism involving monomeric A l alkyls:

Vi R ô A f c ^ R a A l - ^ A l

R A1; L R 2

R

-R AK"\/ 2

R

etc


144

Since the monomer-dimer equilibrium is much faster than propagation, all chains grow at the same rate (Poisson distribution of chain lengths).

Polymerization at A l


RsAl Polymerization with aluminium trialkyls has been described by Martin and Bretinger; they proposed the active species to be monomeric R A1 (2). Under the conditions used, R exchange between R A1 species is rapid, which is hard to reconcile with their observation that only 0.004 mol% of the A l is active. The authors claimed that, with the precautions taken, contamination by transition metals was very unlikely. This suggests that some other A l species, not participating in R exchange, is responsible for the polymerization. 3

3

+

(amidinate)AlR

This was originally thought to be the active species in the (amidinate)AlMe /B(C F5)3 system. Later work showed the system to be more complicated (3). Calculations indicated that monomeric (amidinate)AlR should not polymerize because chain transfer is much easier than propagation (10). 2

6

+

+

(Pyridyl-imine-amide)AlR

Gibson has used the addition of trialkylaluminium to pyridine-diimine ligands to generate dialkylaluminium complexes of anionic pyridine-imineamide ligands. Reaction of these complexes was reported to produce cationic aluminium alkyls capable of polymerizing ethene (to rather l o w - M polymers) (7). This is remarkable, considering the fact that - assuming ^-coordination of the ligand - the cation is formally coordinatively saturated. However, the reported MJM values (2.9-6.3) clearly show that this is not a well-defined single-site catalyst. w

n


145 ΜΑΟ ΜΑΟ, in the absence of transition metal catalyst precursors, is also frequently found to produce some polymer (though with much lower rates than the usual T M catalysts) (6).


Methods Geometries of all complexes were optimized at the B 3 L Y P level using 6-31G(d) basis set. Improved single-point energies were then calculated with the MP2/6-311G(d,p) basis. For several systems, CCSD(T) calculations were also performed, but these did not make a significant difference. D F T (all fiinctionals) appears to underestimate the barriers for chain transfer to monomer at A l (10), therefore we do not report any D F T values here. A l l calculations were carried with the G A M E S S - U K (11) and GAUSSIAN-98 programs (12). A l l geometries were optimized as local minima or saddle points without any symmetry restrictions, and the nature of each stationary point was checked by a frequency calculation. The reported energies do not include any zero-point energy or thermal corrections, nor are they corrected for BSSE.

Results and Discussion

Propagation Table I compares propagation barriers for a number of mono- and dinuclear Al-ethyl species. O f the mononuclear species, M e A l E t has the lowest propagation barrier. Introduction of electron-withdrawing substituents or switching to a cationic species increases the olefin complexation energy and the insertion barrier. The barriers are much higher than those typical for transitionmetal polymerization catalysts, but they are not so high that they exclude the possibility of aluminium-catalyzed olefin polymerization. Transition-metal catalysts typically have very early insertion transition states, with short C=C bond lengths and relatively long metal-olefin distances, as expected for exothermic reactions with low barriers. The A l transition states, in contrast, are more central, with nearly equal lengths for the A l - C bonds being formed and broken, and a rather long C=C bond. This is e.g. illustrated by the insertion transition state for M e A l E t in Figure 1. 2

2


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Table I. Propagation barriers (ethene insertion into AI-Et bond) (kcal/mol). Mononuclear Me Al-Et F A1-Et (Me)(NH )Al-Et 2

2

+

3

1 Ν

21.9 29.1 24.0 33.0

Dinuclear (μ-ΟΗ)(Α1Η ) -Εί (μ-ΟΗ)(Α1Με ) -Εί ^-F)(AlMe )rEt (μ-ΝΗ)[Α1Μβ(ΝΗ )] -Εί

28.9

(μ-ΟΗ) (Α1Η)2-Εί

2

α

2

2

2

2

3

Ε 26.4 26.6 26.4 25.2

+

2


Ν 26.4

+

2

Group 13 Chemistry - American Chemical Society

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