Catalysis in Polymer Synthesis - American Chemical Society

Chapter 7

Migratory Nickel(0)—Phosphorane Catalyst α-Olefin Polymerization by 2,ω-Linkage

Downloaded by STANFORD UNIV GREEN LIBR on October 11, 2012 | http://pubs.acs.org Publication Date: June 22, 1992 | doi: 10.1021/bk-1992-0496.ch007

G. Fink, V. Möhring, A. Heinrichs, and Ch. Denger Max-Planck Institut für Kohlenforschung, Kaiser Wilhelm Platz 1, 4330 Mülheim, Ruhr, Germany

The paper deals with the novel 2,ω-linkage of α-olefins with the catalyst system nickel (0) compound / bis(trimethylsilyl)aminobis -(trimethylsilylimino)phosphorane. When linear α-olefins are poly merized, the polymer contains only methyl branches, regularly spaced along the chain with a seperation corresponding to the chain length of the monomer. The monomer insertion is regioselective; only C -->C coupling of the growing chain with the next monomer takes place. The nickel catalyst complex migrates along the polymer chain between two insertions. A kinetic model has been set up to describe the mutual superposition of the insertion reaction and the migration of the catalyst. In this model the migration steps are treated as preceding equilibria. The use of optically active monomers gives no reaction of the pure enantiomeres, whereas the racemic mixture is well polymerized. This reaction leads to a strongly alternating "copolymer" of the (R) and (S) enantiomeres with regularly spaced chiral centers in the main chain. Polymerization under high pressure increases the molecular weight from 6000 to 90000 g/mol. ω

2

Polymerization of ethylene with the homogeneous catalyst system nickel(0)compound / bis(trimemylsilyl)aminobis(trimemylsilylimino)phosphorane leads, accor ding to Keim et al. (I) to short chain branched polymers. We have found that this system polymerizes a-olefins: surprisingly the structure of the product is consistent not with the usual 1,2-coupling of the monomers to give a comb-like branched product (Figure 1, above), but with a 2,co-coupling (Figure 1, below)(2)(3). Examples of nickel(O) compounds (4) / aminobis(imino)phosphorane (5) catalysts are shown in Figure 2. The catalyst components - preferably in equimolar ratio - are employed in situ in the pure liquid monomer or in aromatic solvents. The nature of the nickel(O) compound has no influence on the structure of the α-olefin polymer. But only the phosphoranes shown in Figure 2 form an active catalyst. Even phosphoranes in which only one silicon atom is substituted by a carbon atom give an inactive catalyst (6). On the other hand, the kinetic behaviour differs much

0097-6156/92/0496-0088$06.00/0 © 1992 American Chemical Society

In Catalysis in Polymer Synthesis; Vandenberg, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

7. FINK ET AK

CH

(CH )

*

2

3

\

© > P o l

©

Θ ^

(£n)

Pol

©

CH @

®

®

3

(CHi)

® 1 , 2 - L i n k a g c


89

Migratory Nickel (0)-Phosphorane Catalyst

®

2,(û-Linkagc

(CHR) 0

1

3

( g ) >

©

Pol^@^(CHR) ©J^ /

R(3) = H;

R(>4)

P o 1

= H,Alkyl

Figure 1: Polymerization of α-olefins under 1,2- and 2,co-linkage

^N-SitCH^

2 =

' N-Si(CH ) 3

Bis(1,5 - Cyclooctadiene) Nickel

3

Phosphorane Ligand

(CO0) Ni 2

^N-SKCH^ ((CH ) Si) N-P^ 3

3

2

'N-Si(CH ) 3

( r , r , t - 1,5.9 - Cyclododecatriene)Nickel

3

Phosphorane Ligand

(CDT)Ni

^N-SiiCH^ ((CH ) Si) N-P^ 3

-.Ni-

(Cyclooctafetraene) Nickel

3

2

' N-Si(CH ) 3

3

Phosphorane Ligand

(COT) Ni

Figure 2: Nickel(O) / Phosphorane catalysts


90

CATALYSIS IN POLYMER SYNTHESIS

with the nickel(O) compound used (7). These findings suggest that the ratedeterming step in the formation of the active species is the release of the ligand from the nickel(O) compound.


It was proved that a 2,o>-linkage takes place by polymerizing 1-deutero-lhexen. All the methyl branches in this polymer are bearing one deuterium. How the 2,00-linkage takes place formally is demonstrated again in Figure 3. In this way, linear α-olefins and singly branched α-olefins can be polymerized, but not α-olefins with quaternary C-Atoms in the chain (in other words: one hydrogen atom at every carbon atom is necessary) or olefins with vinylene or vinylidene groups yield polymers. The structure of the formed poly-a-olefins is unusual. The polymer (see again Figure 1 or 3) contains only methyl branches, regularly spaced along the chain with a separation corresponding to the chain length of the monomer. Thus, in the polymer of a linear α-olefin with η ( - C H 2 - ) groups the distance between two methyl branches is (n+1) (-CH -) groups. Thus, their structure is well defined and can be predetermined by selection of the appropriate α-olefin. Thus, for example, the structure of the polymer synthesized from 1-pentene corresponds to that of a strongly alternating copolymer of ethylene and propylene (see Figure 3). 2

13

The structure was confirmed using C-NMR spectroscopy; this is demonstrated in Figure 4 for the example of poly-2,5-( 1-pentene). The assignment of the signals was carried out with the help of the increment rules of Lindemann and Adams (8). All the signals to be expected for 2,co-linked α-olefin polymers were found in the spectra in corresponding intensity ratios. The details of the chain start and a resulting irregularity therefore in the structure were discussed earlier (3)(7). Results and Discussion

Taking into account all the results, a scheme was developed with which the origin of the special structure of the poly-a-olefins can be explained. This scheme is shown in Figure 5 and its main points are: -

The monomer can only insert into a primary nickel-alkyl bond at the end of the growing chain;

-

the insertion is regioselective, only Ο - C 2 coupling of the growing chain with the next monomer takes place;

-

the nickel catalyst "migrates" along the polymer chain between two insertions. During this "migration" transfer reactions can occur, but not insertions.

ω

These processes are represented schematically in Figure 5 for the polymerization of 1-butene. Looking further on the transfer reaction from left to right in the scheme, we should note: if the transfer reaction occurs immediately after the insertion, then a vinylidene group will be formed. If the transfer reaction occurs during the migration of the nickel catalyst along the polymer chain, a vinylene group will be formed and finally, if the transfer reaction occurs at the end of the growing chain, a vinyl group will be formed. All these double bonds are


FINK ET AL.

Migratory Nickel (0)-Phosphorane Catalyst

2,(û-catalyst >

Pol^ 2


Figure 3:2,co-linkage of 1-pentene , CM,

cr

I /3 3

CM, ι

Λ/

Catalysis in Polymer Synthesis - American Chemical Society

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