Mechanism of Oxirane Coordination Polymerization of Soluble

regular structure, in agreement with the fact that the complexes could not be ... not only does not compete with tBO coordination, but decreases by a ...
0 downloads 0 Views 825KB Size
Download PDF
Chapter 11 Mechanism of Oxirane Coordination Polymerization of Soluble Polynuclear μ-Oxometal Alkoxide Aggregates 1

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.ch011

Ph. Condé, L. Hocks, Ph. Teyssié , and R. Warin Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège, B6, Sart-Tilman, 4000 Liège, Belgium

A detailed NMR study of the behaviour of soluble polynuclear μ-oxometalalkoxides has been performed in view of their high activity and stereoselectivity in oxirane polymerization. The results indicate a rather rigid, globular coordination aggregate structure ("tecto-alkoxides"), undergoing very slow exchanges. When correlated with other kinetic data, they support a general insertion-coordination mechanism, producing lower molecular weight atactic chains on the outermost "freer" Al atoms, and a stereoregular high polymer at higher rate on a few hindered sites located deeper in the aggregate. The design o f c a t a l y s t s d i s p l a y i n g a very high a c t i v i t y f o r the s t e r e o s e l e c t i v e p o l y m e r i z a t i o n o f oxiranes t o high molecular weight polyethers often r e l i e s upon a c t i v a t i o n o f otherwise i n a c t i v e metal d e r i v a t i v e s , by r e a c t i o n with compounds able t o promote t h e formation of Α-bridged complexes (1-3) and i n p a r t i c u l a r /t-oxo ones obtained by c o n t r o l l e d h y d r o l y s i s o r a l c o h o l y s i s of a l k y l - z i n c or aluminum d e r i v a t i v e s . That very fact, together with d i f f e r e n t k i n e t i c and s t r u c t u r a l data, prompted Ε. Vandenberg t o express the hypothesis t h a t such a propagation should take p l a c e by a c o o r d i n a t i v e f l i p - f l o p mechanism (see Scheme 1) on s p e c i f i c s i t e s o f a p o l y n u c l e a r aggregate (4). Along the same l i n e s o f thought, w e l l - d e f i n e d i n i t i a t o r s were synthesized, i . e . polynuclear b i m e t a l l i c JU oxoalkoxides [ ( R O ) A l - 0 - Z n - 0 - A l ( O R ) ] with a mean degree of c o o r d i n a t i v e a s s o c i a t i o n η v a r y i n g from 2 t o 8 (5-7) as usual i n most a l k o x i d e s . They proved t o rank among the most a c t i v e c a t a l y s t s ( t y p i c a l h a l f - p o l y m e r i z a t i o n time 2

1

2

n

Corresponding author

0097-6156/92/0496-0149S06.00/0 © 1992 American Chemical Society

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

150

CATALYSIS IN POLYMER SYNTHESIS

M-X

M . . . . X — M

. . . . M

I

ο

\

\

R

IV

ΐ

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.ch011

ο

where X =0

M . . . .X .

M-X

0 \

0 II

S, N .

: ο—

V c· • III

i

R

Scheme 1. C o o r d i n a t i v e I n s e r t i o n Mechanism f o r Oxiranes Polymerization of 5 minutes a t 50° C i n 1 M s o l u t i o n s f o r R = n-butyl) f o r the p o l y m e r i z a t i o n o f methyloxirane (MO) i n t o a high M.W. (up t o 10 ) , p a r t i a l l y i s o t a c t i c (from 4 t o 80 %) p o l y e t h e r (8) (PMO). C o n s i d e r i n g t h e i r w e l l - c o n t r o l l e d s t r u c t u r e and t h e i r p e r f e c t s o l u b i l i t y i n s e v e r a l types of o r g a n i c media, i t appeared worthwhile t o perform a d e t a i l e d s t r u c t u r a l study o f t h a t f a m i l y o f compounds (9) . T h i s paper w i l l summarize the data obtained d u r i n g a c u r r e n t NMR study, and w i l l concentrate on t h e i r r e l a t i o n with other experimental data as w e l l as t h e i r relevance t o the aggregate c o o r d i n a t i o n mechanism o u t l i n e d above. 6

Experimental The complex a l k o x i d e s were s y n t h e s i z e d (5,8,10) in decalin under nitrogen atmosphere by a direct condensation between zinc acetate and aluminum isopropoxide i n a 1 t o 2 molar r a t i o , a l l reagents being c a r e f u l l y d r i e d before use. Temperature was p r o g r e s s i v e l y raised up to boiling point and the evolved i s o p r o p y l a c e t a t e c o n t i n u o u s l y d i s t i l l e d o f f i n order t o completely d i s p l a c e the e q u i l i b r i u m . The end o f the reaction was monitored by g . l . c . (disappearance o f i s o p r o p y l a c e t a t e ) . D e c a l i n was then r e p l a c e d by heptane or aromatics through d i s t i l l a t i o n - r e d i s s o l u t i o n c y c l e s . The g l a s s y d r y r e s i d u e was t o t a l l y s o l u b l e i n a l l o f these media, yielding yellowish solutions. Complex a l k o x i d e s c o n t a i n i n g d i f f e r e n t R groups were obtained by q u a n t i t a t i v e l y exchanging the i s o p r o p y l d e r i v a t i v e with another a l c o h o l i n excess, i . e . η-butyl, c o n t i n u o u s l y d i s p l a c i n g t h e e q u i l i b r i u m by d i s t i l l a t i o n and monitoring the course o f t h e r e a c t i o n by g . l . c . A l l o f t h e reagents and solvents had t o be thoroughly dried, using c o n v e n t i o n a l methods. In Catalysis in Polymer Synthesis; Vandenberg, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

11.

151

Mechanism of Oxirane Coordination

CONDE ET AL.

The composition o f the complexes obtained was q u a n t i t a t i ­ vely characterized by t h e Zn/Al c o n t e n t - r a t i o , as measured by complexometry, and t h e OR/ΑΙ c o n t e n t - r a t i o obtained from g . l . c . o f a l c o h o l s a f t e r h y d r o l y s i s . Both f i t t e d t h e expected v a l u e s as w e l l as elemental a n a l y s i s data. The mean degree o f c o o r d i n a t i v e a s s o c i a t i o n o f these a l k o x i d e s was determined by c r y o s c o p i c measurements i n benzene o r cyclohexane, with a p r e c i s i o n o f ± 2\^%. NMR s p e c t r a were recorded from -70 up t o 125°C, mainly on s o l u t i o n s i n dg-toluene, u s i n g TMS o r HMDS as a r e f e r e n c e u s i n g a C22,63 MHZ Brucker 90 spectrometer equipped with a v a r i a b l e temperature c o n t r o l . Before s e a l i n g , t h e NMR tubes were prepared by i n t r o d u c i n g , through rubber septums, t h e complex and monomer solutions under r i g o r o u s l y dry n i t r o g e n atmosphere, u s i n g s t a i n l e s s s t e e l c a p i l l a r i e s and s y r i n g e s .

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.ch011

13

R e s u l t s and D i s c u s s i o n S t r u c t u r e o f the complex aggregate. Although some o f t h e complex a l k o x i d e s belonging t o t h i s f a m i l y e x h i b i t r a t h e r simple C-NMR s p e c t r a with narrow resonance bands (8) when they a r e not (or l i t t l e ) a s s o c i a t e d , t h e s p e c t r a o f the ( R O ) ^ A l 0 Z n complexes a r e very d i f f e r e n t , both when R i s η-butyl o r i s o - p r o p y l (n i n CgH^ being 6 o r 4 r e s p e c t i v e l y , and a l s o somewhat dependent on t h e complex c o n c e n t r a t i o n ) . In these cases, one s t i l l observes two different resonances corresponding t o c o o r d i n a t i v e l y bridged and " f r e e " OR groups, i n a r a t i o o f about 1 t o 3, but these bands are broad, p a r t i a l l y o v e r l a p p i n g , and a r e not w e l l r e s o l v e d ever when corresponding t o " f r e e " OR s (see Table I) . In f a c t , they appear as non-symmetrical envelopes, i n c l u d i n g a c e r t a i n number o f badly r e s o l v e d bands 13

2

2

1

1 3

Table I : Mean Values f o r C Chemical S h i f t s i n B i m e t a l l i c ytroxo A l k o x i d e s (d -Toluene, v s . TMS, 25°C) 8

(n-BuO) A l 0 Z n 4

2

OC H

2

bridged

free

t "bridged"

free

66.4 (13)

64.0

38.4

35.7

(iso-PrQ) A l 0 Z n 4

2

C H γ 2

0

•

19.6

OCH

2

/

bridged 67.7

0

CH

3

14.5

CHI*

/ V II

free"

63.9 ; 65.2

bridged

s free

28.7

27.9

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

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.ch011

152

CATALYSIS IN POLYMER SYNTHESIS

corresponding t o an equal number of d i f f e r e n t s i t u a t i o n s of a given nucleus i n the aggregate, t h a t number i t s e l f depending on the value of n. Such a s i t u a t i o n o b v i o u s l y a r i s e s from a c o n s i d e r a b l e " r i g i d i t y " of the aggregate, as confirmed by the f a c t t h a t on the NMR t i m e - s c a l e , no r a p i d exchange of OR groups was ever observed when changing temperature (up t o 125°C) or when adding monomer or l i g a n d s ; a n o t i c e a b l e exception being a l c o h o l s which are known t o e x t e n s i v e l y d i s s o c i a t e these aggregates and indeed promote a b e t t e r r e s o l u t i o n of the s p e c t r a . That r i g i d i t y i s s t r i k i n g l y i l l u s t r a t e d by the broadness of the band corresponding t o the β -carbon of the n-butyl group, i n d i c a t i n g t h a t only the extremity of these a l k y l groups i s f r e e from the i n f l u e n c e of the c o o r d i n a t i v e architecture of the complex aggregate. Also, no i n d i c a t i o n ever appeared of a scrambling process y i e l d i n g ZnOR groups. These aggregates most probably d i s p l a y a more or l e s s g l o b u l a r shape, s i n c e η values are u s u a l l y moderate (< 8) and c l o s e t o i n t e g e r s (whatever the nature of the R groups and the metals); t h a t a l s o e x p l a i n s t h e i r amazing s o l u b i l i t y ( a c t u a l l y , a t o t a l m i s c i b i l i t y when R = n-Bu) i n hydrocarbon s o l v e n t s , due t o the f a c t t h a t a "mineral" core of metal ions l i n k e d b y A - o x o - b r i d g e s i s surrounded by a continuous l a y e r of h i g h l y l i p o p h i l i c a l k y l groups (the corresponding methyl and e t h y l d e r i v a t i v e s being indeed i n s o l u b l e ) . Once b u i l t up from random c o o r d i n a t i v e i n t e r a c t i o n s , they become r a t h e r r i g i d except f o r the carbons a t the top of t h a t e x t e r n a l l i p o p h i l i c l a y e r . The r i g i d i t y might be explained by analogy (necessarily l i m i t e d ) with the s t r u c t u r e of s p i n e l l e s , such as i n Gahnite, where z i n c atoms are i n a t e t r a h e d r a l and aluminum atoms i n an octahedral arrangement. In our case however, exchange of OR groups, although very slow, must e x i s t , so preventing the establishment of a p e r f e c t l y r e g u l a r s t r u c t u r e , i n agreement with the f a c t t h a t the complexes could not be crystallized. Supported by s t r u c t u r a l s t u d i e s (11) of an a l k y l z i n c a l k o x i d e tetramer d i s p l a y i n g a cubane-like s t r u c t u r e , a r e p r e s e n t a t i o n of these aggregates should thus imply a r i g i d core of oxozinc groupings surrounded by oxo-aluminum alkoxide e n t i t i e s l o c a t e d i n d i f f e r e n t c o o r d i n a t i o n environments, as t e n t a t i v e l y sketched i n Figure 1. I t i s worthwhile t o mention here t h a t the hi NMR spectra also exhibit broad, complex resonances suggesting a t l e a s t 2 d i f f e r e n t environments f o r the A l atoms (η-butyl d e r i v a t i v e ) , i . e . a dominant 5-coordinated one (ca. 45 ppm) and a 6coordinated one (ca. 10 ppm) i n a r a t i o of roughly 5 t o 1 (12) . The e x i s t e n c e of such a core i n which e l e c t r o n d e l o c a l i z a t i o n i s p o s s i b l e i s a l s o supported by the f a c t t h a t the corresponding (nBuO) ^^O^Mo"" s o l u t i o n s e x h i b i t a very dark-brown c o l o r and absorbs throughout the v i s i b l e s p e c t r a and c l o s e t o the f a r i n f r a r e d . In summary, these e n t i t i e s f o r which we coined the general denomination of " t e c t o - a l k o x i d e s " (by analogy 2 7

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

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.ch011

11.

CONDE ET AK

153

Mechanism of Oxirane Coordination

ι

R F i g u r e 1. Schematic Representation o f the Main Features of an ( R O ) A l 0 Z n Aggregate (n = 8) 4

2

2

with tacto-silicates), may behave as soluble "heterogenous" c a t a l y s t s , the l i p o p h i l i c "surface" of which may offer particular sites of different configurations, displaying specific activation p r o p e r t i e s . In p a r t i c u l a r , i t might very w e l l e x p l a i n the experimental fact that, despite t h e i r unambiguously defined structure, they produce in simultaneous competitive r e a c t i o n s and by t h e same i n s e r t i o n mechanism i n t o t h e A1-0R bond, l i n e a r random oligomers (D.P. c a . 6 to 24), a t a c t i c PMO (M above 10 ) and p a r t i a l l y isotactic PMO (M ca. 1 0 ) , t h e i r variable ratio depending on t h e a c t u a l s t e r i c environment o f these A1-0R bonds (R being an i n i t i a l a l k y l group o r a growing chain). 5

n

6

n

Fate o f the monomer. In order t o check t h e c o o r d i n a t i v e nature o f t h e process and p o s s i b l y the in-situ environment o f the monomer, an attempt was made t o a l s o observe these phenomena by C-NMR spectroscopy. Since with most monomers the s t a t i o n a r y c o n c e n t r a t i o n o f the c o o r d i n a t e d monomer i s u s u a l l y very low versus free monomer and growing chains (due t o f a s t p o l y m e r i z a t i o n ) , an extremely "slow" s u b s t r a t e was s e l e c t e d f o r t h a t purpose, i . e . t e r t - b u t y l o x i r a n e (tBO), and i t was indeed checked t h a t no s i g n i f i c a n t amount o f polymer was ever formed under t h e c o n d i t i o n s o f an NMR r u n . Table I I summarizes t h e observed C chemical s h i f t s of t h e "monomer" when adding tBO t o (nBuO) A l Z n 0 i n base-mole r a t i o s from 0 t o 2. A number o f r a t h e r s t r i k i n g o b s e r v a t i o n s emerge from an a n a l y s i s o f these experiments: 13

1 3

4

2

2

1) t h e c o o r d i n a t i o n o f the oxirane on t h e complex i s extremely s t r o n g . Up t o a 1:1 molar r a t i o o f oxirane In Catalysis in Polymer Synthesis; Vandenberg, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

154

in

CATALYSIS IN POLYMER SYNTHESIS T a b l e II : " c C h e m i c a l S h i f t s (ppm) of tBO, Alone or the Presence of ( n B u O ) A l 0 Z n (in d - T o l u e n e at 25°C, ref. HMDS) 4

2

8

tBO

CH

CH

alone

59.6 77.7

1 t o 2 equiv. 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.ch011

2

per Zn