Metal-Alcoholate Initiators - American Chemical Society

7 Metal-Alcoholate Initiators

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 14, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch007

Sources of Questions and Answers in Ring-Opening Polymerization of Heterocyclic Monomers P. TEYSSIÉ, J. P. BIOUL, P. C O N D É , J . DRUET, J . H E U S C H E N , R. J É R Ô M E , Τ. O U H A D I , and R. WARIN Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège, Sart Tilman, 4000 Liège-Belgium

Soluble μ-oxo-bimetallic trinuclear alkoxides rank among the best initiators known for oxiranes polymerization into high M.W. polyethers. After an i n i t i a l rearrangement, coordination propagation proceeds on three types of centers: non-selective ones, producing oligomers of spe cific lenghts, and other ones generating high M.W. chains either atactic, or stereoregular (up to 80 % isotactic). The relative impor tance of these three centers, working through similar but competitive pathways, can be deter mined to a large extent by modifying the struc ture of the aggregates, implying "topochemical control" in solution. These features are tentatively related to a detailed H, C and Al NMR study, demonstrating the rigidity of these aggregates which offer different coor dination sites for the monomer. The initiators also polymerize most lactones, the perfectly living character of the process allowing inte resting block copolymerizations. Their acti vity towards cumulenes, i.e. isocyanates and CO , also leads to valuable reactions. 1

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In agreement with a former proposal by E. Vandenbergh (1) and with the mode o f p r e p a r a t i o n o f d i f f e r e n t a c t i v e systems, polynuclear s t r u c t u r e s are c l e a r l y a key feature f o r the design o f c a t a l y s t s able to polymerize s u b s t i t u t e d oxiranes i n t o very high mo l e c u l a r weight p o l y e t h e r s . In an e x p l o r a t o r y approach based on these premises and aimed a t the s y n t h e s i s o f w e l l c h a r a c t e r i z e d and v e r s a t i l e specie o f that type, we have shown indeed (2) that s o l u b l e μ-οχο-bimetallic t r i n u c l e a r alkoxides having the general formula £RO) ni-O-MÏ-O-H (0R)^j_ rank among the best known 1

0097-6156/ 85/ 0286-O097S06.00/ 0 © 1985 American Chemical Society

McGrath; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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i n i t i a t o r s f o r oxiranes p o l y m e r i z a t i o n , y i e l d i n g products having molecular weights i n the 10°range and d i s p l a y i n g i n some cases an amazing degree of s t e r e o r e g u l a r i t y d e s p i tes the p e r f e c t s o l u b i l i t y of the i n i t i a t o r and the p o l y mer. These compounds, p a r t i c u l a r l y those where M =A1 and M= Zn, have proven to be very i n t e r e s t i n g models, a l l o w i n g a b e t t e r understanding of s e v e r a l determinant phenomena i n r i n g opening p o l y m e r i z a t i o n , as w e l l as i n other areas of chemistry(3). I t i s the purpose of t h i s paper to sum marize the c u r r e n t status of the f i e l d , and to i n d i c a t e i t s p o t e n t i a l developments .(see a l s o r e f . 4b).


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Synthesis and p r o p e r t i e s of a c t i v e y - o x o - b i m e t a l l i c alkoxides - Preparation : The simplest method to o b t a i n a s t a b l e poîynucïear"system, c o n t a i n i n g the metal atoms e x p e r i mentally recognized to give a c t i v e systems f o r the high M.W. p o l y m e r i z a t i o n of oxiranes, i s a thermal condensa t i o n process between metal alkoxides and carboxylates, i.e.

Conveniently c a r r i e d out i n a hydrocarbon solvent l i k e d e c a l i n e , t h i s r e a c t i o n i s c h a r a c t e r i z e d by a r a p i d f i r s t step, followed by a more d i f f i c u l t e l i m i n a t i o n (at 200°C) of the second e s t e r molecule; i t i s important to d r i v e the e q u i l i b r i u m to completion by d i s t i l l a t i o n , and to avoid so c o o r d i n a t i o n of by products to the c a t a l y s t . I t must a l s o be r e a l i z e d t h a t too high temperatures may f u r t h e r promote formation of Al-O-Al bonds upon e l i m i n a t i o n of ether (or a l c o h o l + o l e f i n ) , a d e t r i m e n t a l p r o cess i n terms of s o l u b i l i t y and a c t i v i t y . Most of the elements of the p e r i o d i c t a b l e can be used i n t h a t type of condensation r e a c t i o n ; a d d i t i o n a l v e r s a t i l i t y i s o f f e r e d by the p o s s i b i l i t y to exchange OR groups q u a n t i t a t i v e l y (again by d i s p l a c i n g e q u i l i b r i u m through d i s t i l l a t i o n ) . Most of the g l a s s y compounds r e s u l t i n g from these r e a c t i o n s have a composition f i t t i n g the above formula, based on elemental and f u n c t i o n a l a n a l y s i s as w e l l as on mass spectrometry. - Characteristic_structural : A more d e t a i l e d anâïysïs~ôf"thësë"prôdûcts~û measurements i n c o n j u n c t i o n with H, C and A 1 NMR spectroscopy has revealed a number of i n t e r e s t i n g s t r u c t u r a l charac t e r i s t i c s (15) :they are present i n s o l u t i o n as aggregates l

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probably of a g l o b u l a r and thermodynamically favored


type

7.

TEYSSIÉ E T A L .

of s t r u c t u r e ( u s u a l l y low and i n t e g e r a s s o c i a t i o n number n, from c r y o s c o p i c determinations). The NMR spectra i n d i c a t e of course the presence o f bridged and non-bridged (free) OR groups, and o f A l atoms with a t l e a s t two d i f f e r e n t c o o r d i n a t i o n environments, while e l e c t r o n i c spectra ( v i s i b l e range), i n d i c a t e d i f f e r e n t types o f c o o r d i n a t i o n geometries f o r t r a n s i t i o n metals (blue o r r e d c o b a l t d e r i v a t i v e s ) depending on the nature of the R group (normal or branched). The most i m p o r t a n t s t r u c t u r a l i n d i c a t i o n though i s obtained ( ) from 1 C NMR spectra o f Al ZnOj(On.Bul which r e v e a l an unexpected r i g i d i t y i n these aggregates ? the spectra d i s p l a y indeed broad d i f f u s e resonances not only f o r the α but a l s o the & carbon of the a l k y l groups, r a t h e r u n s e n s i t i v e t o s h i f t reagents, d i l u t i o n and tem perature; more narrow bonds (with s l i g h t shift)appear only upon a d d i t i o n o f a l c o h o l s (known t o d i s s o c i a t e the aggre gates) , oxiranes o r very p o l a r s o l v e n t s . These f e a t u r e s o b v i o u s l y imply a very r i g i d core f o r the aggregate i n s o l u t i o n , and a number o f s i m i l a r but s l i g h t l y non-equi v a l e n t c o o r d i n a t i o n s i t u a t i o n s : the name "tecto-compLexes" i s a c c o r d i n g l y proposed f o r t h i s c l a s s o f compounds, which a l s o e x h i b i t a p a r t i c u l a r e l e c t r o n i c behaviour as put i n evidence by e.p.r. and v i s i b l e -I.R. s p e c t r a . Although t h i s complex s i t u a t i o n probably j u s t i f i e s the amorphous g l a s s y character o f these products, a 1:1 acetato-alkoxide has been c r y s t a l l i z e d ( A l M o (Oi .Pr)^[QAc^ and i t s s t r u c t u r e e s t a b l i s h e d by XR c r y s t a l l o g r a p h y . (16) 6


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Metal-Alcoholate Initiators

3

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Polymerization

2

of oxiranes

- The^cataly^st : although most compounds having the composition I n d i c a t e d above i n i t i a t e oxiranes polymeri z a t i o n , t h e i r a c t i v i t y as w e l l as the p r o p e r t i e s ( i . e . M.W. and s t e r e o r e g u l a r i t y ) o f the r e s u l t i n g polymers depend c r i t i c a l l y on t h e i r s t r u c t u r a l c h a r a c t e r i s t i c s : these i n c l u d e not only the composition, but a l s o the s i z e (degree o f a s s o c i a t i o n ) and the shape (coordination number and geometry around the metals) o f these c o o r d i n a t i v e l y aggregated compounds. These parameters, obviously d i f f i c u l t t o determine d i r e c t l y and independently, depend i n a very s e n s i t i v e manner on s l i g h t v a r i a t i o n s i n the s y n t h e s i s c o n d i t i o n s , as shown by changes i n k i n e t i c data and product c h a r a c t e r i s t i c s . The ( n . B u O ) A l 0 Z n compound, with a mean degree o f a s s o c i a t i o n η = 6 i n non p o l a r s o l v e n t s i s p a r t i c u l a r l y e f f i c i e n t : the h a l f - p o l y m e r i z a t i o n time o f methyloxirane (MO) i n heptane at 50°C amounts t o 5 minutes (with £MO] = 1M, and[zn}= 10" .M);the r e a g t i o n f o l l o w s a simple 4

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obtained. These f e a t u r e s , together with the s t a b i l i t y o f the c a t a l y s t , make o f i t one o f the best candidates f o r a


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p o s s i b l e i n d u s t r i a l production rubbers.

o f high M.W.

polyether

- The_active_groups t h a t p o l y m e r i z a t i o n proceeds by insërtiôn"ïnto~the~Al-OR bonds i s c l e a r l y demonstrated by the f a c t t h a t l i n e a r chains produced c o n t a i n ( a f t e r h y d r o l y s i s ) one OR and one OH end-group. Furthermore, the d i r e c t r e l a t i o n s h i p between the c a t a l y t i c p r o p e r t i e s and a y-oxo-bridged m u l t i n u c l e a r b i m e t a l l i c s t r u c t u r e has been a s c e r t a i n e d by using oxo-alkoxides obtained from h y d r o l y s i s of Meerwein c o m p l e x e s : Al (OR)^· M (OR)


:

2

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+ H 0 ^ A 1 M 0 ( O R ) 4 + 2 ROH

Metal-Alcoholate Initiators - American Chemical Society

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