Metal-Alcoholate Initiators - American Chemical Society

Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège,. Sart Tilman, 4000 Liège-Belgium. Soluble μ-oxo-bimetallic trinucle...
0 downloads 0 Views 771KB Size
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

13

27

2

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.

RING-OPENING

98

POLYMERIZATION

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).

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

X

2

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

3

27

η

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

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

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

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

99

Metal-Alcoholate Initiators

3

2

2

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

2

2

2

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

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

RING-OPENING POLYMERIZATION

100

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)

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

:

2

2

2

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