Hybrid InorganicOrganic Polymers Derived from Organofunctional


ween the polymer chains. Copolymerization with organic monomers suggest a similarity of behavior between the vinyloxyphosphazene and vinylacetate...
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Chapter 23

Hybrid Inorganic-Organic Polymers Derived from Organofunctional Phosphazenes

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Christopher W. Allen Department of Chemistry, University of Vermont, Burlington, V T 05405 Carbon chain polymers with pendant inorganic ring systems can be prepared by homo- or copolymerization reactions of cyclophosphazenes with o l e f i n i c exocyclic groups. The resulting polymers and copo­ lymers exhibit flame retardancy and have a large number of reactive s i t e s for further synthetic transformations. In alkenylpentafluorocyclotriphosphazenes the r e a c t i v i t y of the o l e f i n i c center may be mediated by placement of an electron donating group on the o l e f i n or by placing an insu­ l a t i n g function between the o l e f i n and the phosphazene. A broad range of homo- and copolymers have been prepared from vinyloxyphosphazenes. These materials undergo f a c i l e thermal decomposition. The v o l a t i l e products of these processes have been i d e n t i f i e d and some insight has been gained into the k i n e t i c s and mechanism of the polymer ther­ molysis reactions.

The primary approach t o the development o f main group inorganic polymer chemistry has been i n the preparation, c h a r a c t e r i z a t i o n and u t i l i z a t i o n o f polymers with an inorganic backbone which may, or may not, be protected by organic substituents ( l a ) . Important members of t h i s c l a s s of materials which are discussed i n t h i s volume and elsewhere includes R CAB),η

la.

(CHCH ) 2

n

lb.

R

R A B t R S i R S i ; SS; R ^ i O ; R P N ; SN 2

2

2

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0097-6156/88/0360-0290$06.00/0 © 1988 American Chemical Society Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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poly(siloxanes), poly(phosphazenes) and more recently poly( s i l a n e s ) , poly(carbosilanes) and p o l y ( s i l a z a n e s ) . Our approach has been to invert the r o l e s of the organic and inorganic e n t i t i e s thereby producing polymers with carbon chain backbones and inorganic substituents ( l b ) . V i n y l polymers with t r a n s i t i o n metal substituents have been studied extensively.with most i n t e r e s t being shown i n metallocene d e r i v a t i v e s . Inorganic main group e n t i t i e s as substituents on carbon chain polymers have received sporatic study with the most i n t e r e s t being shown i n a c y c l i c substituents involving organosilanes. V i n y l carboranes can also be polymerized and copolymerized to polymeric species having the boron c l u s t e r as a substituent. Our primary focus has been on the use of organofunçtional cyclophosphazenes as precursors to the desired polymers. We have conducted a systematic study of the e f f e c t of v a r i a t i o n of both the phosphazene and o l e f i n i c components within the general structure of an organofunçt i o n a l phosphazene (2). These studies have allowed for both an understanding of the polymerization behavior of t h i s novel c l a s s of monomers and the incorporation of c e r t a i n of the useful prop e r t i e s , such as flame retardency, of the phosphazenes i n t o t r a d i t i o n a l organic polymers.

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9

R X

E-C=CH

X=C1,F R=Me,0Et,0Me Y=N,NPX2N

9

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Y ALKENYL PHOSPHAZENES We i n t i a l l y demonstrated that our basic approach was f e a s i b l e by preparing copolymers (3$ R=Me) of 2-(propenyl)pentafluorocyclotriphosphazene (4) and styrene or related monomers. ' The copolymers, which have good s o l u b i l i t y and thermal s t a b i l i t y , do not burn or produce smoke when exposed to a flame. Nucleop h i l i c s u b s t i t u t i o n reactions of the types which are well known i n phosphazene chemistry can be c a r r i e d out on the phosphazene moiety i n these copolymers thus allowing for the preparation of a broad spectrum of related materials. However, the strong electron withdrawing nature of the N P F c unit transforms the propenyl group into a highly polar o l e f i n which undergoes comp e t i t i v e anionic addition i n the course of i t s preparation and serves as a termination s i t e i n the copolymerization r e a c t i o n . 6

3

3

6

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

INORGANIC AND ORGANOMETALLIC POLYMERS

292

Ph

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The electron d e f i c i e n t nature of the o l e f i n can be mediated i n two ways either by placement of an electron donating substituent on the o l e f i n or by i n s u l a t i n g the o l e f i n from the phosphazene ring by other groups. The introduction of an alkoxy function as the electron donating group on the o l e f i n leads to 2-(l-ethoxy vinyl)pentafluorocvclotriphosphazene, N3P3F5C(0Et)=CH2 ( 5 ) . A comparison of the nmr chemical s h i f t s of the o l e f i n i c βcarbon i n 4 and 5 gives evidence for a s i g n i f i c a n t reduction i n the o l e f i n p o l a r i t y by addition of the electron donating ethoxy function. The change i n o l e f i n p o l a r i t y i s also demonstrasted by the absence of anionic addition to the o l e f i n i c center i n 5. F a c i l e copolymerization of 5 with styrene or methylmethacrylate occurs to y i e l d materials (e.g. 3; R=0Et) with higher degrees of phosphazene incorporation but properties s i m i l a r to the copoly­ mers derived from 4. The use of an a r y l group as an i n s u l a t i n g f u n c t i o n ^ , 14 i exemplified i n α-methylstyrylphosphazenes such as the meta and para isomers of N3P3F5C£H4C(CH3)=CH2 (6) which also have been induced to undergo copolymerization with styrene and methylmethacrylate. These l a t e r compounds have the highest degree of phosphazene incorporation into copolymers which we have yet observed. Reactivity r a t i o and Q,e c a l c u l a t i o n s show t h a t , i n general, ( i . e . , i n 4,5,6) the phosphazene moiety e x h i b i t s a very strong electron withdrawing e f f e c t without any s i g n i f i c a n t mesomeric i n t e r a c t i o n . Thus, the model of the e l e c t r o n i c s t r u c ­ ture of the o l e f i n i c center i n alkenylphosphazenes which a r i s e s from quantitative studies of r e a c t i v i t y i n copolymerization reac­ t i o n s i s very s i m i l a r to that deduced from spectroscopic studies of arylphosphazenes. Further examination of these r e a c t i v i t y and Q,e data allow for a c l e a r understanding of the polymerization behavior and how i t i s modified by the phosphazene. Although most systems are best described by the terminal model, the r e a c t i v i t y patterns exhibited i n the copolymerization of ot-methylstyryl pentafluorocyclophosphazene (6) with methylmethacrylate can only by q u a n t i t a t i v e l y f i t by a pennultimate model.1* 1 1

s

9

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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In order to obtain homopolymers from the organofunctional phosphazenes, monosubstituted rather than 1,1-disubstituted o l e ­ f i n s of the type described above, are required. The styrene d e r i v a t i v e , N ^ F ^ H J ^ C h L , was prepared from the phenylmethyl ether d e r i v a t i v e , N P C H.CrOMe)=CH , v i a hydrostannation and subsequent B-elimination of the t r i a l k y l t i n methoxide. Polymerization of the s t y r y l phosphazene lead to a high molecular weight polymer with pentafluorocyclotriphosphazene units at the para p o s i t i o n of each phenyl group (7; E=C H^, X=F). The TGA of t h i s material shows a s i g n i f i c a n t retention of i n v o l a t i l e material to over 1000°, suggesting the i n t r i g u i n g p o s s i b i l i t y 3

3

6

2

6

(CHCH 3 2

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Ε

n

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Ν

7

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9

Ν of using the p y r o l y s i s of polymers of type l b as a new route to ceramic s o l i d s . VINYLOXYPHOSPHAZENES In an attempt to generate convenient routes to organofunctional phosphazenes which could undergo homopolymerization reactions, we explored the reactions of l i t h i u m enolates, p a r t i c u l a r l y that of acetaldehyde, with halophosphazenes which leads to the series of organofunctional monomers_N3P3Cl6-ni0CH=ÇH2)n i (n=l-6), N P F „ (0CH=CH2Jn(n=l-5) and N ^ C l s . n T o C H ^ Î n (n=l,2). In these systems, an oxygen atom acts as the insul a t i n g function between the phosphazene and the o l e f i n . Other, related monomers, can be prepared by nucleophilic s u b s t i t u t i o n reactions with, for example alkoxide ions, of the chlorine atoms i n N P C1 0CH=CH (8). The appropriate monosubstituted derivat i v e (n=lj i n many cases has been polymerized by r a d i c a l i n i t i a t i o n to y i e l d the l i n e a r high polymer with a cyclophospbazene moiety as part of each repeating u n i t , e.g. 7$ E=0, X=C1. However, not a l l of the vinyloxyphosphazene monomers w i l l undergo r a d i c a l polymerization; those with amino substituents are unreact i v e . The C nmr data indicate that these species e l e c t r o n i c a l l y resemble v i n y l ethers (which do not undergo r a d i c a l polymerizat i o n ) whereas the reactive derivatives resemble v i n y l acetate. These data demonstrate an excellent example of e l e c t r o n i c effect transmission i n cyclophosphazene systems. The large number of reactive s i t e s per monomer unit i n these highly functionalized polymers has allowed for further synthetic 15,

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3

5

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18

3

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Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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transformations based on the chemistry of the phosphazene unit thereby allowing preparation of the amino substituted polymers by reaction of 7 (E=0, X=C1) with the appropriate amine. The broad range of vinyloxyphosphazene monomers which are a v a i l a b l e allow for the formation of copolymers of two d i f f e r e n t organofunctional phosphazene monomers. Consequently, we can apply polymerization r e a c t i v i t y studies to d i r e c t l y explore differences i n chemical behavior based on changes i n substituent, ring s i z e , etc. of the inorganic monomer. We have prepared a series of copolymers of 8 and i t s tetrameric analog, N4P4Cl70CH=CH2 and shown that 8 has the greater r e a c t i v i t y i n t h i s system. In a j o i n t study with Dr. van de Grampel of the University of Groningen, we have explored the copolymerization of 8 with a vinyloxycyclophospha(thia)zene. As opposed to the alkenylphosphazene polymers, the vinyloxychlorophosphazene polymers are thermally l a b i l e undergoing an exothermic decomposition v i a HC1 e l i m i n a t i o n as low as 100° followed by a more complex endothermic process at higher temperatures. The f i r s t step represents a mild thermal c r o s s - l i n k i n g reaction with low weight l o s s . The s o l i d state k i n e t i c s of t h i s f i r s t step have been measured. The a c t i v a t i o n energy for the f i r s t step i s greater for the tetramer than for the trimer d e r i vative. Since t h i s i s the reverse of the behavior observed f o r displacement reactions of the (NPCl2)3 and (NPCl2)4 , i t implies that the b a r r i e r i s p r i m a r i l y associated with the chain reorganization energy needed to bring the phosphorus-chlorine bond i n proximity with a carbon-hydrogen group. After the exothermic c r o s s - l i n k i n g step, there i s a second process which appears to r e s u l t i n the elimination of oxobridged phosphazene dimers, e.g. (N3P3Cl5)2Û and, by i m p l i c a t i o n , to b u i l d an oxo c r o s s - l i n k between the polymer chains. Copolymerization with organic monomers suggest a s i m i l a r i t y of behavior between the vinyloxyphosphazene and v i n y l a c e t a t e . The thermal l a b i l i t y of the polymers cont a i n i n g the vinyloxyphosphazene i s even more pronounced i n the copolymers. In order to overcome some of these problems a larger i n s u l a t i n g function was sought. The reactions of 2-hydroxyethyl methylmethacrylate with N^P^Clr leads to N3P3CI5OCH2CH2OC(0)C(CH3) = CH2 which undergoes r a d i c a l or thermal homo and copolymerization which i f not c a r e f u l l y c o n t r o l l e d i s accompanied by extensive c r o s s - l i n k i n g .

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EXTENSIONS TO OTHER SYSTEMS Our studies have convinced us that t h i s approach to hybrid organic-inorganic polymers i s a valuable one leading to a wide variety of new, i n t e r e s t i n g , materials. In addition to new phosphazenes, we are now expanding our studies to the preparation of pentamethylvinylcyclotriborazene polymers and we are i n v e s t i gating various s i l i c o n containing systems.

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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ACKNOWLEDGMENTS This work was supported i n part by the Office of Naval Research; c o l l a b o r a t i v e e f f o r t s with Dr, van de Grampel are supported, i n part, by a NATO t r a v e l grant,

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LITERATURE CITED 1. Pittmann, J r . , C.U.; Rausch, M.D., Pure Appl. Chem., Pure Appl. Chem., 1986, 58, 617. 2. Dzhardimalieva, G.I.; Zhorin, V.A.; Ivkva, I.N. Pomogailo, A.D.; Enikopolyan, N.S., Dokl. Akad. Nauk SSSR, 1986, 287, 654. 3. Billingham, N.C.; Jenkins, A.D.; K r o n f l i , E.B.; Walton, D.R.M. J . Polym. S c i . , Polym. Chem. Ed., 1977, 15, 683; Lakshmana Rao, V.; Babu, G.N., J . Macromol. S c i . , Chem., 1983, A20, 527; Ledwith, Α.; Chiellini, E.; Solaro, R., Macromolecules, 1979, 12, 240; Carbonaro, Α.; Greco, Α.; Vassi, I.W., Eur. Polym. J . , 1968, 4, 445; Tumanova, I.Α.; Semenov, O.B.; Yanovsky, Yu. G., Inter. J . Polym. Mater., 1980, 8, 225. 4. Wright, J.R.; Kingen, T.J., J . Inorg. Nucl. Chem. 1974, 36, 3667; Klingen, T.J., i b i d , 1980, 42, 1109. 5. Allen, C.W., J . Polym. S c i . , Polym. Symp., 1983, 70, 79. 6. Dupont, J.G., Allen, C.W., Macromolecules., 1979, 12, 169. 7. Allen, C.W.; Dupont, J.G., Ind. Eng. Chem. Prod. Res. Dev., 1979, No.18,80. 8. Allcock, H.R., Phosphorus-Nitrogen Compounds; Academic Press: New York, 1972; Allen, C.W., "Cyclophosphazenes and Related Compounds" i n The Chemistry of Inorganic Rings, Ed. I. Haiduc and D.B. Sowerby, Academic Press, i n press. 9. Allen. C.W.; White, A.J., Inorg. Chem., 1974, 13, 1220; Allen, C.W., J . Organometal. Chem., 1977, 125, 215; Allen, C.W.; Green, J.C., Inorg. Chem., 1980, 19, 1719. 10. Allen, C.W.; Bright, R.P.; Ramachandran, K., ACS Symp. Ser., Phosphorus Chemistry, 1981, 171, 321. 11. Allen, C.W.; Bright, R.P., Inorg. Chem. 1983, 22, 1291. 12. Allen. C.W.; Birght, R.P., Macromolecules, 1986, 19, 571. 13. Shaw, J.C.; Allen, C.W., Inorg. Chem., 1986, 25, 4632. 14. Allen, C.W.; Shaw, J.C., Phosphorus and Sulfur, 1987, 30, 97. 15. Allen, C.W.; Ramachandran, K.; Bright, R.P.; Shaw, J.C., Inorg. Chim. Acta, 1982 64, L109. 16. Ramachandran, K.; A l l e n , C.W., Inorg. Chem. 1983, 22, 1445. 17. Allen, C.W.; Bright, R.P., Inorg. Chim. Acta 1985, 99, 107. 18. Brown, D.E.; Allen, C.W., Inorg. Chem., 1987, 26, 934. 19. Allen, C.W.; Ramachandran, K.; Brown, D.E., Inorg. Syn., i n press. 20. van de Grampel, J.C.; Jekel, A.P.; Dhathathreyan, K.; Allen, C.W.; Brown, D.E., Abstract 319, 193rd American Chemical Society Meeting, Denver, A p r i l , 1987. RECEIVED September 1, 1987

Zeldin et al.; Inorganic and Organometallic Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.