Anionic Polymerization - American Chemical Society

17 Living Anionic Stereospecific Polymerization of 2-Vinylpyridine ALAIN SOUM and MICHEL FONTANILLE

Downloaded by CORNELL UNIV on July 24, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch017

Laboratoire de Recherches sur les Macromolecules, de l'Universite Paris—Nord (ERA 607), 93430 Villetaneuse, France

Living anionic systems are among the most suitable to study the mechanism of polymerizations. Indeed, the high efficiency of anionic i n i t i a t o r s , combined with the good s t a b i l i t y of active centers, allows one to study directly the active species by common physico-chemical methods. Natta et a l . have shown (1,2) that stereospecific polymerization of 2-vinyl pyridine (2VP) can be achieved by using desolvated Grignard reagents as i n i t i a t o r s , in solution in hydrocarbon solvents. Although the essential characteristics were not revealed in this work, there is no doubt that an anionic mechanism is operative in such a polymerization. Recently, Hogen Esch et a l . (3,4,5), obtained the stereospecific oligomerization of 2-VP, by using organolithium derivatives as i n i t i a t o r s in THF solution. They proposed a stereoregulating mechanism mainly based on the analysis of the configuration of low molecular weight products formed. Our study concerns the polymerization of 2-VP, initiated by organomagnesium derivatives of the type R Mg R', in hydrocarbon solvents. In order to obtain a detailed knowledge of the stereoregulating mechanism, we focused this study on the determination of the nature and of the structure of active centers. 1.

I n i t i a t i o n o f the Polymerization

In s p i t e of t h e i r low r e a c t i v i t y towards e t h y l e n i c doublebonds, the organomagnesium compounds a r e able to i n i t i a t e the p o l y m e r i z a t i o n o f 2-VP in THF s o l u t i o n ( 6 ) , but the process is not s t e r e o s p e c i f i c . We have found that, in hydrocarbon s o l v e n t s , the a c t i v a t i n g e f f e c t of the p y r i d i n e r i n g upon the r e a c t i v i t y o f the doublebond, allows n u c l e o p h i l i c a d d i t i o n from non-solvated R Mg R derivatives. Nevertheless, a l l these compounds cannot be used i n d i s c r i m i n e n t l y . Indeed, the p y r i d i n e r i n g of the 2-VP molecule is sens i t i v e t o the attack of n u c l e o p h i l i c reagents, l e a d i n g to s i d e f

0097-6156/81/0166-0239$07.75/0 ©

1981 American Chemical Society

McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

240

ANIONIC

POLYMERIZATION

r e a c t i o n s during the i n i t i a t i o n step as w e l l as during propagat i o n by a t t a c k onto the pVP u n i t s of the polymeric chains. For example, d i b u t y l magnesium leads to high polymers whose M i s much higher than that c a l c u l a t e d from the i n i t i a t o r conc e n t r a t i o n . U.V. spectra of l i v i n g species show the presence of l a r g e amount of a n i o n i c by-products. We used the d i b e n z y l magnesium (Bz2 Mg) t o i n i t i a t e the p o l y m e r i z a t i o n . However t h i s compound i s not s o l u b l e i n hydrocarbon s o l v e n t s and i n i t i a t i o n takes t h e r e f o r e place i n heterogeneous phase. The excess of Bz2 Mg must be f i l t e r e d o f f , i n order t o l i m i t the subsequent a t t a c k of the p y r i d i n e r i n g s . Nevertheless, as i n d i c a t e d by F i g u r e 1 , s i d e r e a c t i o n s s t i l l slowly occur, e x p l a i n i n g the appearance of a shoulder i n the U.V. spectrum, i n the 380 nm r e g i o n . This shoulder corresponds probably t o the formation i n the system of the f o l l o w i n g groups:


n

The corresponding mechanism was already proposed i n a previous paper (6)· The number average molecular weight a n a l y s i s of the polymers obtained, g i v e s values about twice those c a l c u l a t e d from the amount of Bz2 Mg reacted (Table 1 ) . Moreover, v i s c o s i t y measurements on a c t i v e and d e a c t i v a t e d polymers i n s o l u t i o n , show that the v i s c o s i t y of the s o l u t i o n remains roughly constant a f t e r d e a c t i v a t i o n * (Figure 2 ) . To e x p l a i n these unexpected r e s u l t s , we assumed a disymm e t r i c a l s t r u c t u r e of a c t i v e s p e c i e s , H (pVPMgBz)

corresponding t o the m o n o f u n c t i o n a l i t y of the i n i t i a t o r , i n s p i t e of the b i v a l e n t nature of the c a t i o n . * V i s c o e i t y measurements were performed i n THF s o l u t i o n , a f t e r s u b s t i t u t i o n of benzene by THF, i n order t o prevent a p o s s i b l e aggregation of species i n hydrocarbon s o l v e n t s .


SOUM AND FONTANILLE

2-Vinylpyridine

241


17.

Figure 1. Variation of electronic absorption spectra of poly(2-vinylpyridine) MgBz initiated by dibenzylmagnesium as a function of time: (a) 5 min; (b) 70 h; (c)400 h;(d)1000 h.



0.1

0

Oligomers

Oligomers

BPMg

BPMg

Oligomers**)

PS 4

PS 5

PS 6

PS 7

PS 8

a) b) c) d)

0.56

0

3.6

1.84

0.54

0.36

1

13.6

100

80

74

39

12

17

1

I n i t i a l monomer cone. [Mol.lOj mole.l""

Y i e l d s of polymerizations are q u a n t i t a t i v e C o n c e n t r a t i o n in carbanions Oligomers prepared from dibenzylmagnesium Oligomers prepared from benzylpicolylmagnesium

0

Oligomers )

lO(solid)

PS 3

-

Bz2Mg

PS 2

1

s-Bu2Mg

PS 1

Initiator cone.(b) [I].103 mole.l""

100

243

278

435

1370

1083

12

170

[I]

[Mol

10,500

25,500

29,200

45,700

144,000

113,800

1,260

17,800

0

n

Theoretical M c a l c u l a t e d from [M ]/[I]

22,000

49,000

62,000

100,000

300,000

230,000

3,000

145,000

Experimental

Initiator

Sample

a

MOLECULAR WEIGHTS OF POLY(2-VINYL PYRIDINE) PREPARED FROM ORGANOMAGNESIUM COMPOUNDS AT 25°C... IN BENZENE SOLU*TION ,.

TABLE 1:



17.

SOUM AND FONTANILLE

2-Vinylpyridine

243

Figure 2. Effect of deactivation on viscosity of a living solution in THF of poly(2-vinylpyridine) MgBz prepared from dibenzylmagnesium: living polymer (%); dead polymer ( • ).


ANIONIC POLYMERIZATION


244

In order to demonstrate such an assumption, we hydrolyzed a con c e n t r a t e d s o l u t i o n o f l i v i n g oligomers and we obtained an amount of toluene corresponding to one b e n z y l i c carbanion per l i v i n g chain. The d i s y m e t r i c a l s t r u c t u r e of the species i n d i c a t e s that the r e a c t i v i t y of the second b e n z y l i c carbanion of the i n i t i a t o r is a f f e c t e d by the s u b s t i t u t i o n o f the f i r s t one. T h i s behavior was already observed by other authors. Benzyl p i c o l y l magnesium is t h e r e f o r e a model molecule o f the l i v i n g chain and we have prepared such a compound by r e a c t i n g Bz2 Mg w i t h ct-picoline in methylcyclohexane and r e c r y s t a l l i z e d the product in the form of yellow needles. The disymmetrical s t r u c t u r e of t h i s compound was e s t a b l i s h e d by *H NMR spectroscopy and by V.P.C.... a n a l y s i s of the products a f t e r h y d r o l y s i s . The corresponding spectrum and assignments w i l l be discussed l a t e r . As i n d i c a t e d by F i g u r e 3 , a s o l u t i o n of BPMg in cyclohexane or in benzene, absorbs in the U.V. r e g i o n , r e s p e c t i v e l y a t 270 and 285 nm, without any shoulder between 300 and 400 nm. The benzyl p i c o l y l magnesium obtained, as d e s c r i b e d above, can be used as an i n i t i a t o r and produces l i v i n g i s o t a c t i c poly mers on which: - The U.V. spectrum does not present any shoulder between 300 and 400 nm ( F i g u r e 4 ) . - The M v a l u e s a r e in agreement with the ones c a l c u l a t e d assuming i n i t i a t i o n by the p i c o l y l carbanion only (Table 2 ) . n

BPMg is t h e r e f o r e a convenient monofunctional i n i t i a t o r l e a d i n g to pure l i v i n g species with a disymmetrical s t r u c t u r e , and a l l the r e s u l t s reported here were obtained w i t h t h i s mole c u l e as an i n i t i a t o r . 2.

Study o f the Nature of the A c t i v e Centers

In order to determine i f the true a c t i v e species a r e in a predominant c o n c e n t r a t i o n in the system s t u d i e d , we have c a r r i e d out a k i n e t i c study o f the propagation r e a c t i o n . The s t r a i g h t l i n e s in the p l o t of l o g [M ]/[M] vs time, ( F i g u r e 5) i n d i c a t e that the k i n e t i c order is equal to 1 w i t h respect to monomer c o n c e n t r a t i o n and that the c o n c e n t r a t i o n o f a c t i v e centers remains constant. The l i n e a r i t y of the p l o t k f (C...), f o r concentrations between 5.10" -6.10"*% ( F i g u r e 6 ), demonstrates the k i n e t i c order 1 w i t h respect to the a c t i v e c e n t e r s . The r a t e constant of the bimolecular propagation r e a c t i o n is k 3.0 l . m o l e " . s " a t 20 C. From experiments performed a t d i f f e r e n t temperatures ( F i g u r e 7 ) , it is p o s s i b l e to o b t a i n the a c t i v a t i o n energy ( E 71 KJ.mole" ) and the entropy o f a c t i v a t i o n : As* » -58 u.e. T h i s s t r o n g l y negative value of As* i n d i c a t e s a very r i g i d 0

β

s

p

e

38

a

1


2-Vinylpyridine


SOUM AND FONTANILLE


245


246

ANIONIC POLYMERIZATION

Q

ηm Figure 4.

Electronic absorption spectra of poly(2-vinylpyridine) MgBz initiated by benzylpicolylmagnesium in benzene.


Anionic Polymerization - American Chemical Society

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