STEREOSPECIFIC OF DIENES COMPLEXES B.

A.

UNDER

CATALYSIS THE

IN T H E

INFLUENCE

OF T R A N S I T I O N Dolgoplosk

and

POLYMERIZATION OF

lr-ALLYL

METALS E.

I.

UDC 542.97+541.64+547.315

Tinyakova

The s t e r e o s p e c i f i c p o l y m e r i z a t i o n of dienes is the most complex field of p o l y m e r i z a t i o n catalysis, which is due chiefly to the polyvariant c h a r a c t e r of the m i c r o s t r u c t u r e s a r i s i n g in the polymerizalLion p r o cess. A large number of Z i e g l e r - N a t t a catalysts and t h e i r modifications, c h a r a c t e r i z e d by a sufficiently high s t e r e o s p e c i f i c i t y of the action in the p o l y m e r i z a t i o n of various dienes and providing for the production of uniformly c o n s t r u c t e d p o l y m e r s containing various types of units (1,4-cis-, 1 , 4 - t r a n s - , 1,2- o r :3,4-units with syndio- or isotactic arrangement) are now known. All these s y s t e m s consist of salts of transition m e t als and organometallic compounds. The complexity of their composition and the v a r i e t y of the reactions proceeding in the interaction of the components of the s y s t e m has not yet permitted an approach to the study of the nature of the active centers forming one type of units or another and a clarification of the m e c h a n i s m of s t e r e o r e g u l a t i o n . And yet, a study of the relationship between the nature of the catalytic complex and the s t e r e o s p e c i f i c i t y of its action is the key to the problem. It is now c l e a r that the main role in p o l y m e r i z a t i o n p r o c e s s e s is played by compounds of the t r a n s i tion metal, and not organoaluminum compounds. It is well known that alkyl derivatives of the transition metals are e x t r e m e l y unstable and decompose readily at the moment of formation [1] R--M--tl--~ i/_~ + 1%~ + M 2R--MX-+ R_~ + B+~ -t- MX2 + M The h i g h e r stability of 7r-allyl complexes is due to a d o n o r - d a t i v e interaction, in which the 7r-electrons of the double bond and the unshared d - e l e c t r o n s of the transition metal participate [2, 3]. ~r-Allyl derivatives of the t r a n s i t i o n metals are of special i n t e r e s t for a study of the fine f e a t u r e s of the p o l y m e r i z a t i o n p r o c e s s , since they are considered as compounds simulating the s t r u c t u r e of the active centers of the growing chains

CH 3

H

Syn-form

H

CH 3

Anti-form

The syn- and a n t i - f o r m s of the 7r-allyl complex can be c o n s i d e r e d as intermediates for the formaKon of t r a n s - and c i s - c o n f i g u r a t i o n s of the units in the p o l y m e r chain, r e s p e c t i v e l y . The s t e r e o r e g u l a t i n g effect of such s y s t e m s in the p o l y m e r i z a t i o n p r o c e s s can be d e t e r m i n e d both by the nature and valence state of the t r a n s i t i o n metal and by the nature of the ligands coordinated with it. This article p r e s e n t s the r e s u l t s of our e x p e r i m e n t a l investigations of the effectiveness and s t e r e o specificity of 7r-allyl complexes of a number of t r a n s i t i o n metals in the p o l y m e r i z a t i o n of butadiene, A. V. Topchiev Institute of P e t r o c h e m i c a l Synthesis, Academy of Sciences of the USSR. T r a n s l a t e d f r o m Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 344-357, F e b r u a r y , 1970. Original artieIe submitted July 18, 1968. 01970 Consultants Bureaz~, a division of Plenum Pl~blishing Corporatior~, 227 ~'est ]TthStreet, New Yor]c, N. Y. lO0]]. All rights reserved. T/sis article canttot be reprodltced for arty purpose whatsoever without permission of the publisher..4 copy of this article is availat)le from the publisher for $15.00.

291

permitting an outline of approaches to a consideration of the mechanism of stereoregulation.* Both poly-~r-allyl compounds of the transition metals, containing only ~r-allyl groups at the metal atom [Tr-CnH2n_i)mM], and compoundsof the type of (Tr-CnH2n_I)mlV[X, where X represents halogens or other electronegative groups, while CnH2n_i = Call5 and C4H7, were used as the catalysts. Poly~-allyl compounds were produced from alkenyl magnesium halides and anhydrous halides of the transition metals

o

~6

!

O o

o

yo

/2g

t8o

Time, min Fig. i. Kinetics of the polymerization (1) of butadiene in the presence of Cr(CaHs)3 and the kinetics of the consumption of allyl groups (2); solvent toluene, 20~.

nCH2= CH -- CH2MgC]-~ MCIn-+ M(C3H5)~ + nMgCl2 The reaction was conducted in ether medium at low temperatures [3]. Bis-Tr-allylnickel and tris-~r-allyleobalt were purified by sublimation under vacuum, tris-allyl(crotyl)chromium was purified by recrystallization from pentane. In individual cases the poly-~rallyl complexes were produced by the reaction of the stoichiometric hydrides with butadiene [4] MH~ + nC4H6-+ (~-C4Hr).~M

Allyl(crotyl)nickel halides were produced f r o m alkenyl halides and nickel t e t r a c a r b o n y l [5] CnH2n-IX + Ni(CO) 4---~CnH2~-INiX + 4C0 X r e p r e s e n t s chlorine, bromine, or iodine. Complexes of the type of (~r-C3Hs)mMXn were produced by the interaction of poly-Tr-allyl complexes with the c o r r e s p o n d i n g acids, phenols, etc. [6-8]. (~-CaHs) .M + CClaCOOH--~ (z~-CaHs)~-IMOCOCCla-~ Call6 NO2 NO~ \ \

(~-C~m).M§

(~-C~o)~_~--O~__/~--__/--NO~+C~o

Stereospecificity of the Action of 7r-Allyl Compounds of Chromium and Certain Principles of the P o l y m e r i z a t i o n P r o c e s s . With 7r-allyl complexes of c h r o m i u m as an example, it was shown that on the same transition metal, depending on the nature of the ligands bonded to it, all possible s t r u c t u r e s may be r e a l ized, p r e s e r v i n g high s t e r e o s p e c i f i c i t y of the action [6-9] + C,H

//~CH>...~

83% 1,2-units, 1 7~ 1, 4-trans=units

,4-2KCI(CCIsCOOH)

ffCH2

CH "~"CW'")CrC12C4H~ ~ 90% 1,4-cis- units

.93% t,4-trans-units Cr0a on aluminosflicate +C4H~ - - ~ 9 9 o~-trans-units [10] T r i s - a l l y l ( c r o t y l ) c h r o m i u m is a s t e r e o s p e c i f i c catalyst for the formation of crystalline 1,2-polybutadiene [9]. The appearance of a c e r t a i n number of 1 , 4 - t r a n s - u n i t s is apparently due to the penetration of t r a c e s of oxygen into the system, producing the formation of a new active center. In the case of partial displacement of the aHyl groups by chloride or t r i c h l o r o a c e t a t e anions, the s y s t e m b e c o m e s s t e r e o s p e c i f i c for c i s - p o l y m e r i z a t i o n [6, 7]. On the c o n t r a r y , the introduction of oxygen into the s y s t e m in amounts of one atom per mole of t r i s - a l l y l c h r o m i u m leads to the formation of a catalyst e x t r e m e l y stereospecific for the formation of t r a n s - u n i t s [9]. The s t e r e o s p e c i f i c i t y of the action of the latter s y s t e m coincides with the action of the well-known c h r o m i u m oxide s y s t e m [10]. The catalytic effect in the p o l y m e r i z a t i o n of otefins under the influence of a c h r o m i u m oxide catalyst usually is attributed to the p r e s e n c e of Cr 5+ in the s y s t e m [11]. The data cited above indicate that, at least as applied to 1,2- and c i s - p o l y m e r i z a t i o n of butadiene, the catalysis is due to the p r e s e n c e of c h r o m i u m in a more reduced state than Cr 3+. *The experimental r e s u l t s of individual investigations have been published in a number of a r t i c l e s in the Journal Doklady Akademii Nauk SSSR and in Letters to the Editor.

292

T A B L E 1. I n f l u e n c e of t h e D e g r e e of O x i d a t i o n of t h e T r a n s i t i o n M e t a l in the ~r-Allyl C o m p l e x e s on the S t e r e o s p e c i f i c i t y of the A c t i o n in the P o l y m e r i z a t i o n of B u t a d i e n e

Catalyst

Presumed compo-

~ ~

sition of catalyst

~ ~

li

-Cr (g~HT)a ,Cr (C,H 7)~-[-2HC [ "Cr (CtI-I~)~-}-CC13C O O H

Content of units, % 1,4- eis 1,4-trans

91,5

Oligomers 1,5

7

30--57 0 0

14--16 17 9~

27--49 83 6

[61 [9] [61

100 4,5 5,5 Cyclododecat ie ne 91

Co (C~H0, Co (C~H~h-]-21"ICI

(C;H~) CoCI~or C,H,CoCI (C41-Iv)2CoCI

<30 (C~HT)~-~ I-ICI

Ti(C411~)s I~h(C4H0~

~ ~ [91 [61 [71 [61 [61 [13] [141 [71 [61 [61

93

Ni (C~H 7).' .C~H,NiC1 C4HTNiOCOCC I~

1,2

li,5

Cr (C~HT) Cl.

: (~-C;H7)2NbCI

Nb (C~H~) 3 ~xT1)(C4H 7),~HC1

o ~

In a study of the basic principles of polymerization under the influence of Cr(C3Hs)3, it was established that all the allyl groups participate frontally in the reaction of propagation (Fig. 1) [12]. Curve I characterizes the consumption of allyl groups, curve 2 the yield of the polymer. The polymerization process was conducted at 20 ~ in toluene solution. After a definite degree of polymerizationhad been reached, the unpolymerized monomer was removed under vacuum; an aqueous solution of HCI was added to the residue. The gas liberated was analyzed for propylene content. In a study of the consumption of allyl groups at v a r ious stages of the polymerization process, it was also established that c~-butene appears in the gaseous products in amounts of ~10% of the initial C3H5 groups. The yield of butene depends little on the degree of polymerization, and only in the case of completion of the polymerization is it not detected in the reaction products. The formation of butene can be due only to reactions of chain transfer, which occur with p a r t i c i pation of the monomer (reaction 1) or as a result of spontaneous decomposition (reaction 2)

/,CH 2,

.,

-~C4H~

PCH----CH--CH=C ~ -

I

./H2"''- /" mH~O CH Cr - - c ~ "~'CH'"" \ CH3 Her<

-butene

I

CH2P

P = polymer chain

Both p a t h w a y s l e a d to the a p p e a r a n c e of a c o n j u g a t e d s y s t e m of double b o n d s at t h e e n d of the p o l y m e r c h a i n , w h i c h w a s d e m o n s t r a t e d a c c o r d i n g to t h e D i e l s r e a c t i o n [12]. T h e p o s s i b i l i t y of the o c c u r r e n c e of t h e r e a c t i o n a c c o r d i n g to p a t h w a y 2 w a s d e m o n s t r a t e d e x p e r i m e n t a l l y f o r h y d r i d e s of the t r a n s i t i o n m e t a l s , c a p a b l e of r e g e n e r a t i n g t h e ~r-allyl c o m p l e x a f t e r the a d d i t i o n of the d i e n e . T h e f o r m a t i o n of ~10% b u t e n e s a g r e e s with the low a v e r a g e d e g r e e of p o l y m e r i z a t i o n in t h i s s y s t e m . T h e f o r m a t i o n o f ~ - b u t e n e , r a t h e r t h a n f i - b u t e n e s , i n d i c a t e s t h a t d e c o m p o s i t i o n of the 7r-allyl c o m p l e x u n d e r the i n f l u e n c e of w a t e r is a n a l o gous to the e l e m e n t a r y e v e n t s in the r e a c t i o n of p o l y m e r i z a t i o n , l e a d i n g to the f o r m a t i o n of 1 , 2 - u n i t s . T h e p o l y m e r i z a t i o n p r o c e s s p r o c e e d s e f f e c t i v e l y in t h e t e m p e r a t u r e r e g i o n 20-50% T h e s u m m a r y a c t i v a t i o n e n e r g y of t h e p r o c e s s is 19 k c a l / m o l e [12]. Our kinetic investigations indicated that the rate of polymerization is described by the equation w = k[CM]~ ]2, w h e r e [CM] i s the c o n c e n t r a t i o n of the m o n o m e r , w h i l e [CCr ] i s t h e c o n c e n t r a t i o n of t r i s a l l y l c h r o m i u m . T h e i n d e p e n d e n c e of the r a t e on the c o n c e n t r a t i o n of the m o n o m e r c a n be e x p l a i n e d on the b a s i s of t h e f a c t t h a t t h e r a t e d e t e r m i n i n g s t e p is not c o o r d i n a t i o n of the m o n o m e r with the c a t a l y s t but i n s e r t i o n of the c o o r d i n a t e d m o n o m e r into the c h a i n . T h e d e p e n d e n c e of t h e r a t e on the c a t a l y s t c o n c e n t r a t i o n found i s a p p a r e n t l y e v i d e n c e t h a t the d i m e r f o r m of t h e c h r o m i u m c o m p o u n d , e x i s t i n g in e q u i l i b r i u m with t h e m o n o m e r f o r m , is a c t i v e in the p o l y m e r i z a t i o n p r o c e s s .

293

TABLE 2. Influence of the Nature of the Anion in ~-Allyl Complexes of Nickel on the Stereospecificity of the Action in the P o l y m e r i z a tion of Butadiene Method of produc- ~ o }tion 1~ = Method of production Catalyst 1,4- 1,41~ o C~HsNiI C4H~NiI C4H~NiC1

CnH2n_IX~-Ni (CO), X=CI,I (CnH2n_l)2 Ni-{-HX

C4FIrNIOCOCH2C1 C~HsNiOCOCHCI2 C~HsNiOCOCCIs NO2

.--+Cnl-I2n_lNiX-{-Cntt2n w h e r e HX=CH~C1COOH, CHC]2COOH,

1

C3HsNiOSO2C~H~CH3 Br t

C,H~NIO--~--Br

CCI~COOH, ~vc~ / # - - % - - O H I NO, Br

Br--~--OH,

CHaC~H4SOdll

I

9

97 5 6 5 4

3

[14] [14,161 [71 [71 [71~

5

[161

47

[8~

97

[161

Br

Br

Degree of Oxidation of the T r a n s i t i o n Metal and Stereospecificity of the Action. The dependence of the s t e r e o s p e c i f i c i t y of the action on the nature of the ligands surrounding the metal, determined with t r i s a l l y l - c h r o m i u m as an example, is c o m m o n to most of the transition metals. This p r e m i s e is illustrated by a number of examples cited in Table 1. Tris-zr-allyl complexes of chromium, niobium, and titanium are catalysts of 1 , 2 - p o l y m e r i z a t i o n of butadiene, while bis-Tr-allylnickel and t r i s - v - a l l y l c o b a l t are catalysts of oligomerization. The products of the reaction of these compounds with solutions of anhydrous acids are catalysts of 1 , 4 - c i s - p o l y m e r i z a t i o n of butadiene. In the c o u r s e of the interaction of the indicated polyallyl compounds with acids, one mole of the olefin is isolated p e r mole of the acid M (C~H2n_,).~ + HX -+ M (CnH2~_l) m-iX + C~H2~ Although the chemical equivalent of the transition metal is unchanged f r o m polyallyl derivatives of t r a n s i tion metals to the corresponding ehloro-derivatives, the replacement of the 7r-allyl group by an acid residue increases the degree of oxidation of the transition metal, which is illustrated by the number of electrons in the outer shell, calculated for the monomer form of the complex. Thus, the results obtained show that the stereospecificity of the action of 7r-allyl complexes in the polymerization process is directly related to the degree of oxidation of the transition metal. The most "reduced" forms of the transition metal (poly-~-allyl complexes) lead to 1,2-stereoregulation; the most "oxidized" lead to eis-stereoregulation. An exception is tris-erotylrhodium, which leads to a trans-polymer in hydrocarbon solutions. Influence of the Nature of the Anion. The nature of the anion bonded to the transition metal in the vallyl complex

""

"""

, has a substantial influence on the s t e r e o s p e c i f i e i t y of the action and activity of

the system. This phenomenon has been studied in the g r e a t e s t detail for 7r-allyl complexes of nickel with the general formula CnItm_xNiX, where X r e p r e s e n t s a halogen, an anion of a halogenated carboxylic acid, derivatives of phenol, arylsulfonic acid, etc. Table 2 p r e s e n t s data on the m i c r o s t r u e t u r e of polybutadiene, produced in the p r e s e n c e of various complexes. In those e a s e s when 7r-C3H5 is bonded to a residue of mono-, di-, or t r i e h l o r o a e e t i c acid or dinitrophenol, the polybutadiene f o r m e d contains more than 90% 1,4-cis-units; if, however, X r e p r e s e n t s iodine or tribromophenol, 1,4-trans-polybutadiene is obtained. When rr-allylnickeltoluenesulfonate is used, an equibinary p o l y m e r with alternating e i s - t r a n s - u n i t s is obtained. It should be mentioned that 7r-C3HsNiCI itself has v e r y little activity for p o l y m e r i z a t i o n (the yield of the p o l y m e r at 50 ~ in 20-40 h usually does not exceed several percent), and the results obtained on the m i c r o s t r u c t u r e of the chain cannot be c o n s i d e r e d sufficiently reliable in view of the possible influence of i m purities. Some of them, such as, for example, NiC1, lead to c i s - p o l y m e r i z a t i o n [17]; others, as, for e x a m ple, admixtures of water, lead to t r a n s - p o l y m e r i z a t i o n [18]. The results obtained by different authors on

294

the m i c r o s t r u c t u r e of t h e c h a i n f o r t h i s s y s t e m , t h e r e f o r e , m a y n o t a g r e e w i t h one a n o t h e r [14, 15, 19]. T h e e f f e c t i v e n e s s of s y s t e m s f o r p o l y m e r i z a t i o n i s i n c r e a s e d a s w e go to b r o m i n e and t h e n to i o d i n e . Compositi0nl Frequency of carbonyl group, v, Even more effective are systems with other anions, of complex- cm"1 . . . . . in p a r t i c u l a r , w i t h the t r i e h l o r o a e e t a t e anion, w h i c h Acceptor (A) es (CaHTNi- in initial in C ] /xu cm t c a t a l y z e the p r o c e s s of p o I y m e r i z a t i o n at a s u f f i ICl)z: A !accep~or A C TI ' c l e a r l y h i g h r a t e a t 50 ~ T h e p r o d u c t f o r m e d in the //O i n t e r a c t i o n of e q u i m o l a r a m o u n t s of (C4HT)2Ni a n d 1680 1805 125 1:1 CChC--CI CC13COOH is i n s o l u b l e in b e n z e n e a n d t o l u e n e , but //O 1700 68 1768 1:1 a f t e r the i n t r o d u c t i o n of b u t a d i e n e a t the b % j n n i n g CC!~C--It of p o l y m e r i z a t i o n , the s y s t e m b e c o m e s e n t i r e l y h o CCIzCOOH)e 50 1700 1750 1:1 m o g e n e o u s . O u r k i n e t i c s t u d y of p o l y m e r i z a t i o n u n d e r the i n f l u e n c e of 7 r - c r o t y l n i c k e l t r i c h l o r o a c e t a t e i n d i c a t e d t h a t the r a t e of t h e p r o c e s s i s p r o p o r t i o n a l to the n i c k e l c o n c e n t r a t i o n to the f i r s t p o w e r , w h i l e the s u m m a r y a c t i v a t i o n e n e r g y i s 11 k c a l / m o l e [16].

T A B L E 3. C h a r a c t e r i s t i c s of C o m p l e x e s w i t h C h a r g e T r a n s f e r F o r m e d in the I n t e r a c t i o n of [C4HTNiCI] 2 with A c c e p t e r s [24]

It w a s s h o w n f o r 7 r - a l l y l n i c k e l t o l u e n e s u l f o n a t e t h a t a d i s p r o p o r t i o n a t i o n r e a c t i o n o c c u r s at the t e m p e r a t u r e 2 0 - 5 0 ~ l e a d i n g to the f o r m a t i o n of b i s - ~ r - a l l y l n i c k e l [8] 2C3HsNi0SO2C6H4CHa -+ (a-Calls) 2Ni + Ni (0S02C6H4CH3) 2 T h u s , u n d e r d e f i n i t e c o n d i t i o n s two a c t i v e c e n t e r s with d i f f e r e n t s t e r e o s p e c i f i e i t y of a c t i o n c a n b e r e a l i z e d in t h e s y s t e m . T h e i n i t i a l a c t i v e c e n t e r l e a d s to p o l y m e r i z a t i o n , w h i l e b i s - T r - a l l y l n i e k e l l e a d s to c y c l i c t r i m e r i z a t i o n of b u t a d i e n e . C a t a l y s i s u n d e r t h e I n f l u e n c e of C o m p l e x e s with C h a r g e T r a n s f e r (CCT) o r an Anion. ~ - C r o t y l n i e k e l halides, especially ~r-crotylnickel chloride, are relatively ineffective as catalysts of the polymerization of dienes. The polymerization process is sharply accelerated in the presence of electron accepter compounds capable of forming complexes with the dimer or monomer form of ~-allylnickel halides. As applied to the monomer form of the ir-allyl complex, the influence of electron accepters can be schematically depicted in the following way Acceptor o II

C]

el j/CH2...(+) ~H "NiX

Br

~H2P

Br 0

II NilxAI III

cH

~"~H'""

IV

CHa V

II 0 0

Cl Cl Br

[]

Br Br

o

F

II

F Cl

II

Cl Br

F

1t 0

F CI

fl 0

CI Br

12 0 f

CChCOOg, CChC~-H

o I!

Cl

!l 0

11 0 0 N

II

0

CH2

o II

Cl

0

Br

Ii

[]

o

0

0 II

0

Br X

1] 0

Br

I||

I}

0

I \I

0 CCI3C:-CI, CClsCOCCls

(CC18CO0)zM, A1CI~, TiCI~, SaCh, VCI4, ZnCh, MnCh, Mg (C10~)2 CoCL.etc. NiCh, MgCL, MgSO~, Na~COaera.

In c o m p l e x e s 1 and 2, a r i s i n g a s a r e s u l t of c h a r g e t r a n s f e r (1) o r the f o r m a t i o n of an i o n p a i r (2), t h e r e i s an i n c r e a s e in the p o s i t i v e c h a r g e on the t r a n s i t i o n m e t a l a t o m . S i m i l a r e f f e c t s s h o u l d a l s o be e x p e c t e d in c o m p l e x e s a r i s i n g with the p a r t i c i p a t i o n of the d i m e r f o r m of the ~ - a l l y l c o m p o u n d . H a l o d e r i v a t i v e s of q u i n o n e s s h o u l d be c l a s s e d a m o n g g r o u p I of e l e c t r o n a c c e p t e r s f o r m i n g CCT; m o r e o v e r , t h e e f f e c t i v e n e s s of t h e i r a c t i o n d r o p s with d e c r e a s i n g c o n t e n t of the h a l o g e n s , which a g r e e s with t h e d r o p in the h a l f - w a v e

295

T A B L E 4. E f f e c t i v e n e s s and S t e r e o s p e c i f i c i t y of the A c t i o n of C o m p l e x e s [ r - C 4 H T N i C l 2 ] . E l e c t r o n A c c e p t o r in the P o l y m e r i z a t i o n of B u tadiene

Acceptor

tTemperature I Duration of]Yield of Content of units,% I . . . . . . ILiterature of polymeri~ polymeriza~polymer, 1,4- 1,4- {1 2 ~eference zation, ~ [ tion, h /% cis Itrans[ ' ~ .

Without acceptor Fluoranil Chloranil t~omanfl Iodanil ~- Benzoquinone ,5- Dichloroquinone

+5o .-15 -15 --15 --I5 --15 --15

21 10 min

CCI3COOH CClaCHO CC13COC] Zn (OCOCCls)2 Mn (OCOCCla)

--15 --1,5 --15 --15 --15

4 4 4

,20 rnin ,20 rain

TIC14 A1Cls ZnCl~ Mg (Cl04)

--I5 --15 --15 --15

17 17 17 17

MnCl2 COCI~ NiC12

+5 50 50

17 8 8

[201 [21] [20] '[211 [211 [20]

4 4

20 2O 2O 4

[221 [231 [23] [22] [22]

~,5 5

[17] [171 [17] [~7]

3 2 5 3

8

~,5 ,',5 4

[17] [17] [17]

p o t e n t i a l of the a d d i t i o n of the f i r s t e l e c t r o n [20]. T h e d e c r e a s e in the e f f e c t i v e n e s s of the a c t i o n in the s e ries: fluoranil-chloranil-bromanil-iodanil [21] i s a p p a r e n t l y due only to a n i n c r e a s e in t h e s t e r i c h i n d r a n c e s f o r f o r m a t i o n of t h e c o m p l e x . M o l e c u l a r i o d i n e , the e l e c t r o n - a c c e p t o r p r o p e r t i e s of which a r e w e l l known, is a l s o e x t r e m e l y e f f e c t i v e a t low t e m p e r a t u r e s . C o m p o u n d s of g r o u p III, f i r s t u s e d in CCT [22, 23], p r o v e d v e r y e f f e c t i v e . T h e f o r m a t i o n of CCT f o r the i n d i c a t e d g r o u p of c o m p o u n d s h a s b e e n c o n f i r m e d by t h e d a t a of IR s p e c t r o s c o p y a c c o r d i n g to the s h i f t of t h e v i b r a t i o n f r e q u e n c y of the c a r b o n y l g r o u p ( T a b l e 3) [24]. We s h o u l d m e n t i o n the p r e s e n c e of a c o r r e l a t i o n b e t w e e n the s h i f t of the v i b r a t i o n f r e q u e n c y of t h e c a r b o n y l g r o u p and t h e e f f e c t i v e n e s s of the a c t i o n in the p o l y m e r i z a t i o n p r o c e s s . T h e c o m p o u n d s c i t e d i n g r o u p IV a r e t y p i c a l L e w i s a c i d s , and t h e y c a n a p p a r e n t l y give c o m p l e x e s of both t y p e s (1 and 2). T h e c o m p o u n d s c i t e d in g r o u p s I - I V p r o v i d e f o r r a p i d o c c u r r e n c e of the p o l y m e r i z a t i o n p r o c e s s w i t h i n the t e m p e r a t u r e i n t e r v a l f r o m - 1 5 to 20 ~ V a r i o u s s a l t s , e n u m e r a t e d in g r o u p V, a r e e f f e c t i v e at a t e m p e r a t u r e of 50 ~ When e l e c t r o n a c c e p t o r s a r e a d d e d t o a s o l u t i o n of ~ - a l l y l ( c r o t y l ) n i c k e l h a l i d e , a p r e c i p i t a t e i s f o r m ed a f t e r s o m e t i m e , which, h o w e v e r , p a s s e s into s o l u t i o n a f t e r the i n t r o d u c t i o n of b u t a d i e n e , and the p o l y m e r i z a t i o n p r o c e s s b e c o m e s h o m o g e n e o u s . A s t u d y of the c o m p o s i t i o n of the c o m p l e x e s p r e c i p i t a t e d in the a b s e n c e of the m o n o m e r i n d i c a t e d t h a t in m o s t c a s e s t h e c o m p l e x a r i s e s f r o m a d i m e r of the it- a l l y l n i c k e l h a l i d e a n d one m o l e c u l e of the e l e c t r o n a c c e p t o r - t i t a n i u m t e t r a c h l o r i d e [25], c h l o r a n i l [21], n i c k e l t r i c h l o r o a c e t a t e [26]. H o w e v e r , in v i e w of the f a c t t h a t the m a x i m u m r a t e of p o l y m e r i z a t i o n i s r e a c h e d at a r a t i o (C4H7NiCI)2 : A = 1 : 1.5-2 [25, 26], it m a y be a s s u m e d t h a t a f t e r the a d d i t i o n of b u t a d i e n e a p p a r e n t l y an a c t i v e c e n t e r is f o r m e d , c o n t a i n i n g the m o n o m e r f o r m of the 1r-allyl c o m p l e x of n i c k e l . In a s t u d y of the b a s i c k i n e t i c p r i n c i p l e s of the p o l y m e r i z a t i o n of b u t a d i e n e u n d e r the i n f l u e n c e of a c o m p l e x c o n t a i n i n g ~ r - c r o t y l n i c k e l c h l o r i d e - n i c k e l t r i c h l o r o a c e t a t e , it w a s e s t a b l i s h e d t h a t the r a t e of the p r o c e s s i s p r o p o r t i o n a l to the c o n c e n t r a t i o n of the m o n o m e r and of the c a t a l y t i c c o m p l e x to the f i r s t p o w e r . The s u m m a r y a c t i v a t i o n e n e r g y of the p r o c e s s is 6 k c a l / m o l e [26]. In the c a s e of p o l y m e r i z a t i o n u n d e r the i n f l u e n c e of ~ - a l l y l n i c k e l h a l i d e s , on the o t h e r hand, a s w a s shown in [19], t h e r a t e o f the p r o c e s s i s p r o p o r t i o n a l to t h e c o n c e n t r a t i o n of t h e c a t a l y s t to t h e e x p o n e n t 0.5, w h i l e the s u m m a r y a c t i v a t i o n e n e r g y i s 16 k c a l / m o l e . T h e d e p e n d e n c e of the r a t e of the p r o c e s s on the c o n c e n t r a t i o n of t h e c a t a l y s t found i s a t t r i b u t e d by the a u t h o r s to the f a c t t h a t the p o l y m e r i z a t i o n p r o c e s s p r o c e e d s in the m o n o m e r f o r m of ~ - c r o t y l n i c k e l c h l o r i d e and t h a t the s t e p of d i s s o c i a t i o n of the d i m e r into the m o n o m e r is the d e t e r m i n i n g p r o c e s s in the k i n e t i c s . T a b l e 4 p r e s e n t s s o m e d a t a c h a r a c t e r i z i n g t h e r e l a t i v e e f f e c t i v e n e s s and d e g r e e of s t e r e o s p e c i f i c i t y of the a c t i o n of v - a l l y l c o m p l e x e s with c h a r g e t r a n s f e r in t h e p o l y m e r i z a t i o n of b u t a d i e n e .

296

TABLE 5. I n v e r s i o n of Stereospeeifieity of the Action of 7r-Crotytnickel Iodide under the Influence of E l e c t r o n Aeceptors (A) ~ ~:~ 0

I~

~

Contentof

,~a

20 --15 --15

49 6 6

21 28 41

0 95 94

95

1:2 1:2

4 4

[14,27] [271 [271

0_/~\_O

2:1

--15

1,5

15

97

2

[211

CClaCOCgh I2 SnI4

1:1 2:1 l;1

--15 --15 --15

3,5 48 I

20 14 13

92 84 85

5 15

[27] [27] [27]

Without acceptor

CCI3C(O)I{ CC13COCI F F

13

The use of e l e e t r o n - a e e e p t o r compounds leads not only to an a c c e l e r a t i o n of the p o l y m e r i z a t i o n p r o eess, but also to an i n c r e a s e in the s t e r e o s p e e i f i e t t y of the action with r e s p e c t to the f o r m a t i o n of cis-units, the content of which in the p o l y m e r in individual eases r e a c h e s 98~9. It is e x t r e m e l y interesting that under the influence of e l e c t r o n aeeeptors an inversion of the s t e r e o s p e e i f i e i t y of the action is observed. Thus, under the itffluenee of r,-altyl(erotyl)niekel iodide, c r y s t a l l i n e 1,4-trans-potybutadiene is formed, while the same s y s t e m in the p r e s e n c e of e l e c t r o n aeeeptors b e c o m e s stereospeeifie for e i s - p o l y m e r i z a t i o n [27]. The aforementioned is illustrated by the data of Table 5. The opposite effect is exerted by e l e c t r o n - d o n o r impurities, which, together with reducing the rate of polymerization, bring about a shift in the direction of t r a n s - u n i t s [18j WaNe 6). In the p r e s e n c e of w a t e r a complete inversion of the s t e r e o s p e e i f i e i t y of the a c tion is observed (crystalline trans-polybutadiene is formed). Mechanism of Stereoregulation in the P o l y m e r i z a t i o n of Dienes under the Influence of 7r-Allyl Systems. As has a l r e a d y been noted above, the active center of the growing chain r e p r e s e n t s a complex in which the t e r m i n a l ~r-aIlyl unit has a syn- or anti-configuration. It s e e m e d essential to determine f i r s t of all whether the t e r m i n a l unit plays a role in the template effect of the system, i.e., whether the s t r u c t u r e of the t e r m i hal unit in the 7r-allyl-active c e n t e r influences the s t r u c t u r e of the following unit, f o r m e d when butadiene is inserted in the 7r-allyl complex. The method of h y d r o g e n o l y s i s of 7r-erotylniekel chloride [28] was used to show that in this ease 97% t r a n s - p - b u t e n e is formed, w h e r e a s the complex used in the p o l y m e r i z a t i o n led to eis-polybutadiene. This fact, as well as the possibility of obtaining polybutadtene with statistically d i s tributed c i s - and t r a n s - u n i t s under definite conditions, indicates that the s t r u c t u r e of the t e r m i n a l unit is not a deciding f a c t o r in the p r o c e s s of s t e r e o r e g u l a t i o n . It is natural to expect that the insertion of the diene into the complex should be p r e c e d e d by a step of decomposition of the 7r-allyl s y s t e m with the a p p e a r anee of a C - M a - b o n d . The latter s e e m s most probable in the state of coordination of the t r a n s i t i o n m e t al with the e l e c t r o n - d o n o r component - the m o n o m e r . The step of decomposition of the ~r-allyl complex can p r o c e e d at two bonds:

ell2 il CH2.

PCHe--CH=CKCH2MXn,---

c,I-I,

~

CH

C,H,~ C/ H - - g X n ".2MX. __

%c."" [

~ ICH:;P

CH2P

Pathway 1 leads to the formation of 1,4-units, pathway 2 to the formation of 1,2-units. The t h e o r e t i c a l possibility of decomposition of the ~r-allyl complexes a c c o r d i n g to pathway 2 is confirmed by the fact that in the decomposition of 7r-crotyl derivatives of c h r o m i u m and nickel by an aqueous solution of acid, chiefly c~-butadiene is isolated [12, 26]. The m e c h a n i s m of the f o r m a t i o n of c i s - and t r a n s - s t r u c t u r e s was considered on the basis of the decisive role of the nature of the coordination of the m o n o m e r at the active., center

[29]

297

T A B L E 6. I n f l u e n c e of E l e c t r o n - D o n o r C o m p o u n d s on the S t e r e o s p e c i f i c i t y of the A c t i o n a n d A c t i v i t y of t h e S y s t e m s [15] ( T e m p e r a t u r e of p o l y m e r i z a t i o n 50 ~) Content of Duration Yield of [ units, % [of polyElectron-donor 3: Catalytic system {1,4- 1,4'- .... compound (D) Z4HTNiX Imeriza- p~ teis trans,, ilion, h ~o (C4HTNiC1)2+ NiCI2 The same

12

Tetrahyd-_ro,furan (C4Hg)2S

(C,.Hr)9Ni + NiCI~ The same w

I0

.

56 10 , 9,7 78

Ii

(C~Hs)aP H20

10

0,2

CH2-~-CH (~H ~ C H

2

PCH2CH~CHCH~'M'X n

,sCH~ C4H6 /" "',, - ( - - - - CH

95 88 71 95 72 0

4 11 28 a 23 95,5

",, / /z•H--CH•, ...CH~

H~q " " "

C~H6

,MX n ~

pCH2CH~CHCH2NIXn

~%CH "" CH2P

,,/cu.~,

/%, HQ

?<

PCH2CH~CHCH2CH ~

MX n

~,

HC, "% , -MX n C

H

H

CH~CH~CH~--~CHCH~P

Anti-configuration

Syn- configuration

A n i n c r e a s e in the p o s i t i v e c h a r g e on the t r a n s i t i o n m e t a l a t o m , a s w a s shown, l e a d s to c i s - s t e r e o r e g u l a t i o n . If c o o r d i n a t i o n of the d i e n e on the t r a n s i t i o n m e t a l i s a c c o m p l i s h e d with both d o u b l e bonds (a c i s o i d s t a t e of the m o n o m e r ) , t h e n an a n t i - c o n f i g u r a t i o n of the a c t i v e c e n t e r is r e a l i z e d when it is i n s e r t e d into the c h a i n ( p a t h w a y 1). Upon f u r t h e r d i s p l a c e m e n t of the a n t i - ~ - a l l y l unit into the p o l y m e r c h a i n , a c i s unit is r e a l i z e d . If the c o o r d i n a t i o n of the m o n o m e r is a c c o m p l i s h e d with one d o u b l e bond ( p a t h w a y 2), t h e n the d i e n e a c q u i r e s the m o s t t h e r m o d y n a m i c a l l y p r o b a b l e t r a n s o i d s t a t e , w h i l e the 7 r - a l l y l - a c t i v e c e n t e r r e g e n e r a t e d a f t e r i n s e r t i o n of the d i e n e into the c h a i n s h o u l d t h e r e f o r e h a v e a s y n - c o n f i g u r a t i o n . In s u b s e q u e n t b r e a k d o w n of the c o m p l e x u n d e r the i n f l u e n c e of the c o o r d i n a t e d m o n o m e r , a s y n - u - a l l y l unit is r e a l i z e d in the f o r m of a t r a n s - u n i t . P a t h w a y 3 i n d i c a t e s t h e t h e o r e t i c a l p o s s i b i l i t y of s y n - a n t i - i s o m e r i z a t i o n u n d e r the i n f l u e n c e of e l e c t r o n - a c c e p t o r c o m p o n e n t s ; h o w e v e r , it h a s not y e t b e e n d e m o n s t r a t e d e x p e r i m e n t a l l y . T h e p r o p o s e d s c h e m e is g i v e n a r b i t r a r i l y f o r the m o n o m e r s t a t e of the a c t i v e c e n t e r , a l t h o u g h the l i t e r a t u r e d a t a a v a i l a b l e t h u s f a r i n d i c a t e t h a t the i n i t i a l c o m p l e x e s with c h a r g e t r a n s f e r , l e a d i n g to c i s -

formation of the units, contain the dimer form of 7r-crotyl nickel halide - /./.. c H 2 - . . + / ".c~ ' . . . . c H~,N,. .' CH

Ni

Ni

CF

"%,c./i "',,o/ "'.,cHJ [_ C a

CHa

_

Thus, the polymerization process proceeds through successive steps of decomposition of the rr-allyl complex under the influence of the coordinated monomer and regeneration of the 7r-allyl complex after insertion of the monomer into the chain. The proposed scheme outlines only approaches to the consideration of the mechanism of stereoregulation and is in need of experimental verification. C a t a l y t i c S y s t e m s in w h i c h r r - A l l y l C o m p l e x e s a r e G e n e r a t e d d u r i n g the P r o c e s s

of P o l y m e r i z a t i o n

Transition from Metal Hydrides to 7r-AIlyl Complexes. The source of formation of 7r-allyl complexes may be hydrides of the transition metals [4]. Stoichiometric hydrides of nickel, chromium, and cobalt were produced by interaction of alkyl magnesium halides with anhydrous halides of the transition metals in an atmosphere of hydrogen. At the end of the reaction, a solution of butadiene in a benzene-diethyl ether

298

TABLE 7. M i c r o s t r u c t u r e of Polybutadiene Produced by P o l y m e r i z a t i o n in the P r e s e n c e of Systems Based on Hydrides of the T r a n s i t i o n Metals [4], t = 50 ~ Benzene: Butadiene = 1 : 1 i

Initial

hy-

dride

IDuration of IYield of kctivator polymeriza-]polymer, lion, h

Nickel Cobalt Chromium The same

NiC12 CoClz I N~12

~{a

Content of units, %

1,4-cis1,4-trans

4'2 14 69 O1

6 21 11411

1,2 7

77 17

TABLE 8. M i c r o s t r u c t u r e of Polybutadiene Obtained under the Influence of the Systems M X n - A I C I 3 [32, 33] Content of units, %

NiCI~ NiI2 CoCh

97 95 96

1,5

~,5

2

2

Content of units, %

TIC14 TiCla VCI3

IU:- E,;:-

--

cis

1, 2

i trans 99 99

98

Content of units, % MX n

1,4-ll,4cis

VCh

FeCI~

0

38

Itrans 99 59

1,2 1

3

mixture was added to the precipitates formed. The p r e s e n c e of the t r a n s i t i o n metal in solution and the f o r mation of butenes under the action of acid on the solution or precipitate unambiguously indicate the f o r m a tion of 7r-crotyl derivatives [4] MHn + nC4H~-+ M (C4H7) The solution (after liberation f r o m the ether) and precipitates in the p o l y m e r i z a t i o n of butadiene p o s s e s s the same s t e r e o s p e c i f i c i t y of the action as the c o r r e s p o n d i n g 7r-allyl complexes synthesized by other m e t h ods (Table 7). The product of the addition of butadiene to c h r o m i u m hydride leads to the f o r m a t i o n of a p o l y m e r containing the same number of 1,2-units as in the case of p o l y m e r i z a t i o n under the influence of t r a n s - a l l y l chromium. Solutions containing nickel and cobalt initiated c i s - p o l y m e r i z a t i o n of butadiene only after the addition of nickel chloride or cobalt chloride to the s y s t e m . E a r l i e r [6, 7, 13] it was shown that polyallyl derivatives of nickel and cobalt induce only o l i g o m e r i z a t i o n of butadiene, and only after the addition of nickel chloride or other salts is a catalytic s y s t e m s t e r e o s p e c i f i c for c i s - p o l y m e r i z a t i o n formed. The i n v e r sion of the s t e r e o s p e c i f i c i t y of the action in the s y s t e m of crotyl derivative of c h r o m i u m with NiCI 2 is in full a g r e e m e n t with the r e s u l t s obtained a f t e r the addition of NiC12 to t r i s - c r o t y l c h r o m i u m , synthesized through the o r g a n o m a g n e s i u m compounds [9]. This example illustrates the possibility of migration of the 7r-allyl group f r o m one transition metal to the other. The results obtained confirm the fact that in the case of spontaneous decomposition of organometallic compounds of transition metals, f o r m i n g an M - H bond, there is a r e g e n e r a t i o n of a new active c e n t e r on account of addition of the m o n o m e r at the M - H bond. Stereospecific P o l y m e r i z a t i o n under the Influence of Complexes lV[Xn[A[C~ ~ . As is well known, the catalytic p o l y m e r i z a t i o n of dienes under the influence of Lewis acids (A1CI3, BF3, etc.) leads to the f o r m a tion of amorphous cyclized p o l y m e r s with reduced unsaturation (30-70% of the theoretical) with a p r e d o m i nantly t r a n s - c o n f i g u r a t i o n of the units. However, benzene-soluble complexes of aluminum chloride with halides of the t r a n s i t i o n metals are catalysts with high s t e r e o s p e c i f i c i t y [30-33] (Table 8). Complexes containing NiCI 2 or CoC12 catalyze the same s t e r e o s p e c i f i e c i s - p o l y m e r i z a t i o n as the c o r r e s p o n d i n g ~-allyl complexes. Other complexes, containing V 4+, V3+, and Ti 3+, lead to crystalline trans-polybutadiene; m o r e o v e r , in the f i r s t two c a s e s a p o l y m e r with a v e r y high molecular weight is formed. The f o r m a t i o n of cis-polybutadiene in the case of the complex NiI 2 - (AICI3) n is easily understood if we consider that aluminum chloride, as an e l e c t r o n acceptor, changes the s t e r e o s p e c i f i e i t y of the action of ~-allylnicke[ iodine in the direction of c i s - s t r u c t u r e s . The c o o r d i n a t i o n - i o n i c m e c h a n i s m of the p r o c e s s is confirmed both by the peculiarities of the m i c r o s t r u c t u r e of the chain and by the high unsaturation of the p o l y m e r s , close to the theoretical. These s y s t e m s give m o r e reproducible r e s u l t s in the p r e s e n c e of weak e l e c t r o n donors (thiophene, diphenyl ether, etc.), preventing the development of cationic p r o c e s s e s [32j. It is a s s u m e d that the ~r-allyl catalytic complexes f o r m e d as a r e s u l t of addition of the diene at the M - CI bond

299

TABLE 9. P o l y m e r i z a t i o n of Butadiene under the Influence of Salts of the T r a n s i t i o n Metals [36]

Halides

Content of units, %

~o o

NiC12 Ni 12 t~-TiCls FeCI~

CoC12

g

16 17,5 3 80 90

20

50 50

63,5 33 22,5 12 18

CIH6 CI CI M+ / \AI - / - +

/

93 17 81,5 26 89,5

.~,/ "% CH~ , C..H

75 15,5 59 6,5

2 8 3 15 4

%~""

\GI /

cl

1,2

1,4-cis ll,4-trans

o

\m

MCX.Amla

CH2CI

As is well known, the ~-allyl complex is f o r m e d in a s i m i l a r way in the interaction of butadiene with palladium chloride [34]. Analogous considerations have been e x p r e s s e d in an analysis of the mechanism of the t r a n s - p o l y m e r i z a t i o n of butadiene in the aqueous phase under the influence of rhodium salts [35]. Stereospecific P o l y m e r i z a t i o n under the Influence of Imperfect Salts of Metals of Variable Valence. The s t e r e o s p e c i f i c p o l y m e r i z a t i o n of butadiene is also catalyzed by halides of a number of transition m e t als, subjected to heat t r e a t m e n t under vacuum (300 ~ 3 h) [36]. The experimental data cited in Table 9 show that the s t e r e o s p e c i f i c i t y of the action of the indicated compounds is the same as of the c o r r e s p o n d i n g 7r-allyl compounds of t r a n s i t i o n metals. The reaction, as is assumed, may p r o c e e d through a step of a ~allyl ion radical, f o r m e d in the interaction of butadiene with the metal subhalide (defective crystals), for example, CH~ C4[-I~ + Ni-~(---CI--Ni ,

~CH.

I

/

,

"Cil 2

The Role of Organoaluminum Compounds in Ziegler Catalysis. It s e e m s doubtful that the s t e r e o s p e cific p o l y m e r i z a t i o n of dienes under the influence of Ziegler s y s t e m s also p r o c e e d s through a step of ~allyl complexes, which may a r i s e in the alkylation of the transition metal in the p r e s e n c e of a diene, for example, in the case of the s y s t e m s R2AlCl - N i C I 2 and R 2 A I C I - CoCl2, which are just as s t e r e o s p e c i f i c for the c i s - p o l y m e r i z a t i 0 n of butadiene as the c o r r e s p o n d i n g ~-allyl complexes. The f o r m a t i o n of the latt e r may be r e p r e s e n t e d by the s c h e m e R I~AIC1-]-NiCI~-~ Ni/

cl

(A)

R \AI /

. cih, §

C.Z~H

W

"':NiCI'RAICI2

C[ [,R

(B)

The catalyst f o r m e d (B) evidently r e p r e s e n t s a complex with charge t r a n s f e r or an ion in which the role of the e l e c t r o n a c c e p t o r is played by alkylaluminum chlorides. Compound (A) is less stable than compound (B), and therefore, in the absence of the monomer, gradual decomposition is observed with the formation of reduced f o r m s of the transition metal. To an equal degree this pertains to s y s t e m s containing titanium halides, in which ~-allyl complexes can a r i s e only with reduced f o r m s of the metal P~A1 -{- TIC14-~ R~A1CI-{- RTiCla --+ R-,~ + R+~ + TiCla I~A1 + TiCla-+ R2A1C1-{- RTiC12~-~ fiC4H6TiC12

In Ziegler systems in which titanium compounds participate, all possible stereoisomers can be realized, but the nature of the v-allyl complexes in the corresponding catalytic systems has not yet been established.

300

It is known only that t r a n s - a l l y l t i t a n i u m leads to the 1 , 2 - s t e r e o i s o m e r . The a f o r e m e n t i o n e d also p e r t a i n s to m a n y other s y s t e m s of the Z i e g l e r type, containing compounds of vanadium, c h r o m i u m , cobalt, e t c . , for which the nature of the catalytic complex still r e m a i n s unclarifted. Evidently it can be noted with sufficient a s s u r a n c e that o r g a n o a l u m i n u m compounds p e r f o r m two functions: 1) alkylation of the t r a n s i t i o n metal, which e n s u r e s the p o s s i b i l i t y of t r a n s i t i o n to ~-allyl corapiexes; 2) the f o r m a t i o n of c o m p l e x e s with c h a r g e t r a n s f e r or an anion in which a l k y i a l u m i n u m halides p a r t i c i p a t e . The e f f e c t i v e n e s s of the l a t t e r i n c r e a s e s in the s e r i e s : R2A1CI < RAICI 2 < AIC[ 3. The p a r t i c i p a t i o n of Lewis acids (alkylaluminum halides) in the complex evidently leads to stabilization of the c a r b o n - t r a n s i t i o n m e t al bond. T h e r e f o r e complex f o r m a t i o n with alkylaluminum halides can play a significant role in the polym e r i z a t i o n of olefins as well, i n c r e a s i n g the lifetime of the growing p o l y m e r chain. CON

C LUSI

ON

S

1. New catalytic s y s t e m s simulating the s t r u c t u r e of the active c e n t e r s in the p o l y m e r i z a t i o n of dienes w e r e studied, and it was shown that the s t e r e o s p e c i f i c i t y of t h e i r action is r e l a t e d p r i m a r i l y to the e l e c t r o n i c state of the t r a n s i t i o n m e t a l . 2. The m e c h a n i s m of s t e r e o r e g u l a t i o n was c o n s i d e r e d on the b a s i s of the e x p e r i m e n t a l data. The p o s s i b i l i t y of the f o r m a t i o n of v - a l l y l a c t i v e c e n t e r s in v a r i o u s s y s t e m s containing compounds of the t r a n s i tion m e t a l was d e m o n s t r a t e d . LITERATURE 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18.

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