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elements from Groups V-VII of the periodic table. .... 0.5. 0. 0.1. 7. 1.6. 2s. 22p. 6. 4 tetrahedra l. 6 octahedra l. Cl(-I). 1.8. 1. 1.8. 1. 3.2. 3s...
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4 Coordinated Anionic Polymerization and Polymerization Mechanisms FREDERICK J. K A R O L

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UNIPOL Systems Department, Union Carbide Corporation, Bound Brook, NJ 08805

Early Developments (1950-1965) Developments Since 1965 Catalyst Systems and Chemistry Types of Olefin Monomers and Polymers Polymerization Mechanisms O r i g i n of Stereoregulation Chemically Anchored, Supported Catalysts for Olefin Polymerization General Features of Polymerization C a t a l y s t s , Future Research and Related Areas,

Early Developments (1950-1965) Discoveries i n the l a b o r a t o r i e s of Z i e g l e r and Natta (1-5) in the early 1950s caused a revolution i n polymer and o r g a n o m e t a l l i c c h e m i s t r y . The ability t o p o l y m e r i z e e t h y l e n e a t atmospheric pressure and room temperature was a r e s u l t of extensive studies by Z i e g l e r over many years i n the field of reactions of organometallic compounds with o l e f i n s (6). P r i o r to t h i s discovery extremely high pressure (>20,000 lb/in2) and temperatures (approximately 250 °C) were required to convert ethylene to solid polyethylene. Direct p o l y m e r i z a t i o n of e t h y l e n e by this h i g h p r e s s u r e route had been achieved in the 1930s. This f r e e - r a d i c a l process normally produces branched polyethylenes of the low-density type. Z i e g l e r claimed the discovery of a new process for polyethylene, but acknowledged he had not d i s c o v e r e d a new product. He r e c o g n i z e d its identity with p o l y m e t h y l e n e made from catalyzed decompositions of diazomethane. Ziegler and Gellert (6) in 1949 showed t h a t aluminum h y d r i d e r e a c t s w i t h e t h y l e n e at 60-80 °C t o yield t r i e t h y l a l u m i n u m . At 100-120 °C reaction with a d d i t i o n a l ethylene leads to formation of h i g h e r alkyls of aluminum ( R e a c t i o n 1). At temperatures above 120 C h i g h e r aluminum alkyls r e a c t w i t h e t h y l e n e through a 0097 6I56/85/0285-0069$07.50/0 © 1985 American Chemical Society

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displacement r e a c t i o n to g i v e o l e f i n s and t r i e t h y l aluminum ( R e a c t i o n 2). These r e a c t i o n s r e p r e s e n t a c a t a l y t i c process f o r the conversion of ethylene i n t o higher a - o l e f i n s . 100-120 °C al-C H 2

5

+ nCH =CH 2

> a l -«fCH -CH >-C H

2

2

2

2

(1)

5

120-250 °C a l - 4 C H - C H } — C H + CH =CH n Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 24, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch004

2

2

2

5

CH =CH-4CH -CH ^ ri-1 2

2

2

2

>

2

C H + al-C H 2

5

2

(2)

5

where a l = 1/3 A l In the course of these i n v e s t i g a t i o n s , an experiment was carried out to prepare hexyl and o c t y l d e r i v a t i v e s of aluminum by reaction of triethylaluminum with ethylene. Instead of the a n t i cipated aluminum a l k y l s , an almost q u a n t i t a t i v e y i e l d of 1-butene was o b t a i n e d . A f t e r a strenuous i n v e s t i g a t i o n , Z i e g l e r and h i s coworkers found that an extremely s m a l l trace of m e t a l l i c n i c k e l caused t h i s change i n the course of the r e a c t i o n . The n i c k e l , present from a p r e v i o u s hydrogenation experiment, c a t a l y z e d the displacement reaction (Reaction 2) of 1-butene from butylaluminum ( R e a c t i o n 3). 100-120 °C al-C H 2

5

+ C H 2

> al-CH -CH -C H

4

2

2

2

5

Ni C H 2

(3)

4

al—C Htj + CH or hydride compound (Group I-IV) (Group IV-VIII)

active Z i e g l e r catalyst (4)

Z i e g l e r and h i s coworkers were p r i m a r i l y interested i n ethylene polymerization and copolymerization with a - o l e f i n s . After Z i e g l e r

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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4.

KAROL

Coordinated Anionic Polymerization and Mechanisms

71

revealed d e t a i l s of h i s work to Montecatini, Natta, working with combinations of Z i e g l e r - t y p e c a t a l y s t s , discovered stereoregular polymers of p r o p y l e n e , 1-butene, s t y r e n e , e t c . (2, 3}. Z i e g l e r c a t a l y s t s containing h i g h l y ordered ( c r y s t a l l i n e ) t r a n s i t i o n metal s a l t s i n a lower v a l e n c e s t a t e , f o r example, T i C l g and V C l g , polymerize a - o l e f i n s to c r y s t a l l i n e stereoisomeric polymers. Under the d i r e c t i o n of N a t t a , b a s i c p r i n c i p l e s of c o n t r o l l i n g stereoregularity were established ( 7 - 9 ) . For contributions i n t h i s a r e a , Z i e g l e r and N a t t a were awarded the 1963 Nobel P r i z e f o r chemistry. Independent c a t a l y s t research, c a r r i e d out by s e v e r a l U. S. o i l companies i n the e a r l y 1950s with t r a n s i t i o n metal oxides supported on r e f r a c t o r y metal o x i d e s , l e d to the d i s c o v e r y of some of the e a r l i e s t low-pressure c a t a l y s t s for o l e f i n polymerization (10-13). These c a t a l y s t s g e n e r a l l y c o n s i s t of o x i d e s of t r a n s i t i o n metal elements from Groups V-VII of the periodic t a b l e . For c a t a l y t i c a c t i v i t y the t r a n s i t i o n metal oxides are supported on high-surfacearea s o l i d s such as s i l i c a , a l u m i n a , s i l i c a - a l u m i n a , and c l a y . Silica-supported chromium t r i o x i d e (Cr0g/Si02) c a t a l y s t i s the most important t r a n s i t i o n metal oxide c a t a l y s t for ethylene p o l y m e r i z a t i o n (12). E t h y l e n e horaopolymers made w i t h these c a t a l y s t s are predominantly l i n e a r , h i g h - d e n s i t y p r o d u c t s . With propylene and higher l i n e a r and branched a - o l e f i n s , polymerization rates, polymer y i e l d s , and degree of c r y s t a l l i n i t y are much lower than for the polymerization of ethylene. Developments Since 1965 Developments toward higher a c t i v i t y Q>200 kg polymer/g T i vs. 15 kg polymer/g T i ) Z i e g l e r - N a t t a c a t a l y s t s during the l a s t 15 years have, to a considerable extent, been based on reaction of s p e c i f i c magnesium, t i t a n i u m , and aluminum compounds (14-19). C a t a l y s t s , chemically anchored on Mg(0H)Cl-type supports, provided some of the e a r l y impetus i n the area of high a c t i v i t y systems (20, 21). Other studies concentrated on the use of MgCl2 as a substrate. Grinding of MgCl2 and treatment w i t h T i C l ^ p r o v i d e d one route to a h i g h e r s u r f a c e area s u b s t r a t e of magnesium and t i t a n i u m (22-24). Some developments focused on reaction products of magnesium a l k y l s and titanium compounds (25-27). Other workers described the advantages of p r e p a r i n g t r i m e t a l l i c sponges by the a d d i t i o n of c e r t a i n aluminum compounds to a magnesium substrate that had been treated w i t h a t i t a n i u m compound (28). C a t a l y s t s based on r e a c t i o n products of magnesium a l k o x i d e s w i t h t r a n s i t i o n metal compounds have a l s o received attention (29). During c a t a l y s t preparation the o r i g i n a l s t r u c t u r e of the a l k o x i d e i s d e s t r o y e d , and a new c r y s t a l l i n e species of higher surface area i s formed. The d i s c o v e r y of e t h e r - t r e a t e d TiClo-based c a t a l y s t s of high a c t i v i t y and s t e r e o s p e c i f i c i t y , p a r t i c u l a r l y for the polymerization of propylene, has been of considerable importance (30, 31). These c a t a l y s t s are prepared by reduction of T i C l ^ with aluminum a l k y l s and subsequent treatment w i t h Lewis bases such as e t h e r s . The t r e a t e d T i C l g product can be transformed to a h i g h l y a c t i v e , stereospecific c a t a l y s t by treatment with T i C l ^ . A t t r a c t i v e , h i g h - a c t i v i t y c a t a l y s t s for propylene polymerizat i o n have a l s o been d e s c r i b e d (32, 33). M o n t e d i s o n / M i t s u i

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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c a t a l y s t s comprise an aluminum a l k y l complexed w i t h a e l e c t r o n donor such as e t h y l benzoate, and a s o l i d m a t r i x c o n t a i n i n g the reaction products of halogenated magnesium compounds with a Ti(IV) compound and an e l e c t r o n donor. The s p e c i f i c surface area of the s o l i d matrix after treatment i s i n the range of 100-200 n r / g . High c a t a l y s t p r o d u c t i v i t i e s based on titanium have been reported for polymerizations with these c a t a l y s t s . The d i r e c t use of organometal l i e compounds of t r a n s i t i o n metals for the preparation of s o l i d c a t a l y s t s for o l e f i n polymerization, p a r t i c u l a r l y e t h y l e n e p o l y m e r i z a t i o n , d e v e l o p e d i n the 1960s. C a t a l y s t s obtained by supporting ir-cyclopentadienyl, i r - a l l y l , and O-organometallic compounds of t r a n s i t i o n metals, such as titanium, zirconium, and chromium, proved to be h i g h l y a c t i v e for ethylene p o l y m e r i z a t i o n (34). A chromocene c a t a l y s t , ( C ^ H ^ o C r / S i C ^ , has been d e s c r i b e d i n some d e t a i l (35). C a t a l y s i s by supported complexes of t r a n s i t i o n metals has recently been w e l l documented (19), and much a c t i v e research continues i n t h i s area. Catalyst Systems and Chemistry Before the 1970s, Z i e g l e r - N a t t a c a t a l y s t s for a - o l e f i n production were normally prepared from c e r t a i n compounds of t r a n s i t i o n metals of Groups I V - V I of the p e r i o d i c t a b l e ( T i , V , C r , e t c . ) i n combination w i t h an organometal l i e a l k y l or a r y l ( T a b l e I ) . P r a c t i c a l l y a l l subhalides of t r a n s i t i o n metals have been claimed as c a t a l y s t s i n stereoregular polymerization. Only those elements with a f i r s t work function TiCl

3

3

3

+ R A1C1 2

+ [R*]

(5) (6)

The fate of the a l k y l fragment [R*] remains uncertain. There i s s t i l l c o n t r o v e r s y c o n c e r n i n g the p r e c i s e extent of r e d u c t i o n that i s reached with t r i a l k y l a l u m i n u m compounds at different r a t i o s of aluminum to titanium. However, s c i e n t i s t s g e n e r a l l y agree that the more a c t i v e organometal l i e compounds such as t r i a Iky laluminum p r o v i d e more e x t e n s i v e a l k y l a t i o n , and r e d u c t i o n o f t h e i n t e r m e d i a t e 3-form of T i C l may occur ( R e a c t i o n s 7-10). T h i s r e d u c t i o n process l e a d s to lower v a l e n c e s t a t e s presumable by reactions of the f o l l o w i n g type: 3

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4.

KAROL

Coordinated Anionic Polymerization and Mechanisms RT1CI3 + R3AI R TiCl

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3

2

2

+ R A1C1 2

(7)

> RTiCl

2

+ [R*]

(8)

+ R3AI

> RTiCl

2

+ R A1C1

(9)

RTiCl

>TiCl +[R-]

2

B-TiCl

> R TiCl

73

2

2

2

2

(10)

Intermediate a l k y l t i t a n i u m h a l i d e s and the titanium subhalides remain, i n most cases, t i g h t l y complexed w i t h the product organoaluminum compounds. A l a r g e number of compounds and complexes are p o s s i b l e , of which o n l y a few are c a t a l y t i c a l l y active. Because l o w - v a l e n t t r a n s i t i o n m e t a l compounds are e l e c t r o n - d e f i c i e n t m o l e c u l e s , they w i l l attempt to expand t h e i r coordination number by sharing l i g a n d s between two metal c e n t e r s with the formation of b i m e t a l l i c complexes. One cannot be c e r t a i n whether the compounds and complexes that are i s o l a t e d are the true c a t a l y s t s or are merely precursors of other compounds that are the true c a t a l y s t s . By analogy w i t h these r e a c t i o n s , the s o l i d c r y s t a l l i n e s u r f a c e of t i t a n i u m d i c h l o r i d e or t r i c h l o r i d e and organoaluminum compounds i n s o l u t i o n might be expected to undergo s i m i l a r reactions. B i m e t a l l i c complexes have been suggested to be formed p r i m a r i l y at s i t e s of s t r u c t u r a l defects such as edges, step f a u l t s , and c h l o r i n e vacancies where the hexacoordination a b i l i t y of exposed ions would be incompletely s a t i s f i e d (37). For the h i g h e r a c t i v i t y Z i e g l e r - N a t t a c a t a l y s t s ( T a b l e I I ) based on r e a c t i o n products of s p e c i f i c magnesium, t i t a n i u m , and aluminum compounds, the s i m i l a r i t y i n s i z e , coordination preference, e l e c t r o n i c s t r u c t u r e , and e l e c t r o n e g a t i v i t y of T i ( I V ) , Mg(II), and A l ( I I I ) ions i s r e f l e c t e d i n s t r u c t u r a l parameters and c h e m i c a l p r o p e r t i e s (38) ( T a b l e I I I ) . The s i m i l a r i t y i n s i z e between Mg(II) and T i ( I V ) p r o b a b l y permits an easy s u b s t i t u t i o n between ions i n a c a t a l y s t framework. The r o l e of magnesium i o n s i n h i g h a c t i v i t y Z i e g l e r - N a t t a c a t a l y s t s has r e c e i v e d some recent a t t e n t i o n w i t h p a r t i c u l a r emphasis on four points (39): 1.

2. 3. 4.

T i t a n i u m c e n t e r s may be d i l u t e d by magnesium i o n s t h a t influence the number of a c t i v e centers. This d i l u t i o n effect i n c r e a s e s the number of a c t i v e c e n t e r s t h a t tend to be i s o l a t e d . The a c t i v e centers are at l e a s t an order of magnitude higher than the e a r l i e r Z i e g l e r - N a t t a c a t a l y s t s . The presence of magnesium i o n s s t a b i l i z e s a c t i v e t i t a n i u m c e n t e r s from d e a c t i v a t i o n processes r e l a t i v e to s o l u b l e systems. The presence of magnesium i o n s enhances c h a i n - t r a n s f e r p r o cesses because the number-average m o l e c u l a r weight decreases when the Mg/Ti r a t i o increases. The presence of magnesium ions leads to c a t a l y s t s that provide p o l y e t h y l e n e s w i t h a narrow m o l e c u l a r weight d i s t r i b u t i o n (fflw/Mn ca.3-5).

Several reviewers have attempted to summarize e x i s t i n g data on the determination of propagation rate constants (k ) and the number of a c t i v e centers (C*) i n o l e f i n polymerization (40-43). Although

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Table I .

Selected First-Generation Ziegler-Natta Catalysts

T r a n s i t i o n Metal Compound TiCl

4

TiCl

3

Polymer

Metal A l k y l

polyethylene

(C H ) A1 2

5

3

(C H ) A1C1

i s o t a c t i c polypropylene

2

5

2

vci

4

(CoHc^oAlCl

syndiotactic polypropylene

vci

4

(i-C H ) Al

poly-4-methyl-l-pentene

(C H ) A1C1

cis-1,4-polybutadiene

4

Soluble cobalt s a l t

Table I I .

2

9

5

3

2

High-Activity Ziegler-Natta Catalysts

Titanium/Magnesium Composition

Metal A l k y l

TiCl /MgC H Br

(C H ) A1

polyethylene

(C H ) A1

polyethylene

(C H ) A1

polyethylene

T i C l / M g C l / e l e c t r o n donor ( C H ) A 1

polyethylene

4

8

17

TiCl /Mg(0C H ) 4

2

TiCl /MgCl 4

4

5

2

2

2

(activated)

2

3

2

3

5

2

TiCl /MgCl / ethyl-p-toluate

3

5

3

(C H ) A1C1 2

3

2

3

5

2

6TiCl /AlCl /ether 4

5

j (

5

2

(C H ) A1ethyl-p-toluate 2

5

3

Polymer

i s o t a c t i c polypropylene i s o t a c t i c polypropylene

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985. 6

6

6

6

2

2s 2p

2

2s 2p

2

2

1.3

1.6

3.2

0.33

0.17

1.81

0.65

0.50

1.81

Mg(II)

Al(III)

Cl(-I)

3s 3p

3s 3p 3d°

1.5

0.17

0.68

Electronic Structure

Ti(IV)

Electronegativity

Radius (A)

Size/ Charge

tetrahedral octahedral tetrahedral octahedral

4 6 4 6

bent-bridginj

octahedral

6

2

trigonal bypyrimidal

5



tetrahedral

4

1

Geometry

Coordination Number

Geometric and E l e c t r o n i c Properties of Ions of Catalyst Components

Ion

Table I I I .

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the a b s o l u t e v a l u e s f o r the r a t e c o n s t a n t s and a c t i v e c e n t e r s sometimes d i f f e r from one r e v i e w e r to a n o t h e r , some c o n c l u s i o n s appear to be g e n e r a l l y a c c e p t e d . R e s u l t s of experiments to determine the number of a c t i v e c e n t e r s have i n d i c a t e d t h a t f o r f i r s t - g e n e r a t i o n Z i e g l e r - N a t t a c a t a l y s t s only a s m a l l f r a c t i o n of the t o t a l amount of t r a n s i t i o n metal compound i s c a t a l y t i c a l l y a c t i v e at any s p e c i f i c time. Generally for titanium c a t a l y s t s of low p r o d u c t i v i t y , C* values range from 1 0 ~ - l C f m o l / m o l titanium compound. The a c t i v e s i t e c o n c e n t r a t i o n i n T i C l o - b a s e d Z i e g l e r Natta c a t a l y s t s has been c a l c u l a t e d to be 1 0 ~ - l u mol/mol T i C l ^ (44, 45). S i m i l a r l y , an a c t i v e s i t e concentration ( 5 x 1CT m o l / m o l T i C l ^ ) was c a l c u l a t e d f o r an a l k y l a l u m i n u m - t i t a n i u m t e t r a c h l o r i d e c a t a l y s t (46). I f the a c t i v e s i t e s are assumed to be titanium centers, only a very s m a l l proportion of these centers i n the T i C l g s o l i d i s a c t i v e as p o l y m e r i z a t i o n s i t e s . Some p o t e n t i a l l y a c t i v e s i t e s may not be used because they are i n a c c e s s i b l e or deactivated by i m p u r i t i e s i n the system. Titanium c a t a l y s t s of h i g h e r p r o d u c t i v i t y ( s e c o n d / t h i r d generation c a t a l y s t s ) frequently show higher C* values of 7 x l C f ^ - l O " m o l / m o l t i t a n i u m compound. Some u n c e r t a i n t y c o n t i n u e s to e x i s t about the v a l u e of k i n h i g h a c t i v i t y c a t a l y s t s because of the decay of c a t a l y t i c a c t i v i t y with time (47, 48). 2

2

3

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2

2

Types of Olefin Monomers and Polymers Z i e g l e r - N a t t a c a t a l y s t s can polymerize a v a r i e t y of s t r u c t u r a l l y different monomers. Examples of stereoregular homopolymers (Table I V ) , e l a s t o m e r i c or c r y s t a l l i n e c o p o l y m e r s , as w e l l as b l o c k copolymers may be found i n the patent and open l i t e r a t u r e (4^, 4 9 51). E t h y l e n e p o l y m e r i z e s e a s i l y w i t h many s o l u b l e and heterogeneous Z i e g l e r c a t a l y s t s . Some e t h y l e n e - a c t i v e c a t a l y s t s , for example, Cp2TiCl2 + aluminum a l k y l (52), are not a c t i v e for ao l e f i n polymerizations. However, a l l known Z i e g l e r c a t a l y s t s that polymerize propylene are a l s o a c t i v e i n ethylene polymerization. Many a - o l e f i n s , i n a d d i t i o n to propylene, have been polymerized to i s o t a c t i c polymers. The r e a c t i v i t y of the o l e f i n diminishes as the s i z e of the o l e f i n i n c r e a s e s , f o r example, e t h y l e n e > propylene > 1-butene > 4-methy 1 - 1 - p e n t e n e . Reactivity also d i m i n i s h e s as branching comes c l o s e r to the double bond, f o r example, 4-methy1-1-pentene > 3-methy1-1-pentene. The lower polymerization a c t i v i t y of higher a - o l e f i n s has been ascribed to d i f f i c u l t y i n approach or coordination to the a c t i v e s i t e . Reports of p o l y m e r i z a t i o n of i s o b u t y l e n e (2-methylpropene) w i t h T i C l ^ Z i e g l e r c a t a l y s t are now a t t r i b u t e d to a c a t i o n i c polymerization i n i t i a t e d by r e s i d u a l T i C l ^ present i n the p a r t i c u l a r Z i e g l e r catalyst. Attempts to polymerize i n t e r n a l , n o n c y c l i c o l e f i n s such as c i s - and trans-2-butene have not been successful (53). However, under s p e c i a l c o n d i t i o n s N a t t a and coworkers (54) were a b l e to copolymerize ethylene with c i s - and trans-2-butene, cyclopentene, cyclohexene, cycloheptene, and cyclooctene. Homopolymerization of many c y c l o o l e f i n s has been reported (55). Polymerization can occur with these o l e f i n s by 1,2-addition to the double bond or by various r i n g opening p r o c e s s e s . C h o i c e of c a t a l y s t components and polymerization conditions determine the mode of polymerization.

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Table I V . Polymer Nomenclature and Structure Structure

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Nomenclature

Isotactic

Polypropylene

VW\,CH -CH-CH -CH-CH -CH'WVA, 2

2

CH

2

CH

3

CH

CH3

3

3

I Syndiotactic

Polypropylene wv\,CH -CH-CH -CH-CH -CH'vw\, 2

2

CH

cis-1

Poly butadiene

CH

3

C = C

vwbCH

2

WA,CH -CH-CH -CH-CH -CH 2

2

2

I

I

CH

CH

II CH

Syndiotactic

1,2-Polybutadiene

3

C = C

trans-1,^f-Polybutddiene

Isotactic 1,2-Polybutadiene

2

' W W C H

2

II CH

2

- C H - C H

CH

2

II CH

2

- C H - C H

2

2

- C H '

CH

I

CH

I CH

II 2

CH

2

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

W

V

^

78

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Some Z i e g l e r - t y p e c a t a l y s t s have been used to p o l y m e r i z e terminal acetylenes (56, 57). Highly dispersed or s o l u b l e c a t a l y s t s based on T i C l , Ti(0R) , metal chelates (Co, N i , V, Fe) p l u s A l E t g have been most s u c c e s s f u l . A c e t y l e n e s , u n l i k e a - o l e f i n s , polymerize with Group VIII t r a n s i t i o n metal compounds. With some a c e t y l e n e s , t r i m e r i z a t i o n r e a c t i o n s leading to the corresponding substituted benzene d e r i v a t i v e s take place. Nonconjugated dienes of the type ^C^CH^CH^jjCHsCH? have been p o l y m e r i z e d by 1,2- and c y c l o a d d i t i o n r o u t e s (58). conjugated dienes are r e a d i l y p o l y m e r i z e d by Z i e g l e r c a t a l y s t s (59). By proper s e l e c t i o n of c a t a l y s t i t i s f e a s i b l e to prepare polymers h a v i n g any d e s i r e d s t r u c t u r e ( T a b l e V ) . I t i s p o s s i b l e i n some cases to change the type of s t r u c t u r a l u n i t s i n the polymer by merely a l t e r i n g the r a t i o of c a t a l y s t components. F a c t o r s t h a t determine stereoregulation i n these polymers include the types of metal l i g a n d s , c r y s t a l s t r u c t u r e of the t r a n s i t i o n metal s a l t , s p e c i f i c t r a n s i t i o n m e t a l , r e l a t i v e c o n c e n t r a t i o n s of c a t a l y s t components, and experimental conditions. Polymerization studies with polar monomers indicate that some of these monomers can be polymerized at Z i e g l e r - t y p e s i t e s (4, 5). F r e q u e n t l y secondary r e a c t i o n s often prevent propagation from o c c u r r i n g . The p o l a r monomer may complex or r e a c t i r r e v e r s i b l y w i t h one or both of the c a t a l y s t components, or e l s e one of the c a t a l y s t components may serve as a r a d i c a l or c a t i o n i c i n i t i a t o r f o r p o l y m e r i z a t i o n of the monomer. P r e v e n t i o n of these s i d e reactions permits a more favorable Z i e g l e r - t y p e polymerization.

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4

4

Polymerization Mechanisms The chemistry of c a t a l y s t formation and the nature of a c t i v e s i t e s have been e x t e n s i v e l y debated s i n c e the i n i t i a l d i s c o v e r i e s of Z i e g l e r and Natta (4, 5^, 36, 60). The k i n e t i c s and mechanism have been the subject of numerous studies. Nearly a l l p o s s i b i l i t i e s i n a s s i g n i n g the nature of the a c t i v e s i t e have been exhausted by p r o p o s a l s of d i f f e r e n t t h e o r i e s . A c o n s i d e r a b l e amount of experimental evidence has been reported frequently to c l a i m proof that a p a r t i c u l a r mechanism i s operative. One may c l a s s i f y these proposed mechanisms a c c o r d i n g to the charge d i s t r i b u t i o n i n the t r a n s i t i o n s t a t e of the propagation r e a c t i o n , v i z , c o o r d i n a t e d anionic, coordinated c a t i o n i c , and coordinated r a d i c a l , i n accordance w i t h whether the propagating polymer c h a i n i s c o n s i d e r e d a c a r b a n i o n , a carbonium i o n , or a r a d i c a l s p e c i e s . " C o o r d i n a t e " indicates the common feature of complexation of the o l e f i n before i n t r o d u c t i o n i n t o the growing c h a i n (18). In the case of a o l e f i n s , proposals that have received most attention are those of the coordinate-anionic type i n which the coordinated monomer enters the chain through a catalyst-polymer bond p o l a r i z e d i n the sense M - R ° ~ . P r o p o s a l s f o r c o o r d i n a t e anionic polymerization may be f u r t h e r d i s t i n g u i s h e d i n accordance w i t h whether the t r a n s i t i o n metal, or the base metal center, or a b i m e t a l l i c complex i n v o l v i n g both centers i s considered the s i t e of chain growth. Propagation at one metal center of a b i m e t a l l i c complex would be c l a s s i f i e d i n the monometallic category. B i m e t a l l i c mechanisms that employ two a +

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Coordinated Anionic Polymerization and Mechanisms

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different metals have been proposed by different workers (37, 61, 62). One p r o p o s a l f o r an a c t i v e s i t e model f o r the b i m e t a l l i c mechanism would be

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C l \ ^CK ^ R j;Tr' ^ A l ^ OAT ^'Pn-'' ^ R where Pn i s the growing polymer c h a i n . P r o p a g a t i o n occurs by coordination of the o l e f i n to the titanium center with cleavage of the t i t a n i u m - p o l y m e r p a r t i a l bond. The polymer c h a i n i n t h i s mechanism i s always bound, at l e a s t p a r t i a l l y , to aluminum (Reaction 11). CH -P

CH -P

2

2

.Al

TIC

+

CH = CH ?

Tu

2

CH kH 2

CH -P

CH -CH 2

xJ + ) ^ C H ^ ^Ti ^Al 2

2

2

(-) ( + )

2

|

CH =CH

^Al,

x

< +)

2



•» T i .CH -CH -P ' ^R — - A l ^ N

2

(11)

2

2

CH -CH -CH -P

CH -CH -CH -P 2



2

3

2

Si-OH

Reduction

Si-Q\ I 0 1 •Si-0

\Cr + oxidation products ( (C0 , H 0) / 2

2

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89

Initiation Reaction uncertain, but believed to result in formation of divalent chromium hydride or a l k y l . Propagation Jr-R + nCH =CH 2



2

^Cr-4CH -CH ^R, etc. 2

(19)

2

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R = hydride or alkyl The r a t e and e x t e n t to which s i l i c a - b a s e d c a t a l y s t s fracture d u r i n g the p o l y m e r i z a t i o n of e t h y l e n e have been d i s c u s s e d (96). Fragmentation of the c a t a l y s t was complete within the f i r s t minute or two of polymerization, whereas the rate of reaction continued to increase for more than an hour. With the Z i e g l e r c a t a l y s t , R3AI + T i C l ^ supported on magnesium hydroxychloride, chemisorption can be represented by Reaction 20. TiCl

+ Mg(0H)Cl

4

> Cl TiOMgCl + HC1

(20)

3

Reduction and c h a i n p r o p a g a t i o n p r o b a b l y occur i n a manner described e a r l i e r for unsupported Z i e g l e r c a t a l y s t s . Chromocene deposited on s i l i c a supports forms a h i g h l y a c t i v e c a t a l y s t f o r p o l y m e r i z a t i o n of e t h y l e n e (35, 89). The c a t a l y s t formation step l i b e r a t e s c y c l o p e n t a d i e n e and l e a d s to a new d i v a l e n t chromium s p e c i e s c o n t a i n i n g a cyclopentadienyl l i g a n d . P o l y m e r i z a t i o n i s b e l i e v e d to occur by a c o o r d i n a t e d a n i o n i c mechanism (Reaction 21) o u t l i n e d e a r l i e r .

2

(21)

Cr-4CH -CH2^R, etc.

nCH =CH

2

2

The presence of the cyclopentadienyl ligand at the chromium center p r o v i d e s a c a t a l y s t w i t h a unique h i g h response to hydrogen as a chain transfer agent (97). A number of TT- and a-bonded t r a n s i t i o n metal compounds i n s o l u t i o n or supported (34) have been described as polymerization c a t a l y s t s . Unsupported and supported t r a n s i t i o n m e t a l - a l l y l compounds have been proposed to i n i t i a t e polymerization by reaction with monomer i n a manner i l l u s t r a t e d by Reaction 22 for ( a l l y l ) Z r B r / S i 0 and ethylene. 3

2

CH

CH

2

-Si-0 \

/ C H Zr / / P C M , Br 5?—Si-0

CH =CH 2

2

\ >0 I / -Si-0

2

/ CH Zr | CH -CH=CH Br 2

N

2

2

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General Features of Polymerization Catalysts The current statue of o l e f i n and diene polymerizations catalyzed by t r a n s i t i o n metal compounds that function by a coordinated anionic mechanism suggests a number of general conclusions. 1.

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2.

3.

Many d i s t i n c t c a t a l y s t types are p o s s i b l e . The voluminous patent l i t e r a t u r e and s c i e n t i f i c publications indicate the vast scope of t r a n s i t i o n metal catalyzed polymerization of monomers. Propagation occurs by monomer coordination and i n s e r t i o n i n t o a t r a n s i t i o n metal-carbon bond. The t o t a l experimental evidence strongly supports the proposal that the a c t i v e s i t e i n Z i e g l e r c a t a l y s t s i s a t r a n s i t i o n m e t a l - c a r b o n bond, and t h e propagation r e a c t i o n c o n s i s t s of repeated i n s e r t i o n of the o l e f i n into t h i s bond. S e v e r a l r o u t e s to t r a n s i t i o n metal-carbon bond e x i s t . Trans i t i o n metal carbon bonds may be generated by a l k y l a t i o n of a t r a n s i t i o n metal compound w i t h a metal a l k y l . L o w - v a l e n t t r a n s i t i o n metal compounds, per se, t h a t i s , T i C ^ t may f u n c t i o n as c a t a l y s t s . Reduction by the o l e f i n t h a t occurs with the C r 0 / S i 0 2 c a t a l y s t may a l s o provide s i t e s for p o l y m e r i z a t i o n . F i n a l l y , t r a n s i t i o n metal compounds i n s o l u t i o n (34) or supported may function as polymerization s i t e s . Ligand environment at a c t i v e s i t e s plays a s i g n i f i c a n t r o l e i n polymerization behavior. Ligand e f f e c t s i n diene p o l y m e r i z a t i o n (8_7, .88) and work w i t h supported chromocene c a t a l y s t s (98) d r a m a t i c a l l y i l l u s t r a t e t h i s point. I s o t a c t i c placements o r i g i n a t e from catalyst-monomer i n t e r a c t i o n s . These placements do not r e q u i r e the p a r t i c i p a t i o n of metal a l k y l i n the a c t i v e s i t e . Syndiotactic placements originate from nonbonded i n t e r a c t i o n s between the monomer molecule undergoing i n s e r t i o n and ligands on the vanadium atom. C a t a l y s t supports can lead to or improve polymerization a c t i v i t y by generating or increasing a c t i v e s i t e concentration. Magnesium compounds or i o n s i n h i g h a c t i v i t y c a t a l y s t s p l a y s e v e r a l r o l e s by increasing the number and s t a b i l i t y of a c t i v e t r a n s i t i o n metal centers. The presence of magnesium enhances c h a i n t r a n s f e r processes and can p r o v i d e p o l y e t h y l e n e s of narrow molecular weight d i s t r i b u t i o n . 3

4.

5. 6. 7. 8.

Future Research and Related Areas A f t e r 30 y e a r s , o l e f i n p o l y m e r i z a t i o n by a c o o r d i n a t e d a n i o n i c mechanism continues to receive worldwide attention as evidenced by a voluminous patent and j o u r n a l l i t e r a t u r e . Much a t t e n t i o n has been d i r e c t e d to c a t a l y s t and process o p t i m i z a t i o n and unders t a n d i n g of key r e a c t i o n v a r i a b l e s . The development of h i g h a c t i v i t y Z i e g l e r - N a t t a c a t a l y s t s has spurred a renewed i n t e r e s t i n s i m p l i f i e d processes requiring no post-treatment of the polymers. Recent announcements by Union Carbide of a low-pressure, f l u i d bed

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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process to produce granular, low-density polyethylenes have caused a r e v o l u t i o n i n the p o l y e t h y l e n e f i e l d (99, 100). A whole new generation of low-density polyethylenes, described as l i n e a r lowdensity polyethylenes, has appeared (101, 102). C a t a l y s t research i n t h i s area continues to be intense with considerable emphasis on copolymerization k i n e t i c s , and a l s o c o n t r o l of c a t a l y s t morphology to r e g u l a t e polymer morphology. Announcements by Montedison and M i t s u i P e t r o c h e m i c a l s and by o t h e r s i n regard to h i g h - m i l e a g e c a t a l y s t s for i s o t a c t i c polypropylene w i l l a l s o continue to receive worldwide attention (32). The advent of the energy c r i s i s has caused us to examine t r a d i t i o n a l views of the r e l a t i v e costs of different monomers and to consider the p o t e n t i a l of l e s s c o s t l y monomers for polymerizat i o n . One can expect t h a t c a t a l y s i s of the c o o r d i n a t e d a n i o n i c type w i l l play a major r o l e i n any new developments i n o l e f i n and diene polymerizations. F i n a l l y , one s h o u l d r e c a l l t h a t Z i e g l e r c a t a l y s t s have found many uses i n other areas of chemistry such as m e t a t h e s i s of o l e f i n s , o l i g o m e r i z a t i o n , i s o m e r i z a t i o n , h y d r o genation, and a l k y l a t i o n . The vast scope of these c a t a l y s t s w i l l almost c e r t a i n l y a c h i e v e a wider range as these types of s t u d i e s continue i n the future. L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Z i e g l e r , K.; Holzkamp, E . ; Briel, H . ; Martin, H. Angew. Chem. 1955, 67, 541. N a t t a , G. J. Polym. Sci. 1955, 16, 143. N a t t a , G.; P i n o , P.; Corradini, P; Danusso, F . ; M a n t i c a , E . ; M a z z a n t i , G.; M o r a g l i o , G. J. Am. Chem. Soc. 1955, 77, 1708. Boor, J., Jr. In " M a c r o m o l e c u l a r Reviews"; Peterlin, A . , et al., Eds.; I n t e r s c i e n c e : New York, 1967; Vol. 2, pp. 115-268. Boor, J., J r . " Z i e g l e r - N a t t a Catalysts and P o l y m e r i z a t i o n s " ; Academic: New York-San Francisco-London, 1979. Z i e g l e r , K. In "Advances in Organometallic Chemistry"; Stone, F. G. A . ; West, R, Eds.; Academic: New York-San F r a n c i s c o London, 1979. Natta, G. Angew. Chem. 1956, 68, 393. Natta, G. Mod. P l a s t i c s 1956, 34(4), 1969. Natta, G. Angew. Chem. 1964, 76, 553. P e t e r s , E. F . ; Zletz, A.; E v e r i n g , B. L Ind. Eng. Chem. 1957, 49, 1879. C l a r k , A . ; Hogan, J. P.; Banks, R. L.; L a n n i n g , W. C. Ind. Eng. Chem. 1956, 48, 1152. Hogan, J. P.; Banks, R. L. U. S. Patent 2 825 721, 1958. Sailors, H. R.; Hogan, J . P. Macromol. Sci. Chem. 1981, A(7), 1377. F r i e d l a n d e r , H. N. In "High Polymers"; R a f f , R. A. V . ; Doak, K. W., Eds.; I n t e r s c i e n c e : New York, 1965; Vol. XX, pp. 215-66. K a r o l , F. J . In " E n c y c l o p e d i a of Polymer Science and Technology, Supplement I"; Mark, H. F . ; Bikales N. M . , Eds.; Interscience: New York, 1976; pp. 120-46. W e i s s e r m e l , K . : Cherdron, H.; B e r t h h o l d , J.; D i e d r i c h , B . ; Keil, K. D.; Rust, K.; Strametz, H . ; T o t h , T. J. Polym. S c i . Symp. 1975, 51, 187 and references therein.

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16.

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