Applied Polymer Science - ACS Publications - American Chemical

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

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

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A P P L I E D P O L Y M E R SCIENCE

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



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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



72


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


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Coordinated Anionic Polymerization and Mechanisms RT1CI3 + R3AI R TiCl


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


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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

(CoHcôAlCl

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. 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 .


76


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


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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11

Table I V . Polymer Nomenclature and Structure Structure


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


W

V

^

78


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.


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 +


4.

79


KAROL

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


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 Âl 2

2

2

(-) ( + )

2

|

CH =CH

Âl,

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


(18)

4.


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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


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


(22)

90


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.


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



4.

KAROL


91

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.


92



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D i e d r i c h , B. A p p l . Polym. Symp. 1975, 26, 1 and r e f e r e n c e s therein. 17. Pino, P.; Mulhaupt, R. Angew. Chem. Int. Ed. Engl. 1980, 19, 857. 18. Galli, P.; Luciani, L . ; C e c c h i n , G. Angew. M a k r o l . Chemie 1981, 94, 63. 19. Yermakov, Y. I.; Kuznetsov, B. N . ; Zakharov, V. A. " S t u d i e s in Surface Science and C a t a l y s i s " ; E l s e v i e r : AmsterdamOxford-New York, 1981; Vol. 8, Chap. 5. 20. Dassesse, P.; Dechenne, R. U. S. Patent 3 400 110, 1968. 21. Stevens, J. Hydrocarbon Process 1970, 179-82. 22. Hewett, W. A.; Shokal, E . C. British Patent 904 510, 1960. 23. L o n g i , P . ; Giannini, U . ; C a s s a t a , A. British Patent 1 335 887, 1970. 24. G i a n n i n i , U . ; C a s s a t a , A . ; L o n g i , P.; M a z z o c h i , R. British Patent 1 387 890, 1971. 25. Haward, R. N . ; Rober, A. N . ; Fletcher, K. L. Polymer 1973, 14, 365. 26. Lassalle, D.; Vidal, J . L . ; Roustant, J . C.; Mangin, P. 5th Conf. European Plastics Caoutch. Soc. Chim. Ind.; P a r i s , France, 1978. 27. Boucher, D. C.; Parsons, I . W.; Haward, R. N. M a k r o l . Chem. 1971, 175. 3461. 28. Berger, E . ; Derroitte, J. L . U.S. Patent 3 901 863, 1975. 29. Delbouille, A . ; Derroitte, J . L . ; Berger, E . ; G e r a r d , P. British Patent 1 275 641, 1972. 30. Hermans, J. P . ; H e n r i o u l l e , P. U . S . Patent 3 769 233, 1973. 31. Hermans, J. P . ; Henrioulle P. U . S . Patent 4 210 738, 1980. 32a. Giannini, U.; Albizzati, E.; Parodi, S. U . S . P a t e n t 4 149 990, 1979. b. L u c i a n i , L . ; Kashiwa, N . ; Barbe, P. C.; Toyota, A. U.S. Patent 4 226 741, 1980. c. C r e s p i , G. Paper presented at symposium on " F r o n t i e r s i n Organic Chemical Technology"; Baroda, I n d i a , March 28-29, 1979; pp. 333-42. d. C r e s p i , G. Pet. Chem. I n d . , Dev. Ann. 1979, 121. e. DiDrusco, G . ; R i v a l d i , R. Hydrocarbon Proc. 1981, 153-55. 33a. G o o d a l l , B. L . Paper presented at M i d l a n d M a c r o m o l e c u l a r Symposium; Midland, Mich., August 17-21, 1981. b. G o o d a l l , B. L.; van der S a r , J. C.; Keuzenkamp, A. European Patent A p p l . 0 029 623, 1981. 34. Ballard, D. G. H. Advan. Catal. 1973, 23, 263. 35. K a r o l , F. J.; K a r a p i n k a , G. L.; Wu, C.; Dow, A. W.; Johnson, R. N . ; Carrick, W. L . J. Polym. Sci., A-1 1972, 10, 2621. 36. Hoeg, D. F. In "The S t e r e o c h e m i s t r y of M a c r o m o l e c u l e s " ; K e t l e y , A. D., Ed.; Dekker: New York, 1967; Chap. 2. 37. Natta, G . ; Mazzanti, G. Tetrahedron 1960, 8, 86. 38. Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry," 4th ed.; Interscience: New York, 1980. 39. Greco, A . ; Bertolini, G.; Cesca, S. J . A p p l . Polym. Sci. 1980, 25., 2045. 40. T a i t , P. J. T. In "Developments in P o l y m e r i z a t i o n " ; Haward, R. M . , Ed.; A p p l . Sci. Publ.: London, 1979; Vol. 2.


4.

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41. Ziegler-Natta 42.


43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74.


93

T a i t , P. J. T. " S t u d i e s on Active Center D e t e r m i n a t i o n i n P o l y m e r i z a t i o n " ; Paper presented at M i d l a n d Macromolecular Symposium; Midland, Mich., August 17-21, 1981. Caunt, A. D. "Specialist Periodical Report, C a t a l y s i s " ; Chemical Society: London, 1977; V o l . 1. See reference 19, Chapter 3. Natta, G . ; Pasquon, I . Advan. C a t a l . 1959,11,1. Grieveson, B. M. Makromol. Chem. 1965, 84, 93. Feldman, C. F.; Perry, E. J. Polym. Sci. 1960, 46, 217. C h i e n , J . C. W. In "Preparation and Properties of Stereoregular Polymers"; Lenz, R. W.; Ciardelli, F., Eds.; D. Reidel P u b l . : Holland-Boston-London, 1980. G i a n n i n i , U. Makrol. Chem. Suppl. 1981, 5, 216. See reference 5, Chapters 19-21. B r e u e r , F . W.; Geipel, L. E.; Loebel, A. B. I n " H i g h Polymers"; Raff, R. A. V . ; Doak, K. W., Eds.; I n t e r s c i e n c e : New York, 1965; Vol. XX. Lukach, C. A.; Spurlin, H. M. In "High Polymers"; Ham, G. E . , Ed.; Interscience: New York, 1964; V o l . XVIII. B r e s l o w , D. S.; Newburg, N. R. J. Am. Chem. Soc. 1959, 81, 81. Natta, G. Experientia 1963, 19, 609. N a t t a , G.; Dall'Asta G.; M a z z a n t i , G. Angew. Chem. 1964, 76, 765. See reference 4, pp. 241-43. Reikhsfeld, V. O.; Makovetskii, K. L. Dokl. Akad. Nauk. SSSR 1964, 155, 414. N i c o l e s c u , I . V . ; A n g e l e s c u , E. M. J. Polym. Sci. 1965, 3, 1227. Marvel, C. S.; G a l l , E. J. J. Org. Chem. 1960, 25, 1784. M a r c o n i , W. In "The S t e r e o c h e m i s t r y of M a c r o m o l e c u l e s " ; K e t l e y , A. D, Ed.; Dekker: New York, 1967; Chap. 5. S c h i n d l e r , A. 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, Chap. 5. Patat, F.; Sinn, H. Angew. Chem. 1958, 70, 496. Uelzmann, H. J. Polym. Sci. 1958, 32, 457. L u d l u m . B. D.; A n d e r s o n , A . W.; A s h b y , C. E . J. Am. Chem. Soc. 1958, 80, 1380. K a r o l , F. J.; C a r r i c k , W. L. Ibid. 1961, 83, 2654. M a t l a c k , A. S.; B r e s l o w , D. S. J. Polym. Sci., P a r t A 1965, 2853. Werber, F. X . ; Benning, C. J.; Wszolek, W. R.; Ashby, G. E. J. Polym. S c i . , Part A-1 1968, A-1, 743. Benning, C. J.; Wszolek, W. R.; Werber, F. X. J. Polym. Sci. Part a-1 1968, 755. Hoeg, D. F . ; Liebman, S. Ind. Eng. Chem. Process Design Develop. 1962, 1, 120. Boor, J., Jr. J. Polym. Sci., P a r t A-2 1964, 265. Boor, J., Jr. Polym. Preprints 1965, 6, 890. Porri, L . ; N a t t a , G.; Gallazzi, M. C. Chim. Ind. 1964, 46, 428. Cossee, P. J. C a t a l . 1964, 3, 80. Arlman, E. J. Ibid., 1964, 3, 89. Arlman, E . G. Ibid., 1964, 3, 99.


94 75. 76. 77. 78.


79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 101. 102.


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