Applied Polymer Science - ACS Publications - American Chemical

16

Polyolefins FRANK M. McMILLAN

Downloaded by MONASH UNIV on December 6, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch016

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Discovery of High-Pressure (Low-Density) Polyethylene Polyisobutylene Discovery of Linear (High-Density) Polyethylene Use of Comonomers Linear Low-Density Polyethylene Stereospecific Polymerization Discovery of Polypropylene Manufacturing Processes High-Pressure LDPE Low-Pressure, Linear HDPE LLDPE Polypropylene Other Polyolefins Commercial Production Properties and Applications Consumption of Polyolefins New Monomers and Polymers Growth of Understanding Catalysts and Kinetics Structure vs. Properties The Future Polymerization Problems Postpolymerization Processing

A good reason to look at history is to select those parts we should like to repeat. One part of science history that both polymer chemists and businessmen would surely like to see emulated is the unprecedented, explosive burst of creativity, invention, and successful development that occurred in the 1950s and gave the world a new class of polymers (stereoregular), a family of new plastics (linear and stereoregular polyolefins), a family of new synthetic rubbers, including the first duplication of a natural high polymer 0097-6I56/85/0285-0333$08.25/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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APPLIED POLYMER SCIENCE

(stereoregular polydienes), a new class of catalysts (Ziegler/Natta coordination catalysts), and a new nomenclature (isotactic, syndiotactic, atactic, etc.). This decade of discovery also spawned two Nobel prizes, thousands of patents, and thousands of scientific papers, not to mention dozens of lawsuits. Because the stereoregular synthetic rubbers are the subject of a separate chapter, this discussion will be concerned only with the polyolefins. Although the newer and more exciting members of this family are stereoregular polyolefins, the first member (high -pressure polyethylene) is not a stereoregular polymer; moreover, the first synthetic stereoregular polymer (polyvinyl isobutyl ether) is not even a polyolefin. Polymer science may be said to have begun in the 1920s with the Svedburg's pioneering work with the ultracentrifuge and Herman Staudinger's revolutionary theory that high polymers consist of giant, long-chain molecules rather than colloidal aggregates. After a decade of challenge and even ridicule, Staudinger's basic idea gradually was accepted, and further progress became possible. An important advance was the recognition that cohesion in high polymers depends primarily on the entanglement of long chains and that additional strength and hardness are gained through any of three supplemental mechanisms (Figure 1): van der Waals attractions between polar groups, formation of crystalline domains among well-fitting chains, and chemical cross-linking. Discovery of High-Pressure (Low-Density) Polyethylene Had these and other basic relationships of composition and chain configuration to polymer properties been well understood fifty years ago, we could visualize a polymer chemist of that day—a day when Bakelite was well known but polystyrene and poly(vinyl chloride) (PVC) were novelties—setting out deliberately to design and synthesize a new high-utility, general-purpose, low-cost plastic with the aid of these principles. He might have reasoned thusly: first, the polymer must have a high enough molecular weight for long-chain entanglement (but not so high as to make processing difficult). Secondly, strength and toughness should be added, not by introducing polar groups (for they would add to weight and to volume cost and would be apt to impair thermal stability), nor by introducing chemical cross-links (for that would require curing cycles, would probably introduce color, and would produce nonrecyclable scrap), but by the formation of crystalline domains. The advantages include no additives, no color problems, relatively sharp melting/molding range, fast and reversible molding by heating and cooling, and no extra cost. Properties could be varied considerably by varying the size and number of the crystalline domains, controlled in turn by the degree of regularity and hence the fit between adjacent chains. Having forsworn polar substituents, the prescient polymer chemist would quickly see that by elimination, the logical composition would be a polymeric hydrocarbon. The chains would then have to be very smooth and well-fitting to achieve crystallization, because the forces of attraction between chains would be quite weak. The smoothest chain would be one of methylene groups. On consulting the literature, the chemist would be encouraged by the fact that the



16. MCMILLAN Polyolefins

335

straight-chain paraffins become c r y s t a l l i n e s o l i d s as soon as the c h a i n l e n g t h g e t s above 10 carbons, and he would note t h a t polymethylene had, i n fact, been made i n the previous century by von Pechmann and o t h e r s Q_) by decomposing diazomethane ( F i g u r e 2). Thus encouraged, he would seek an e c o n o m i c a l l y f e a s i b l e r o u t e t o approximate such a polymer, and he would be l e d i n e v i t a b l y t o ethylene as the most p r a c t i c a l monomer. The remaining problem being how t o f o r c e the r e l a t i v e l y u n r e a c t i v e e t h y l e n e m o l e c u l e t o polymerize to a high polymer, he would l o g i c a l l y turn to elevated temperatures and extreme p r e s s u r e t o f o r c e the r e a c t i o n i n the desired d i r e c t i o n (Figure 2). Is t h i s a reasonable approximation of the manner i n which h i g h pressure polyethylene was a c t u a l l y discovered? Not i n the remotest degree, as we a l l know. J . C. S w a l l o w of I m p e r i a l Chemical I n d u s t r i e s ( I C I ) , who was p e r s o n a l l y i n v o l v e d i n t h a t h i s t o r i c discovery, made the f o l l o w i n g observation: "As time goes on the s t o r y of i n v e n t i o n s and d i s c o v e r i e s t e n d s t o become i d e a l i z e d and o v e r r a t i o n a l i z e d and presented i n a way which suggests that everything f o l l o w s l o g i c a l l y from the f i r s t experiment. H i s t o r y shows t h a t t h i s r a r e l y happens and t h a t chance always p l a y s a b i g p a r t - What does, however, seem t o be important i s the s i g n i f i c a n c e of the r e c o g n i t i o n of a d i s c o v e r y which may often be more important than the discovery i t s e l f . " ( 2 ) We s h a l l see how f r e q u e n t l y the same comment c o u l d be made about subsequent d i s c o v e r i e s . Without r e p e a t i n g i n d e t a i l the often t o l d s t o r y of how t h i s h i s t o r i c " f i r s t " i n p o l y o l e f i n s came i n t o being, l e t us note that i t s t a r t e d w i t h a b o l d d e c i s i o n on the p a r t of I C I t o e x p l o r e the new f r o n t i e r s of u l t r a p r e s s u r e r e a c t i o n s opened up by the p i o n e e r i n g work of Bridgeman i n the U n i t e d S t a t e s and M i c h e l s i n H o l l a n d . Among 50 reactions t r i e d by the ICI research group without success was the a l k y l a t i o n of benzaldehyde with ethylene, suggested by S i r Robert Robinson (3) and attempted by R. 0. Gibson i n March 1933. The intended r e a c t i o n d i d not take p l a c e , but a t r a c e of e t h y l e n e polymer was formed, thanks (as i t was l e a r n e d afterwards) t o the f o r t u i t o u s presence of p e r o x i d e i n the benzaldehyde. S u c c e s s f u l d u p l i c a t i o n s of the p o l y m e r i z a t i o n had t o await a second "happy a c c i d e n t " — a l e a k t h a t r e s u l t e d i n the i n t r o d u c t i o n of excess ethylene containing enough oxygen impurity to act as c a t a l y s t . A l t h o u g h f o r t u i t o u s circumstance p l a y e d a key r o l e more than once i n these and subsequent e v e n t s , f u l l c r e d i t i s due Fawcett, Gibson, S w a l l o w , and o t h e r s of I C I (4) f o r l a u n c h i n g the r e s e a r c h and f o r being a l e r t enough to r e c o g n i z e the s i g n i f i c a n c e of the unexpected r e s u l t s and a g g r e s s i v e enough t o pursue the l e a d t o a successful conclusion. As a r e s u l t of t h e i r e f f o r t s , polyethylene was a v a i l a b l e t o i n s u l a t e c a b l e s f o r radar and thus h e l p win the B a t t l e of B r i t a i n and World War I I . The phenomenal postwar growth of polyethylene, r e s u l t i n g from the advantageous properties noted i n the "predictions" made e a r l i e r , have made i t a m u l t i b i l l i o n - p o u n d p l a s t i c and a household word around the world.



336


Chain Entanglement

Polar Bonds

Figure 1.

CH N 2

Crosslinking by C h e m i c a l Bonds

Crystalline Polymers

Source of cohesion i n long-chain polymers.

CH CH CH CH CH CH CH CH CH CH --

2

3

2

2

2

2

2

2

2

2

2

CH^CH CH CH CH CH CH CH CH CH CH — 2

2

2

2

2

2

2

2

2

2

CH =CH ^ 2

2

C H C H C H C H C H C H C H CU UH C H C H C H C H Q

0

0

0

0

2 0

2, 0

0

0

0

CH. CH.

Figure 2.

F o r m a t i o n of p o l y e t h y l e n e by d e c o m p o s i t i o n diazomethane and by polymerization of ethylene.


of


337


Polyisobutylene Polyisobutylene i s not u s u a l l y even mentioned when p o l y o l e f i n s are discussed. Yet i t i s obviously a p o l y o l e f i n , and i t ranks at l e a s t a c l o s e second i n the chronology of discovery. In the e a r l y 1930s, chemists a t the I . G. Farben l a b o r a t o r i e s i n Germany "stumbled on" the finding that BFg would e x p l o s i v e l y polymerize isobutylene to a " s n o w b a l l " o f h i g h p o l y m e r (_5). But t h e p o l y m e r i s n o t s t e r e o r e g u l a r (no asymmetric carbon atom), c r y s t a l l i z e s o n l y s l i g h t l y even when stretched, and has no unsaturation; consequently, i t i s n e i t h e r a h a r d , c r y s t a l l i n e p l a s t i c nor a v u l c a n i z a b l e elastomer. I t has t h e r e f o r e found o n l y l i m i t e d commercial use. C o p o l y m e r i z a t i o n w i t h a s m a l l amount of diene (butadiene or isoprene) introduces r e s i d u a l unsaturation and forms the well-known and h i g h l y useful v u l c a n i z a b l e synthetic rubber, "butyl rubber." Discovery of Linear (High-Density) Polyethylene Coming back to our hypothetical prescient polymer pioneer, we s h a l l assume that, had he i n fact invented high-pressure polyethylene, he would not have r e s t e d on h i s l a u r e l s , even a f t e r c o n t r i b u t i n g t o winning the war and c r e a t i n g one of the w o r l d ' s most u s e f u l p l a s t i c s . I n s t e a d , he would d o u b t l e s s p e r c e i v e t h a t a v a l u a b l e v a r i a n t of p o l y e t h y l e n e would be one w i t h more and s t r o n g e r c r y s t a l l i n e domains, because these would add hardness and strength and r a i s e the m e l t i n g p o i n t . He would by now have found out t h a t h i g h - p r e s s u r e p o l y e t h y l e n e forms imperfect c r y s t a l s because the c h a i n s c a r r y a number of s h o r t branches ( F i g u r e 2) so the r i g h t course would be obvious. He must find a way to polymerize ethylene w i t h o u t forming branches. L o g i c a l l y , t h i s would r e q u i r e m i l d conditions and hence an e x c e p t i o n a l l y a c t i v e and s e l e c t i v e c a t a l y s t . T h i s s o r t of r e a s o n i n g would have formed a l o g i c a l p r e l u d e to the discovery of l i n e a r polyethylene; but of course i n a c t u a l fact i t had nothing whatever to do with that discovery. I n 1 9 5 3 , K a r l Z i e g l e r , a f t e r many y e a r s o f r e s e a r c h on o r g a n o m e t a l l i e compounds, had d i s c o v e r e d the "Aufbau" r e a c t i o n , whereby ethylene was converted to low polymers (oligomers) by the action of aluminum a l k y l s (6). The polymers were s t r i c t l y l i n e a r , but the p o t e n t i a l s i g n i f i c a n c e of t h i s fact was l a r g e l y overlooked by the s c i e n t i f i c and t e c h n i c a l community. Then one of Z i e g l e r ' s g r a d u a t e s t u d e n t s , H o l z k a m p , d i s c o v e r e d through a c c i d e n t a l c o n t a m i n a t i o n of the r e a c t i o n system w i t h n i c k e l t h a t t h i s metal acted as a c o c a t a l y s t to stop the r e a c t i o n a t the dimer (1-butene) stage. A f t e r t h a t , a c c o r d i n g to Z i e g l e r , i t was s i m p l y a matter of s y s t e m a t i c a l l y working through the periodic chart u n t i l a different c o c a t a l y s t was found that had the opposite effect: suppressing the displacement reaction and thus favoring continued chain growth and the production of high polymers. The best cocatalyst proved to be t i t a n i u m , and thus was born the Z i e g l e r c a t a l y s t system ( t i t a n i u m c h l o r i d e and aluminum a l k y l ) and Z i e g l e r polyethylene (7). Fortuitous circumstance had once again rewarded a l e r t observat i o n and d i l i g e n t follow-up. A c r i t i c a l factor, impressive i f one considers the time and circumstances, was that Z i e g l e r immediately r e c o g n i z e d t h a t h i s f i r s t crude polymer had properties d i s t i n c t l y



338


superior to those of the well-known high-pressure polyethylene. On the strength of t h i s fact and the a l l u r e of a low-pressure process, he undertook a v i g o r o u s program of p a t e n t i n g , p u b l i s h i n g , and p u b l i c i z i n g h i s discovery, with r e s u l t s that made him a m i l l i o n a i r e Nobel l a u r e a t e and Z i e g l e r p o l y e t h y l e n e an important commercial product i n many countries. Unfortunately for Z i e g l e r , who wrote h i s own patent a p p l i c a t i o n , he c o n f i n e d h i s coverage to e t h y l e n e — a n e x p e n s i v e mistake that l e d to disagreements and l i t i g a t i o n when stereoregular p o l y o l e f i n s were discovered. Perhaps Z i e g l e r ' s greatest good fortune was i n timing. I t seems no g r e a t feat today to r e c o g n i z e the m e r i t s of an a l l - l i n e a r polymer, but the 1950s were about the e a r l i e s t years when the i n s t a n t enthusiasm t h a t greeted Z i e g l e r c o u l d p o s s i b l y have occurred. I t i s not s u r p r i s i n g t h a t von Pechmann's l i n e a r p o l y m e t h y l e n e roused no commercial i n t e r e s t and s c a r c e l y any s c i e n t i f i c c u r i o s i t y at the t u r n of the c e n t u r y ; however, i t i s s i g n i f i c a n t that Marvel made l i n e a r polyethylene with a metal a l k y l c a t a l y s t i n 1929, and t h a t n o t h i n g came of i t (8.). Two major chemical companies looked at that process at that time and concluded that i t had no commercial p o s s i b i l i t i e s . Z i e g l e r ' s t i m i n g was a l s o f o r t u n a t e i n b r i n g i n g out a new product at a time when the chemical industry was i n an aggressive, expansionist mood. Had he been a few years e a r l i e r , that wave would not yet have crested; had he been even a year or two l a t e r , he might have l o s t out to the other l i n e a r , l o w - p r e s s u r e p o l y e t h y l e n e processes that were developed independently and contemporaneously. By a s t r i k i n g coincidence, during the same period (1953), Hogan and Banks of P h i l l i p s Petroleum were endeavoring to find a p r a c t i c a l way of making g a s o l i n e - r a n g e hydrocarbons from e t h y l e n e . Like Z i e g l e r , they s t a r t e d from known d i m e r i z a t i o n r e a c t i o n s and used c a t a l y s t systems such as c o b a l t on c h a r c o a l . In t e s t i n g many c o m b i n a t i o n s , they were nonplussed to f i n d t h a t one c a t a l y s t , chromium oxide supported on s i l i c a - a l u m i n a , caused the reaction tube to become plugged with a white s o l i d . Other workers had made s i m i l a r o b s e r v a t i o n s e a r l i e r , but had d i s c a r d e d t h e i r plugged tubes and r e p o r t e d the experiment as a f a i l u r e . Hogan and Banks did not discard t h e i r plugged tubes, but worked up the polymer to see what they had. What they had was l i n e a r polyethylene of almost perfect l i n e a r i t y (9). In s t i l l another p a r a l l e l i s m of t i m i n g and r e s u l t , another research team at Standard O i l was i n v e s t i g a t i n g the same s t a r t i n g c a t a l y s t , c o b a l t on c h a r c o a l , at about the same time ( a c t u a l l y , somewhat e a r l i e r [1951]), although for a different reaction. Their work l e d t o s t i l l a n o t h e r a c c i d e n t a l d i s c o v e r y o f l i n e a r polyethylene (10). Z l e t z was a t t e m p t i n g to use the supported c o b a l t c a t a l y s t f o r a l k y l a t i o n w i t h e t h y l e n e and found, to h i s s u r p r i s e , t h a t c o n s i derable s o l i d polymer was formed. Like Hogan, he was not a trained polymer chemist, but had enough c u r i o s i t y , i n i t i a t i v e , and freedom to pursue the i n t e r e s t i n g bypath. Other metals and other supports were t e s t e d by Z l e t z and h i s coworkers and l e d e v e n t u a l l y to improved c a t a l y s t s such as molybdenum on a l u m i n a t h a t formed the b a s i s f o r development of a p r a c t i c a l l o w - p r e s s u r e p o l y e t h y l e n e process.


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These two c o m p e t i t i v e p r o c e s s e s , d i s c o v e r e d i n d e p e n d e n t l y of each other and of Z i e g l e r , were developed aggressively by P h i l l i p s , l e s s so by Standard. The P h i l l i p s process i s now the w o r l d ' s leading producer of l i n e a r polyethylene. Once again, i t may be said t h a t f r u i t f u l i n n o v a t i o n was the r e s u l t of an unexpected r e s u l t encountering a prepared mind.


Use of Comonomers In the course of e x t e n s i v e market development, i t was d i s c o v e r e d t h a t p e r f e c t c h a i n r e g u l a r i t y , an i d e a l t h a t c o u l d be approached quite c l o s e l y with the P h i l l i p s process, i s not the i d e a l morphology for commercial grades of polyethylene. Painful experience, to be d e s c r i b e d more f u l l y l a t e r ( s e e s e c t i o n on P r o p e r t i e s and A p p l i c a t i o n s ) taught t h a t a s m a l l and c o n t r o l l e d amount of c h a i n i r r e g u l a r i t y i s r e q u i r e d t o a c h i e v e t h e optimum d e g r e e o f c r y s t a l l i n i t y and the most u s e f u l b a l a n c e of p r o p e r t i e s . This condition was accomplished by the incorporation of s m a l l amounts of a h i g h e r o l e f i n , such as 1-butene, as comonomer, t o i n t r o d u c e a c e r t a i n amount o f s h o r t - c h a i n b r a n c h e s t h a t i n t e r f e r e w i t h c r y s t a l l i z a t i o n to the desired degree. Linear Low-Density Polyethylene A f t e r r e v i e w i n g the h i s t o r y of the development of p o l y o l e f i n p r o c e s s e s , a t h o u g h t f u l polymer chemist might decide t h a t a worthwhile goal would be to develop a product that would combine the best features of low-density and l i n e a r , high-density polyethylenes, or at l e a s t one that would be intermediate i n properties between the two extremes. A l o g i c a l approach would be t o attempt t o employ a Z i e g l e r - or P h i l l i p s - t y p e c a t a l y s t i n the high-pressure process to convert i t from a f r e e - r a d i c a l to an i o n i c i n i t i a t o r . A change i n the type and amount of b r a n c h i n g of the polymer c h a i n s might be expected to r e s u l t . Whatever the i n s p i r a t i o n , that exact r e s u l t has, i n fact, been a c h i e v e d . The Union C a r b i d e Company was the f i r s t t o announce i n the U n i t e d S t a t e s ( i n 1977) a " l i n e a r , l o w - d e n s i t y p o l y e t h y l e n e " (LLDPE) process and product, although somewhat s i m i l a r products had been made f o r some years by Du Pont i n Canada. A c o m p e t i t i v e process to the C a r b i d e process was soon announced by Dow Chemical Company. S e v e r a l other companies i n the United S t a t e s and Europe have subsequently either taken l i c e n s e s from Carbide or developed t h e i r own LLDPE processes and have e i t h e r begun or planned production. As of 1980, the s t a t u s of v a r i o u s processes f o r production of LLDPE throughout the world was reported to be as shown in Table I. In a d d i t i o n t o C a r b i d e and Dow, four other companies a r e producing LLDPE i n the U n i t e d S t a t e s : Exxon, M o b i l , N a t i o n a l P e t r o c h e m i c a l s , and E l Paso ( T a b l e I I I ) . Others who have been reported to be doing development work include ARCO, C i t i e s S e r v i c e (now O c c i d e n t a l ) , P h i l l i p s , S o l t e x , and USI. S e v e r a l a d d i t i o n a l p l a n t s and plant expansions are expected i n the near future. A s t r i k i n g (and i r o n i c a l ) f e a t u r e of the LLDPE development i s t h a t i t has g i v e n " Z i e g l e r p o l y e t h y l e n e " a new l e a s e on l i f e . Whereas the P h i l l i p s process a l l but displaced the Z i e g l e r process


340


Table I .

Linear Low-Density Polyethylene Processes (34)


LLDPE Process Type Gas Phase Fluidized Bed Gas Phase S t i r r e d Bed

Liquid Phase Slurry

Liquid Phase Solution

High Pressure a

Pressure (lb/in. )

Catalyst

Union Carbide

300

Cr

P i l o t Plant

Naphtachimie

350

Z

P i l o t Plant

Amoco

300

Z

P i l o t Plant

C i t i e s Service

300

Z

Commercial

Phillips

450

Cr

P i l o t Plant

Solvay

400

Z

Commercial

Du Pont, Canada

400-600

Z

Commercial

Dow

400-600

Z

Commercial

M i t s u i Petrochemical

600

Z

P i l o t Plant

DSM

600

Z

Commercial

CdF

Status

Developing Companies

Commercial

2

20,000

Z=Ziegler type, Cr=Chromium type


Z

3



341

for high-density polyethylene i n most countries, the majority of the new (or converted o l d ) LLDPE plants w i l l use Z i e g l e r - t y p e c a t a l y s t s . LLDPE i s g e n e r a l l y intermediate i n p r o p e r t i e s between c o n v e n t i o n a l l o w - d e n s i t y p o l y e t h y l e n e (LDPE) made by h i g h - p r e s s u r e processes and l i n e a r , h i g h - d e n s i t y p o l y e t h y l e n e (HDPE) of the Z i e g l e r or P h i l l i p s type. A q u a l i t a t i v e comparison of the more s i g n i f i c a n t p h y s i c a l p r o p e r t i e s i s g i v e n i n T a b l e I I (some quant i t a t i v e data are given i n Table IV). The property d i f f e r e n c e s c i t e d i n T a b l e I I are a t t r i b u t e d p r i m a r i l y to differences i n the degree and kind of branching i n the polymer chains. LLDPE i s reported to have fewer but longer branches than LDPE. As w i t h HDPE, the branching i s f u r t h e r m o d i f i e d by the introduction of higher a - o l e f i n s as comonoraers ( t y p i c a l l y , about 8% of 1-butene, 1-hexene, or 1-octene). LLDPE has been e n t h u s i a s t i c a l l y r e c e i v e d i n the m a r k e t p l a c e because of a number of f a v o r a b l e f a c t o r s . One f a c t o r i s the advanced s t a t e of p o l y e t h y l e n e t e c h n o l o g y , which made i t easy to recognize the advantages of the new product and to pursue i t s best applications. The o r i g i n a t o r s were l e a d i n g p r o d u c e r s o f c o n v e n t i o n a l p o l y e t h y l e n e and were i n good p o s i t i o n s to mount aggressive development, manufacturing, and marketing programs. Also favorable was the p o s s i b i l i t y to convert some e x i s t i n g polyethylene (and even polypropylene) p l a n t s to LLDPE production, with obvious s a v i n g s i n time and c o s t s . These f a c t o r s , combined w i t h an a t t r a c t i v e b a l a n c e o f p h y s i c a l p r o p e r t i e s , good p r o c e s s i n g c h a r a c t e r i s t i c s , and the a v a i l a b i l i t y of the process for l i c e n s i n g , have r e s u l t e d i n r a p i d b u i l d up of p r o d u c t i o n c a p a c i t y , which i n 1982 reached a t o t a l of 2.5 b i l l i o n l b / y r i n the U n i t e d S t a t e s . Great optimism i s e v i d e n t i n t h i s f i g u r e because demand, a l t h o u g h growing at a record pace, i s running behind capacity by a factor of about two. Much of the growth i n LLDPE demand w i l l be at the expense of other p o l y o l e f i n s , e s p e c i a l l y conventional high-pressure, LDPE. I t s proponents c l a i m t h a t LLDPE can r e p l a c e LDPE i n 70 to 80% of i t s markets and that "the l a s t conventional polyethylene plant has been b u i l t . " Another s i g n of the times i s the f a c t t h a t I C I , o r i g i n a t o r of the high-pressure process, has now ceased production. Faced w i t h s h r i n k i n g demand f o r the c o n v e n t i o n a l p r o d u c t s , s e v e r a l LDPE producers have undertaken p r o d u c t i o n of s p e c i a l copolymers (some u s i n g v i n y l e s t e r s as comonomers) and m o d i f i e d grades that more c l o s e l y approach LLDPE i n properties. Thus, with the a d d i t i o n a l c a p a b i l i t i e s and s t i m u l a t i o n provided by the LLDPE processes, i t has become p o s s i b l e to manufacture polyethylenes with almost any desired degree and type of branching, molecular weight, and molecular weight d i s t r i b u t i o n . C o n t r o l over these v a r i a b l e s e n a b l e s products to be " t a i l o r made" f o r optimum performance i n a host of d i f f e r e n t a p p l i c a t i o n s . As a consequence, the borders between different types of polyethylene are becoming i n d i s t i n c t , and the remaining gaps i n the ranges of a t t a i n a b l e properties are being f i l l e d in. Much of the r e p o r t e d c a p a c i t y f o r p o l y e t h y l e n e p r o d u c t i o n must now be c o n s i d e r e d c o n v e r t i b l e from one type to another.


342



Stereospecific Polymerization I f our imaginary chemist who could have achieved t h i s sort of r e s u l t by l o g i c a l processes had a c t u a l l y done so, he would s u r e l y by now have gone as f a r as he c o u l d w i t h e t h y l e n e . But i f he read the l i t e r a t u r e thoughtfully, i t would have impressed him that s e v e r a l of the pioneers of polymer s c i e n c e , s t a r t i n g w i t h Staudinger i n 1932 and l a t e r Huggins (11), Schildknecht (12), and F l o r y (13), had each recognized the possible existence of an a d d i t i o n a l degree of order, and hence of c r y s t a l l i z a b i l i t y , i n high-polymer chains containing asymmetric carbon atoms. They thought t h a t such s u b s t i t u t e d e t h y l e n e polymers as p o l y ( v i n y l bromide) ( F i g u r e 3) ought to be c r y s t a l l i n e i f each polymer c h a i n were i n an " a l l d" or " a l l 1" s t e r i c configuration and that, because the known polymers were, i n fact, n o n c r y s t a l l i n e , they must be i n random order. S c h i l d k n e c h t f i r s t made a new c r y s t a l l i n e h i g h polymer and a s c r i b e d i t s c r y s t a l l i n i t y to s t e r i c r e g u l a r i t y . In 1947 he r e p o r t e d t h a t p o l y ( v i n y l i s o b u t y l e t h e r ) i s e i t h e r amorphous or h i g h l y c r y s t a l l i n e , depending on conditions of polymerization, and he b o l d l y proposed that the c r y s t a l l i n e form had a regular s t e r i c structure (Figure 4): either ddddddddd or more l i k e l y (he thought), d l d l d l d l d l d l (14). I t was shown l a t e r by Natta (15) that the " a l l d" structure (now known as i s o t a c t i c ) i s the correct one. With a l l these clues and portents i n mind, i t would have been but one more l o g i c a l step for our imaginary chemist to conceive that a hydrocarbon polymer containing asymmetric carbon atoms c o u l d a l s o e x h i b i t a regular s t e r i c structure. Adjacent chains would then f i t together e x c e p t i o n a l l y snugly to form stronger and higher melting c r y s t a l s than those i n l i n e a r polyethylene. Such a polymer would have c o r r e s p o n d i n g l y h i g h e r s t r e n g t h , h a r d n e s s , s o f t e n i n g temperature, and solvent resistance. I t would then have been l o g i c a l f o r the chemist to s e t t l e on propylene as the simplest and most r e a d i l y a v a i l a b l e monomer that could form such a polymer (Figure 5). I t would have remained only to t r y to adapt a Z i e g l e r - t y p e c a t a l y s t or some other type t h a t might overcome the tendency of propylene, i n common with a l l a l l y l i c compounds, to form only low polymers. Discovery of Polypropylene Once a g a i n , i n the r e a l w o r l d , f o r t u i t y and a c u i t y p l a y e d f a r g r e a t e r r o l e s than c o l d reason. Natta had made an agreement t h a t gave him access to Z i e g l e r ' s new chemistry and c a t a l y s t s , and he promptly set one of h i s chemists, C h i n i , to try Z i e g l e r c a t a l y s t s on p r o p y l e n e ( Z i e g l e r had a l r e a d y t r i e d p r o p y l e n e , but under the c o n d i t i o n s used the r e s u l t s were so poor t h a t he was l e d to c o n c l u d e , p r e m a t u r e l y , t h a t the h i g h e r o l e f i n s "would not work"). N a t t a was, of course, hoping f o r a h i g h polymer, but what he expected was an amorphous, rubbery product, because the concept of s t e r e o s p e c i f i c i t y was not i n h i s mind at the b e g i n n i n g . In any e v e n t , he and h i s company sponsor, M o n t e c a t i n i , were at the time more interested i n synthetic rubber than i n p l a s t i c s . I t was a great surprise, therefore, that the product of C h i n i ' s very f i r s t experiment, made on March 11, 1954, consisted mostly of a hard, h i g h l y c r y s t a l l i n e t h e r m o p l a s t i c . N a t t a made a s i m p l e ,


343



Table I I .

Differences Between Linear Low-Density, High-Density, and Low-Density Polyethylenes (34, 35)

Property

LLDPE v s . LDPE

LLDPE v s . HDPE

Tensile strength

higher

lower

Elongation

higher

higher

Impact strength

higher

similar

Heat resistance

higher

lower

Stiffness

higher

lower

Warpage

lower

similar

Processability

harder

easier

Haze

worse

better

Gloss

worse

better

Clarity

worse

better

Melt strength

lower

lower

Melting range

narrower

narrower

2

X

H

X

H

X

I

I

I

I

I

t

2

H

i

X

2 i H

2

i

X

2 i H

H I

2 i X

dldldldldldldldl

X X X X X X --CH -C*-CH -C*-CH -C*-CH -C*-CH -C*-CH -C*--2 i 2 i 2 i 2 i 2 i 2 i H H H H H H 0

0

0

0

0

9

dddddddddddddddd

Figure 3.

Stereospecif i c structures p o s s i b l e w i t h polymer c h a i n s containing asymmetric carbon atoms (R = i-C^Hg).


344


OR OR OR OR OR OR OR OR --CH CCH CCH CCH CCH CCH CCH CCH CCH 9

£

0

\

£

H 9L

0

£

\

0

£

H

0

£

I

H

I

0

£

H

\

0

£

H

%

H

0

£

%

£

H

OR H OR I H I OR I H I ORI H I I I —CH CCH CCH CCH CCH CCHoCCH CCH CCH 0

0

£ \

0

0

0

£ \

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0

%

H

£ \

£\

OR

H

£ \

OR

£ %

H

0

0

£ \

£\

OR

Crystalline

0

H

£

OR

form

(j)R OR H (j)R H H H (j)R —CH CCH CCH CCH CCH CCH CCH CCH CCHo £ \ £% £% £\ 2I 2I 2I £ I £ 0

0

H Figure 4.

0

0

H

0

0

H

OR

0

OR

0

OR

H

OR

Amorphous f o r m (Random o r d e r ) Possible s t e r i c structures for polymers of v i n y l i s o b u t y l ether.

CHo t

Isotactic:

CHQ

J

I

CHq

CHQ

J

I

J

I

J

-CH C-CH -C-CH -C-CH -C-Cr £ I £ I 2 I 2 I H H H H CH3 H CH3 H Syndiotactic:-CH C-CH -6-CH -C-CH -C-Cr H CH H CH^ CH3 CH3 H CH3 Atactic: --CH C-CH -C-CH -C-CH -C-Cr 0

0

2

0

2

0

2

2

CHq CH3 C-CH -C-H1 H 2

CH3 C-CH

H1 2

-6— Cl H

H

Q

3

9

£

0

I

£

0

I

£

0

I

£

I

H H 1 C-CH -C-1 CH CH 3 3 0

0

H Figure 5.

H

CH^

H

Polypropylene.


0


16.


345

l a c o n i c entry i n h i s p e r s o n a l notebook: "Today we made polypropylene." But h i s group l o s t no time i n determining that the c r y s t a l l i n i t y r e s u l t e d from the remarkable fact that polypropylene had a stereoregular, " a l l - d " structure, which Natta, at h i s wife's suggestion, christened " i s o t a c t i c " (16). Another regular form, with the side groups appearing a l t e r n a t e l y on opposite sides of the main c h a i n ( d l d l d l d l d l ) was c h r i s t e n e d " s y n d i o t a c t i c , " and the random configuration was christened "atactic" (Figure 5). Natta and h i s coworkers proceeded r a p i d l y and b r i l l i a n t l y t o synthesize other stereoregular p o l y o l e f i n s (from 1-butene, styrene, 4-methy1-1-pentene, e t c . ) and t o e s t a b l i s h t h e i r c h a i n configurations and c r y s t a l habits. They found that the chain always assumes a s p i r a l configuration i n the c r y s t a l , most commonly three monomer units per complete r e v o l u t i o n (the "threefold h e l i x " ) (17). This impressive body of work r e s u l t e d i n Natta's sharing the Nobel P r i z e i n chemistry with Z i e g l e r i n 1963 and i n a race for commercial production of polypropylene by numerous companies i n many countries. Completely i s o t a c t i c polypropylene i s too h i g h l y c r y s t a l l i n e to be u s e f u l f o r most a p p l i c a t i o n s . Depending on c a t a l y s t and c o n d i t i o n s , some amorphous a t a c t i c polymer i s formed i n a l l p r a c t i c a l commercial processes, and l e a v i n g a c e r t a i n amount i n the product modifies the c r y s t a l l i n i t y to some extent. For a p p l i c a t i o n s t h a t demand v e r y good i m p a c t s t r e n g t h and l o w - t e m p e r a t u r e f l e x i b i l i t y , however, something more i s needed. The best approach i s again that taken with l i n e a r polyethylene: the introduction of s m a l l amounts of higher a - o l e f i n s as comonomers. Copolymerization w i t h s m a l l amounts of e t h y l e n e i s a l s o used t o improve impact strength. As with a l l commercial polymers, nonpolymeric a d d i t i v e s are a l s o employed to improve such properties as s t a b i l i t y , c l a r i t y , and processing c h a r a c t e r i s t i c s . Manufacturing Processes A recent r e v i e w of p o l y o l e f i n p r o d u c t i o n processes (18) p r o v i d e s i n f o r m a t i o n on types of equipment used and o p e r a t i n g c o n d i t i o n s . Summarized i n the f o l l o w i n g s e c t i o n s are the major processes i n current use. High-Pressure LDPE. The o r i g i n a l ICI (and, l a t e r , Du Pont) process uses s t e e l autoclave reactors, operating at pressures ranging from 15,000 to 40,000 psig. and temperatures ranging from 150 to 250 °C, w i t h oxygen or a p e r o x i d e as a ( f r e e - r a d i c a l ) p o l y m e r i z a t i o n initiator. The Union Carbide Company i n the U n i t e d S t a t e s and BASF (Badische) i n Germany developed processes u s i n g t u b u l a r r e a c t o r s . These processes g i v e products that d i f f e r i n molecular weight and branching from the ICI process. Low-Pressure, L i n e a r HDPE. The Standard of I n d i a n a process i s a s o l u t i o n process t h a t uses a molybdena-on-alumina c a t a l y s t and operates a t p r e s s u r e s up to 1000 p s i g . and temperatures up t o 300 °C. The o r i g i n a l P h i l l i p s process i s a s o l u t i o n process t h a t operates i n s t i r r e d reactors and employs a suspended s o l i d c a t a l y s t . The polymer s o l u t i o n i s f i l t e r e d f o r c a t a l y s t removal and then



346


stripped for solvent removal. The l a t e r P h i l l i p s " p a r t i c l e form" process i s a s l u r r y process i n which the polymer p r e c i p i t a t e s as i t forms. T h i s process uses a c i r c u l a t i n g - l o o p r e a c t o r . Because of improved c a t a l y s t use e f f i c i e n c y , c a t a l y s t removal from the polymer i s unnecessary. The Z i e g l e r process uses a f i n e l y d i v i d e d c a t a l y s t ( t i t a n i u m h a l i d e and an aluminum a l k y l ) i n a s o l v e n t and operates at temperatures ranging from 50 to 120 °C and p r e s s u r e s r a n g i n g from 150 to 300 psig. The product i s treated to deactivate the c a t a l y s t and remove residues. Gas-phase processes (monomer vapor p o l y m e r i z e d i n absence of l i q u i d phase) have been d e v e l o p e d by BASF and a number of other companies. These processes depend on h i g h c a t a l y s t e f f i c i e n c y to obviate c a t a l y s t d e a c t i v a t i o n and removal. LLDPE. Few process d e t a i l s have been revealed by the manufacturers, but from patent d i s c l o s u r e s and other i n f o r m a t i o n i t i s apparent that LLDPE i s made by a v a r i e t y of processes: s o l u t i o n , s l u r r y , and gas phase. For the s o l u t i o n process, c a t a l y s t s of the Z i e g l e r type, for example, a combination of t i t a n i u m t r i c h l o r i d e w i t h aluminum t r i c h l o r i d e and an aluminum a l k y l , are used. Reaction temperature i s a p p r o x i m a t e l y 160 °C, and pressure i s a p p r o x i m a t e l y 300 p s i g . C a t a l y s t p r o d u c t i v i t y i s q u i t e h i g h , and c a t a l y s t removal i s unnecessary. S l u r r y processes may use a combination of organo-aluminum and organo-magnesium compounds with titanium t e t r a c h l o r i d e . Phillipstype c a t a l y s t s (supported chromium compounds) are a l s o used. Pressures and temperatures are moderate. A h y b r i d process ( I C I ) uses two s t i r r e d a u t o c l a v e stages and adds a f r e e - r a d i c a l i n i t i a t o r i n the f i r s t (high-pressure) stage and a Z i e g l e r - t y p e c a t a l y s t i n the second (lower-pressure) stage. Polypropylene. Polypropylene i s u s u a l l y manufactured by a s l u r r y process that uses a titanium trichloride-aluminum a l k y l c a t a l y s t i n a s t i r r e d reactor at temperatures of 60 to 90 °C and pressures of a few hundred pounds per square inch. The product s l u r r y i s flashed to remove p r o p y l e n e and t r e a t e d to d e a c t i v a t e the c a t a l y s t and remove r e s i d u e s . I f necessary, amorphous polymer i s removed by s o l v e n t extraction. S o l u t i o n , bulk (no solvent) and gas-phase processes have a l s o been developed and are i n commercial use. Recent improvements i n c a t a l y s t a c t i v i t y have reduced requirements to a p o i n t such t h a t c a t a l y s t removal i s no longer necessary i n some processes. A r e c e n t l y announced development promises to extend the technology used i n making LLDPE to polypropylene manufacture. That objective i s being pursued i n a j o i n t program of S h e l l Chemical, the second l a r g e s t U.S. producer of p o l y p r o p y l e n e , and Union C a r b i d e , one of the o r i g i n a t o r s of LLDPE. The process operates i n gas phase w i t h a h i g h l y e f f i c i e n t c a t a l y s t and i s expected to a c h i e v e s u b s t a n t i a l savings i n c a p i t a l and operating costs. In 1984, t h i s p r o j e c t had reached p i l o t p l a n t s t a g e , w i t h commercial production targeted for the f o l l o w i n g year i n a converted high-pressure polyethylene p l a n t .


16.


347


Other P o l y o l e f i n s . V i r t u a l l y every a v a i l a b l e a - o l e f i n has been polymerized with Ziegler/Natta-type c a t a l y s t s . The polymers are of c o n s i d e r a b l e s c i e n t i f i c i n t e r e s t (17), but few have a c h i e v e d commercial s t a t u s . The f o l l o w i n g o l e f i n s a r e of p a r t i c u l a r interest. P o l y ( l - b u t e n e ) . The i s o t a c t i c polymer i s not p a r t i c u l a r l y h i g h m e l t i n g (125-130 °C) (19, 24), but i s e x c e p t i o n a l l y s t r o n g and tough. I t i s manufactured on a r e l a t i v e l y s m a l l s c a l e by S h e l l Chemical i n the U n i t e d S t a t e s and by H u l s i n Germany (18). I t s a p p l i c a t i o n s i n c l u d e heavy-duty bags and p i p i n g . The polymer e x h i b i t s polymorphism (20) and undergoes a gradual phase t r a n s i t i o n that complicates i t s a p p l i c a t i o n technology. An examination of i t s unusual s t r e s s - s t r a i n curve suggests p o t e n t i a l u t i l i t y as a leather substitute (perhaps i n either f i b e r mat or r e t i c u l a t e d foam form), but t h i s p o s s i b i l i t y apparently has not been a c t i v e l y pursued. Poly (4-methy 1-1-pentene). The i s o t a c t i c polymer, as d e s c r i b e d by N a t t a (21), S a i l o r s and Hogan (18), and S c h i l d k n e c h t (25), i s a hard, high-melting, transparent, and chemically r e s i s t a n t p l a s t i c . I t i s manufactured by I C I i n Japan (18) and i s s o l d f o r s p e c i a l t y uses, notably laboratory and medical g l a s s w a r e . " Commercial Production P o l y o l e f i n manufacturing p l a n t s now e x i s t i n about 45 c o u n t r i e s (18). As of e a r l y 1984, the U n i t e d S t a t e s had 11 each of h i g h p r e s s u r e LDPE producers; l i n e a r HDPE producers; and polypropylene producers. In a d d i t i o n , t h e r e a r e e i g h t producers of LLDPE w i t h s e v e r a l more p l a n t s planned or under construction, and one producer of p o l y ( l - b u t e n e ) . A (January 1984) l i s t of producers and p l a n t capacities i s given i n Table I I I . However, any such enumeration i s rendered obsolete immediately by the numerous changes i n corporate s t r u c t u r e , s t r a t e g i e s , and m a n u f a c t u r i n g c a p a b i l i t i e s t h a t characterize the p o l y o l e f i n industry today and for the near future. In addition to the expansions, conversions, and c l o s i n g s announced by c u r r e n t producers, s e v e r a l other companies are known t o have plans for entering p o l y o l e f i n production. Growth c u r v e s f o r LLDPE, p o l y p r o p y l e n e , and HDPE are shown i n F i g u r e 6. S t a r t i n g a year or two l a t e r than HDPE, p o l y p r o p y l e n e p r o d u c t i o n has grown a t a comparable r a t e , and LLDPE p r o d u c t i o n , which started two decades l a t e r , i s s e t t i n g a s i m i l a r pace. In Figure 7, the rates of growth of the three major p o l y o l e f i n s a r e compared w i t h t h o s e o f t h e o t h e r two l a r g e s t v o l u m e thermoplastics: PVC and p o l y s t y r e n e . For t h i s comparison, the growth c u r v e s were s h i f t e d on the time a x i s to a common s t a r t i n g point, and year zero for each product i s the year i n which i t f i r s t attained a volume of 50 m i l l i o n l b . Several s t r i k i n g facts are made evident by these comparisons: 1.

The p o l y o l e f i n s as a c l a s s have by far the highest growth rate and g r e a t e s t t o t a l volume of a l l t h e r m o p l a s t i c s . R e c e s s i o n induced f l u c t u a t i o n s aside, there i s l i t t l e evidence of l e v e l i n g off of t o t a l demand.

American Chemical Society Library In Applied Polymer Science; Tess, R., et al.;

ACS Symposium Series; American Chemical 1155 16th st f Society: N.W. Washington, DC, 1985.


348

Table I I I .

U . S . P o l y o l e f i n Producers and Capacities (January 1984) P o l y o l e f i n Capacity (thousands of metric tons per year)


Producer Allied American Hoechst Amoco (Std. of Ind.) ARCO ( A t l a n t i c R i c h f i e l d ) Chemplex Dow Du Pont E l Paso Exxon Gulf Himont Mobil N a t l . Petrochemicals Northern Petro. Phillips Shell Soltex Texas Eastman Union Carbide USI USS Chemicals Total

LDPE

190 440 320 170 265 410

HDPE

LLDPE

320 110 160 155 130 170 405

Polypropylene

b a

260 15 170 250 195 340

2965

100 610

2690

2450

10030

a

625 180 180 330

1925

150 180 180 590

a

a

255

235

a

15 270

225

320 110 390 335 320 925 725 335 715 850 605 395 250 550 725 350 430 345 790 330 235

230 180 315

b

Total

100 100 350 90 65

a

* Plant can produce LDPE instead Plant can produce HDPE instead (Reproduced with permission from Ref. 37.)




Figure 6.

349

U.S. p r o d u c t i o n of high-density polyethylene, polypropylene, and l i n e a r low-density polyethylene.



350

M i l l i o n s of pounds •^ ° Low-density P E ' V ' * 0O


14,000

Figure 7.

x

"

Comparison of growth rates of major thermoplastics (Time axis adjusted to common o r i g i n ) .


16. MCMILLAN Polyolefins 2. 3.

351

The growth h i s t o r i e s of the p o l y o l e f i n s have been remarkably s i m i l a r , despite a spread of nearly h a l f a century i n the times of i n c e p t i o n . There i s a near o v e r l a p of the growth c u r v e s f o r the p o l y o l e f i n s , i n one c l a s s , and f o r PVC and p o l y s t y r e n e i n another c l a s s . T h i s c o i n c i d e n c e i s s u r p r i s i n g i n view of the d i s p a r i t i e s of properties, uses, and time s c a l e s , and suggests that there may be some c o n t r o l l i n g factors that are g e n e r a l l y unrecognized.


Properties and Applications P h y s i c a l properties of the p o l y o l e f i n s are summarized i n Table I V . Considerable v a r i a t i o n i s , of course, possible among different grades of the same p o l y o l e f i n , and useful modifications are obtained by copolymerization, blending with other polymers, and compounding w i t h v a r i o u s a d d i t i v e s . As w i t h other c r y s t a l l i n e p l a s t i c s , the p o l y o l e f i n s are not benefited by a d d i t i o n of l i q u i d p l a s t i c i z e r s . Such "foreign" molecules are g e n e r a l l y not accepted by the c r y s t a l l a t t i c e , or, i f incorporated, cause excessive weakening. Once the polymers have been made, the "chemistry" of p o l y o l e f i n s i s e s s e n t i a l l y "nonchemistry," i n the sense t h a t e v e r y t h i n g i s d i r e c t e d toward s u p p r e s s i n g u n d e s i r a b l e r e a c t i o n s . Thus, p u r i f i c a t i o n processes are a p p l i e d to remove harmful c a t a l y s t r e s i d u e s , and a v a r i e t y of s t a b i l i z e r s are added t o prevent o x i d a t i o n , m e t a l - c a t a l y z e d d e c o m p o s i t i o n , UV l i g h t d e g r a d a t i o n , thermal decomposition, etc. The effectiveness of these techniques can be gauged by the f a c t t h a t u n s t a b i l i z e d p o l y o l e f i n s cannot be f a b r i c a t e d i n o r d i n a r y manufacturing equipment without s i g n i f i c a n t d e g r a d a t i o n , and an unpigmented, u n s t a b i l i z e d f i l m w i l l become b r i t t l e i n a few days of e x t e r i o r exposure; y e t when s u i t a b l y compounded formulations are used f o r a p p l i c a t i o n s such as i n d o o r outdoor carpeting or molded stadium seats, they withstand years of such exposure. The easy p r o c e s s a b i l i t y of the p o l y o l e f i n s permits ready f a b r i c a t i o n by nearly a l l conventional processes. This v e r s a t i l i t y , plus t h e i r low cost, low density, and e x c e l l e n t balance of p h y s i c a l properties, gives them wide a p p l i c a b i l i t y i n many different end-use f i e l d s . We now encounter p o l y o l e f i n s i n our d a i l y l i v e s i n the form of f i b e r s ( e . g . , i n d o o r - o u t d o o r c a r p e t i n g , u n d e r w e a r , and u p h o l s t e r y ) , f i l m (packaging), blow-molded c o n t a i n e r s (squeeze b o t t l e s , detergent b o t t l e s ) , i n j e c t i o n molded a r t i c l e s (housewares, a p p l i a n c e housings, auto p a r t s ) , and e x t r u s i o n s ( w i r e i n s u l a t i o n , pipe). S t a t i s t i c s on major a p p l i c a t i o n s and accounts of new developments are published r e g u l a r l y (22). But what are not apparent i n the published descriptions are the c o u n t l e s s and g r i e v o u s p r o b l e m s encountered and s o l v e d by a p p l i c a t i o n s research groups and process engineers to bring each new p o l y o l e f i n i n t o s u c c e s s f u l p r o d u c t i o n and use. For example, polyethylene i s subject to s o - c a l l e d " s t r e s s c r a c k i n g " or " s t r e s s corrosion," which i s worse the higher the degree of c r y s t a l l i n i t y . L i n e a r p o l y e t h y l e n e was a l s o found to be prone to s e v e r e warping; moreover, moldings would even disintegrate i f exposed to hot a i r for some hours. These problems reached c r i s i s proportions i n the e a r l y marketing phase and nearly caused abandonment of some polyethylene


352


Table I V .

Typical Properties of P o l y o l e f i n s High-Pressure LDPE


Property

3

Linear HDPE

LLDPE

Polypropylene

S p e c i f i c gravity (g/cra )

0.92

0.96

0.93

0.90

Modulus of e l a s t i c i t y (tens.) (10 l b / i n . )

0.2

1.0

NA

1.5

Tensile strength (lb/in. )

1500

4000

3000

5000

Impact strength (ft-lb/in.)

very high

s

2

2

Softening temp. (°C)

100

Max. service temp. (°C)

80-90

B r i t t l e temp. (°C) E l e c . resistance (ohm-cm) Stress crack time Clarity

a

120

very low 1 6

10 -10

1

1 7

21 days translucent

110-120 -90

lO^-lO

150

110

130-140

90-105

-10

-100 1

3 min

1 6

NA

10 -10

24 h

high

semi translucent to opaque transparent

3

1 7

translucent to transparent

3

o r i e n t e d or nucleated

Note:

Properties vary widely with different grades of each p o l y o l e f i n .



16.


353

ventures. The day was saved temporarily, though i n e l e g a n t l y , by the onset of the "hula-hoop" c r a z e , which p r o v i d e d f o r a time a n o n c r i t i c a l o u t l e t for polymer. E v e n t u a l l y , permanent s o l u t i o n s were found by the polymer c h e m i s t s , who v a r i e d m o l e c u l a r weight d i s t r i b u t i o n and introduced a second monomer to modify c r y s t a l l i n i t y and thus modify behavior. P u r i s t s may have been disappointed to learn that the t h e o r e t i c a l i d e a l of a monodisperse, "chemically pure" stereoregular polymer i s not the p r a c t i c a l i d e a l . But we are a l l b e n e f i c i a r i e s of the m o d i f i c a t i o n s and even " a d u l t e r a t i o n s " t h a t a l l e v i a t e d t h e d e f i c i e n c i e s that would otherwise have kept the pure polymers from becoming useful p l a s t i c s . A f a s c i n a t i n g b i t of s c i e n t i f i c d e t e c t i o n was p r o v i d e d by e l e c t r o n microscope s t u d i e s of c r y s t a l l i t e s c a r r i e d out a t B e l l L a b o r a t o r i e s by K e i t h and Padden (23). T h e i r h i g h - m a g n i f i c a t i o n e l e c t r o n micrographs of very c a r e f u l l y prepared samples of h i g h l y c r y s t a l l i n e p o l y e t h y l e n e r e v e a l e d t h a t the spaces between the d e n d r i t i c c r y s t a l l i t e s contain many t h i n filaments stretching from one c r y s t a l to another. These appear to be bundles of o r i e n t e d molecules drawn out into f i b e r s as material moved i n t o the c r y s t a l lites. Such f i l a m e n t s , or " t i e m o l e c u l e s , " are presumably what holds the c r y s t a l l i z e d material together, and i t i s probably t h e i r s c i s s i o n by oxidation that caused rapid l o s s of strength i n h i g h l y c r y s t a l l i n e samples exposed to hot a i r . C r i s e s were a l s o encountered when p o l y p r o p y l e n e was f i r s t introduced: inherent high s u s c e p t i b i l i t y to oxidation, r e l a t i v e l y high b r i t t l e temperature, void formation i n moldings, etc. One by one, these problems were surmounted by polymerization, processing, or compounding v a r i a t i o n s worked out by numerous unsung heroes i n laboratories and plants. Their successes are recorded i n the trade and patent l i t e r a t u r e , but t h e i r enduring monument i s the tremendous growth history of p o l y o l e f i n p l a s t i c s . Consumption of P o l y o l e f i n s Recent consumption patterns for polyethylene and polypropylene are shown i n T a b l e V . Data f o r LLDPE and LDPE are combined i n t h i s t a b u l a t i o n . Because of r a p i d l y s h i f t i n g market p a t t e r n s and the f l e x i b i l i t y provided by dual-purpose plants, the r a t i o of LLDPE to LDPE varies from time to time, but the long-term trend i s strongly upward. Comparison of the t o t a l estimated consumption with production capacities indicates that the p o l y o l e f i n plants have been operating at s l i g h t l y over 80% of capacity. This represents some improvement o v e r p r e v i o u s y e a r s , and f u r t h e r improvement may r e s u l t from the conversion or c l o s i n g of a d d i t i o n a l LDPE plants. C e r t a i n f i e l d s of use are u n i q u e l y s u i t e d t o p o l y p r o p y l e n e because of i t s h i g h e r hardness, s t r e n g t h , and m e l t i n g p o i n t , and p a r t i c u l a r l y because of i t s c a p a b i l i t y of being h i g h l y oriented by drawing at temperatures just below the c r y s t a l melt point to produce f i b e r s , monofilaments, or o r i e n t e d f i l m s . Drawing enhances and r e a l i g n s the c r y s t a l l i n e domains so e f f e c t i v e l y that strength i n the draw d i r e c t i o n i s d r a m a t i c a l l y i n c r e a s e d . In f i l m s , strength, s t i f f n e s s , and c l a r i t y are a l l improved by o r i e n t a t i o n , p a r t i c u l a r l y when i t i s two-dimensional.


354


Table V.

U . S . Consumption of P o l y o l e f i n s (1983) (36, 38)


Fabrication Method

Typical Products

P o l y o l e f i n Consumption (thousands of metric tons) LLDPE,LDPE HDPE Polypropylene

Blow molding

B o t t l e s , other containers, housewares, toys

20

817

39

Extrusion

Film (packaging and i n d u s t r i a l ) , pipe, wire & cable, coated paper, fibers

2517

540

741

Injection molding

Containers, packages, housewares, toys, appliances, auto parts

289

548

43

25

298

248

284

3167

2178

1603

409

468

359

2646

1962

Rotomolding

Large containers

Other Total U . S . Consumption Export Total sales Total s a l e s , a l l polyolefins

3576

539

8184



16.


355

The m e l t i n g p o i n t of p o l y p r o p y l e n e f i b e r i s not q u i t e h i g h enough to permit i r o n i n g of f a b r i c s ; hence, i t s use i n garments i s l i m i t e d , and the major f i b e r o f f t a k e s at present are i n c a r p e t i n g and u p h o l s t e r y . Another important " f i b e r " use i s as a s u b s t i t u t e for j u t e or other coarse f i b e r s i n woven bags. The " f i b e r s " are made by drawing extruded, s l i t f i l m u n t i l f i b r i l l a t i o n occurs. New markets are being c r e a t e d , and o l d ones expanded, at such rates that the exponential growth shown i n Figure 7 seems l i k e l y to c o n t i n u e f o r some time. S t a n f o r d Research I n s t i t u t e has even p r e d i c t e d , on the b a s i s of l o n g - r a n g e e x t r a p o l a t i o n s , t h a t by the year 2000 U.S. production of p o l y o l e f i n s w i l l reach the staggering t o t a l of almost 100 b i l l i o n l b . I t i s predicted that p o l y o l e f i n s w i l l then (as they do a l r e a d y ) c o n s t i t u t e w e l l o v e r h a l f of a l l thermoplastics production. New Monomers and Polymers The o r i g i n a l , simplest p o l y o l e f i n s , polyethylene and polypropylene, c o n t i n u e to dominate the scene, even a f t e r two decades, t o such an e x t e n t t h a t no other p o l y o l e f i n even appears on the p r o d u c t i o n charts. Nevertheless, a great many (we may assume a l l ) a v a i l a b l e o l e f i n s have been tested, and many have been found capable of being c o n v e r t e d to s t e r e o r e g u l a r polymers. As was mentioned above, p o l y ( l - b u t e n e ) and poly(4-methy1-1-pentene) are being o f f e r e d commercially and may be expected to achieve s i g n i f i c a n t volume i n the f u t u r e . I s o t a c t i c and s y n d i o t a c t i c p o l y s t y r e n e are of much t h e o r e t i c a l i n t e r e s t (26) but are not yet commercial products. As we have a l r e a d y seen, many of the d e f i c i e n c i e s of the o r i g i n a l s t e r e o p o l y o l e f i n s have been c o r r e c t e d by j u d i c i o u s i n c l u s i o n of s m a l l amounts of comonomers. The advancing techniques for making b l o c k and g r a f t copolymers open e x t e n s i v e l i n e s f o r a d d i t i o n a l improvements. One of the e a r l i e s t and most noteworthy c o n t r i b u t i o n s i n c o p o l y m e r i z a t i o n i s the e t h y l e n e - p r o p y l e n e copolymer rubbers (27). However, these have been l a r g e l y superseded by t e r p o l y m e r systems i n which r e s i d u a l u n s a t u r a t i o n has been introduced by i n c l u s i o n of s m a l l amounts of a t h i r d monomer which i s an unconjugated d i o l e f i n . These are no longer s t r i c t l y p o l y o l e f i n s but f a l l c l e a r l y i n the category of s y n t h e t i c rubbers and are discussed i n a separate chapter. Growth of Understanding On the s c i e n t i f i c side, considerable progress has been made on two fronts: learning how the c a t a l y s t s operate to make stereoregular polymers and l e a r n i n g how the p r o p e r t i e s of the polymers are c o n t r o l l e d by chain shape, chain length, and, p a r t i c u l a r l y , c r y s t a l habit. C a t a l y s t s and K i n e t i c s . Hundreds of variants and combinations of c a t a l y s t s , c o c a t a l y s t s , c a t a l y s t pretreatments, and r e a c t i o n conditions have been discovered and described, mostly i n the patent l i t e r a t u r e (28). I t i s now g e n e r a l l y agreed that most coordination p o l y m e r i z a t i o n s are heterogeneous, but that some are c l e a r l y homogenous. The b a s i c c h a r a c t e r i s t i c t h a t d i s t i n g u i s h e s a l l Z i e g l e r / N a t t a - t y p e s t e r e o r e g u l a r polymerization c a t a l y s t s i s that



356


they i n v o l v e coordination of the entering monomer with a t r a n s i t i o n metal i n an organometallie complex ( t y p i c a l l y , titanium t r i c h l o r i d e and aluminum t r i e t h y l ) , with monomer i n s e r t i o n occurring between the metal atom and the growing c h a i n . Thus, the polymer c h a i n "grows l i k e a h a i r " ( F i g u r e 8) i n marked c o n t r a s t to f r e e - r a d i c a l polymerization, i n which monomer addition occurs at the opposite end of the chain from the i n i t i a t o r . The precise nature of the a c t i v e s i t e s and the exact mechanism of the reaction are, after two decades of a c t i v e research, s t i l l the s u b j e c t s of i n v e s t i g a t i o n and l i v e l y debate. The d e t a i l e d d e s c r i p t i o n of the p o l y m e r i z a t i o n process g i v e n 20 years ago by Arlman and Cosee (29) s t i l l has current v a l i d i t y , although a great d e a l more d e t a i l and some f u r t h e r i n s i g h t h a v e been added subsequently. A r e v i e w of c a t a l y s t s t r u c t u r e s and r e a c t i o n mechanisms i s the subject of a separate chapter. Structure vs. Properties. In the area of polymer behavior, knowledge of the effects of molecular weight d i s t r i b u t i o n and of shortand long-range branches has been developed for the p o l y o l e f i n s more or l e s s i n common w i t h other h i g h polymers to which the same p r i n c i p l e s apply. The factor of greatest interest and importance i n the morphology of p o l y o l e f i n s , however, i s the c r y s t a l l i n e domains, because t h e i r s i z e , number, s t r u c t u r e , and t r a n s f o r m a t i o n s have a dominant i n f l u e n c e on r h e o l o g y and p h y s i c a l p r o p e r t i e s . Here progress has p a r a l l e l e d and b e n e f i t e d from the work w i t h other c r y s t a l l i n e polymers, p a r t i c u l a r l y p o l y e t h y l e n e t e r e p h t h a l a t e (PETP). S i n c e the p i o n e e r i n g (and i n i t i a l l y c o n t r o v e r s i a l ) work of K e l l e r (30), who made the s t a r t l i n g proposal that the c r y s t a l a x i s i n the l a m e l l a e of oriented PETP f i b e r s i s perpendicular to the draw a x i s , many workers have studied l a m e l l a r and s p h e r u l i t i c structures i n c a r e f u l l y prepared s i n g l e c r y s t a l s and i n b u l k samples. Much l i g h t ( i f e l e c t r o n microscope i l l u m i n a t i o n may be classed as l i g h t ) has been c a s t on how such c r y s t a l s respond to s t r e s s and on how t h e i r t r a n s f o r m a t i o n s are i n v o l v e d i n the s t r i k i n g a l t e r a t i o n of physical properties that occur on u n i a x i a l (fiber) or b i a x i a l ( f i l m ) orientation. Studies such as those published by G e i l (31) and the work of K e i t h , et a l . mentioned above (23), are the headwaters of a stream of c o n t i n u i n g c o n t r i b u t i o n s t h a t has s w e l l e d to a volume s u f f i c i e n t to support a s p e c i a l i z e d journal (32). These e x p l o r e r s of the c r y s t a l domains have presented us w i t h f a s c i n a t i n g p i c t u r e s of a m i c r o w o r l d f u l l of macro s u r p r i s e s and have brought our knowledge of t h i s ruorphology to the point where i t should have p r e d i c t i v e v a l i d i t y and should generate v a l u a b l e new technology. I t i s true of polypropylene as i t i s of metals that i t s highly useful properties derive p r i m a r i l y from i t s m i c r o c r y s t a l l i n e structure, and so do some of i t s d e f i c i e n c i e s . Hence the a b i l i t y to modify t h a t s t r u c t u r e by s t r e s s e s imposed under c o n t r o l l e d c o n d i t i o n s , e i t h e r e x t e r n a l or i n t e r n a l — s k i l l at m a n i p u l a t i n g raicrocrystals—is a v i t a l key to o b t a i n i n g an optimum b a l a n c e of p r o p e r t i e s for a g i v e n a p p l i c a t i o n . A s t r i k i n g demonstration of t h a t f a c t i s afforded by a comparison of i s o t a c t i c w i t h a t a c t i c polypropylene, which i s e n t i r e l y amorphous and for which no l a r g e volume p r a c t i c a l use has yet been found.





357

358



The Future As p o l y o l e f i n s celebrate t h e i r 50th birthday, one might expect to observe the c h a r a c t e r i s t i c s of s t a b i l i t y , low growth r a t e s , and absence of change or innovations that are commonly associated with m a t u r i t y i n e i t h e r an i n d u s t r y or a s c i e n t i f i c f i e l d . But the a c t u a l s i t u a t i o n i s f a r d i f f e r e n t , so much so t h a t a r e c e n t p u b l i c a t i o n by SRI I n t e r n a t i o n a l i n t r o d u c e s the s u b j e c t w i t h the statement that recent advances have spotlighted p o l y o l e f i n s as one of the most e x c i t i n g areas of development i n the chemical industry, and that the outlook for the p o l y o l e f i n s industry through the next decade i s one of change. The c u r r e n t wave of p l a n t c l o s i n g s , expansions, s a l e s , conversions, and modifications are evidence that the changes are w e l l under way. I t i s t h e r e f o r e p e r t i n e n t to c o n s i d e r what d i r e c t i o n s f u t u r e advances i n s c i e n t i f i c understanding and t e c h n o l o g i c a l progress i n the p o l y o l e f i n f i e l d might take. As has been pointed out repeatedly i n t h i s review, even 40 years ago enough was known about polymers f o r any number of chemists to have made the necessary mental connections between known p r i n c i p l e s and e x p e r i m e n t a l f a c t s t o p o s t u l a t e and t h e n d e m o n s t r a t e stereoregular polymerization. Today there are many more who, armed with the knowledge of what has happened i n the past, b e n e f i c i a r i e s of the l e g a c y of understanding d e v e l o p e d and bequeathed by Staudinger, Mark, Tobolsky, Huggins, F l o r y , Schildknecht, Z i e g l e r , N a t t a , and a host of o t h e r s , and equipped w i t h a l l the powerful t o o l s of modern science, are i n a p o s i t i o n to put mind and s k i l l to work and a t l e a s t make t h e h a c k n e y e d p h r a s e , " t a i l o r - m a d e m o l e c u l e s , " a r e a l i t y by d e l i b e r a t e l y c r e a t i n g the next major advance i n p o l y o l e f i n s or i n some r e l a t e d f i e l d . Enumeration of problems i s far easier than s o l v i n g them, but i t i s a necessary f i r s t s t e p . The f o l l o w i n g are some q u e s t i o n s , the answers to which might represent s i g n i f i c a n t advances i n knowledge and, e v e n t u a l l y , i n p r a c t i c a l a p p l i c a t i o n s : Polymerization Problems 1. 2.

3.

Can e f f e c t i v e s t e r e o s p e c i f i c c a t a l y s t s t h a t work i n aqueous systems be found? Now that the d i f f i c u l t feat of c o n t r o l l i n g chain configuration has been mastered, can we achieve s i m i l a r l y precise c o n t r o l over chain length? Offhand, t h i s would have seemed to be an easier task, but i t has not yet been accomplished, except i n respect to average chain l e n g t h s . P e r h a p s t h e r e has not been a s u f f i c i e n t l y s t r o n g i n c e n t i v e , even though m o l e c u l a r weight average and d i s t r i b u t i o n have long been recognized as of prime importance i n many f i e l d s of t e c h n o l o g y . For example, recent work has shown that narrowing the molecular weight d i s t r i b u t i o n of polypropylene improves i t s processing c h a r a c t e r i s t i c s (33). What w o u l d be t h e e f f e c t of a much c l o s e r a p p r o a c h t o monodisperse chain length? What other monomers can be s e l e c t e d or s y n t h e s i z e d t h a t would give predictably unique and useful properties? Anyone can spot an asymmetric carbon atom, but the a b i l i t y to v i s u a l i z e i n


16.

4.


5.

6. 7.

8.


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advance the s p e c i f i c configuration of the stereoregular form of a new hydrocarbon polymer ( i d e n t i t y p e r i o d , dimensions of the unit c r y s t a l c e l l , c r y s t a l melt point, etc.) i s only poorly and p a r t l y evident. Can that c a p a b i l i t y be greatly expanded? C o p o l y m e r i z a t i o n i s w i d e l y used but p o o r l y c o n t r o l l e d . Can p r e c i s e c o n t r o l be e s t a b l i s h e d , f o r example, c o u l d we make a r e g u l a r l y a l t e r n a t i n g copolymer (ABABABABAB) i n s t e r e o r e g u l a r form? A d d i t i o n a l p o s s i b i l i t i e s for advances i n f u l l y c o n t r o l l e d copolymers, t e r p o l y m e r s , g r a f t and b l o c k copolymers, e t c . are too numerous to enumerate. C o n t r i b u t i o n s to the b a s i c understanding of c o o r d i n a t i o n c a t a l y s i s w i l l b r i n g us g r a d u a l l y c l o s e r to the day when we cannot only e x p l a i n e m p i r i c a l l y discovered effects but predict r e s u l t s and d e l i b e r a t e l y design new c a t a l y s t systems. Is there some s t i l l unrecognized source of s t i l l higher order i n hydrocarbon polymers that could be demonstrated and applied to y i e l d a useful new c l a s s of polymers? Does the w o r l d have need, or even room, f o r another l a r g e volume, general purpose p l a s t i c ? I t seems very u n l i k e l y that i t does, but so i t seemed a l s o on the eve of the Z i e g l e r - N a t t a discoveries. Years ago, i t was p r e d i c t e d (by Herman Mark) t h a t an important advance would come from the combination of t h e r m o p l a s t i c s technology with thermosetting technology, that i s , by use of a l l four of the strengthening mechanisms named e a r l i e r (Figure 1) i n a s i n g l e polymer. Numerous laboratories have worked on various aspects of t h i s approach, but the big breakthrough i s apparently s t i l l to be made.

Postpolymerization Processing 1.

2.

3.

Manipulation of m i c r o c r y s t a l l i n e domains appears to provide such diverse phenomena and so many p o t e n t i a l l y useful effects that we may be sure they have not a l l been found; nor have those already found a l l been turned to maximum advantage. We now c o n t r o l s p h e r u l i t e growth to some extent by annealing, nucleation, etc. Solid-phase forming i s a technique .of perpetual promise, long known to be c a p a b l e of producing u s e f u l l y , even r e m a r k a b l y , enhanced properties i n p o l y o l e f i n s , but i s s t i l l awaiting l a r g e scale commercial a p p l i c a t i o n . What further p o s s i b i l i t i e s l i e i n t h i s direction? Enormous i n d u s t r i e s are based on the e m p i r i c a l l y developed processes and moderately w e l l - u n d e r s t o o d phenomena of oned i m e n s i o n a l o r i e n t a t i o n of polymers ( f i b e r drawing); t h e r e i s a l s o some use and some understanding of t w o - d i m e n s i o n a l o r i e n t a t i o n ( b i a x i a l f i l m s t r e t c h i n g , oriented blow molding). I s i t out of the q u e s t i o n to t h i n k t h a t there might be some way of a c h i e v i n g t h r e e - d i m e n s i o n a l o r i e n t a t i o n , with i n t e r e s t i n g s c i e n t i f i c and v a l u a b l e p r a c t i c a l r e s u l t s ? One t h i n k s , f o r example, of c o n t r o l l e d foaming at temperatures just below the c r y s t a l melt point. Learning to c o n t r o l or take advantage of the phase t r a n s i t i o n s that occur i n c e r t a i n stereospecif i c polymers ( p o l y ( l - b u t e n e ) , polystyrene) should lead to i n t e r e s t i n g and useful r e s u l t s .



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Examples in these and other categories could be multiplied indefinitely, but these few w i l l suffice. The fields of compoun ding, additives, blends, grafts, and composites a l l see continual small advances that eventually add up to major gains and w i l l undoubtedly continue to do so. Similarly, new applications and new shaping techniques will continue to be developed (e.g., rotational molding is old hat but not exhausted, solid-phase forming has yet to come into its own, and polyolefin foams have enormous potentials that have scarcely been tapped). So the imaginative chemist does not lack for opportunities to use his imagination or for new worlds to conquer. As we have seen, the unexpected result, the happy accident, has played a key role in nearly a l l of the major advances that have been made to date, and we may expect that fortune will continue to smile occasionally on the alert experimenter in the future. Literature Cited 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

von Pechmann, H. Chem. Ber. 1898, 31, 2643. Bamberger, E . ; Tschirner, F. ibid 1900, 33, 955. Hertzwig, J.; Schonbach, R. Monatsch. 1900, 33, 677. Meerwein, H.; Burneleit, W. Ber. 1928, 618, 1840. Swallow, J. C. In "Polythene"; Renfrew and Morgan, Eds.; Interscience: New York; Chap. 1. Robinson, R. personal communication. Fawcett, E. W.; Gibson, R. O. J. Amer. Chem. Soc., 1934, 386. Mark, H. "Giant Molecules"; Time, Inc.: New York, 1966; p. 129. Ziegler, K. Brennstoff Chemie B-C Bd. 1954, 35(21-21), 321. Petr. Refiner 1955, 34, 111. Ziegler, K.; Martin, H. Angew. Chem. 1955, 67(19/20), 541. McMillan, F. "The Chain Straighteners"; Macmillan: London, 1979; pp. 56-68. Friedrich, M.; Marvel, C. S. J. Amer. Chem. Soc. 1930, 52, 376. Clark, Α.; Hogan, J. P.; Banks, R. L. Papers presented, American Chemical Society Petroleum Chem. Div., April, 1956, p. 211. Peters, Ε. I.; Zletz, Α.; Evering, B. L. Ind. Eng. Chem. 1957, 49, 1879. Huggins, M. J. Amer. Chem. Soc. 1944, 66, 1991. Schildknecht, C. "Vinyl and Related Polymers"; Wiley: New York, 1952. Flory, P. "Principles of Polymer Chemistry"; Cornell Univ. Press, 1953. Schildknecht, C. et a l . , Ind. Eng. Chem. 1948, 40, 2108. Ind. Eng. Chem. 1949, 41, 1998. Natta, G. et a l . , J . Amer. Chem. Soc. 1955, 77, 708. J. Polym. Sci. 1955, 16, 143. Natta, G.; Danusso, F. J . Polym. Sci. 1959, 34, 3. "Stereoregular Polymers and Stereospecific Polymerization"; Natta, G.; Danusso, F., Eds.; Pergamon: New York, 1967. Sailors, H. R.; Hogan, J . P. Macromolecular Sci.-Chem. 1981, A15(7), 1377. Natta, G. Makromol. Chemie. 1955, 16, 213. Natta, G. Ref. 17, p. 268.


16. MCMILLAN

Polyolefins


21. 22. 23. 24.

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Natta, G. Makromol. Chemie. 1960, 35, 93. See, for example, the January issues of Modern Plastics. Keith, H. D. et a l . , J . Polym. Sci. 1966, 4, 267. Schildknecht, C. "Allyl Compounds and Their Polymers (Including Polyolefins)"; Wiley-Interscience: New York, 1973; p. 98. 25. Ibid., p. 103. 26. Natta, G.; Corradini, P. Rend. Accad. Naz. Lincei 1955, (8), 18, 19. Makromol. Chemie. 1955,16, 77. 27. Garrett, Rubber and Plastics Age 1964, 45, 1492. Crespi, G. et a l . , ibid., 1181. 28. Ref. 24, p. 68 et seq. 29. Arlman, E. J . ; Cosee, P. J. of Catalysis 1964, 3, 80-104. 30. Keller, A. J. Polym. Sci. 1955, 17, 291, 351, 447. 31. Geil, P. H. "Polymer Single Crystals"; Interscience: New York, 1963. Chem. and Eng. News Aug. 16, 1955, p. 70. 32. J. Macromolecular Sci.-Physics: Geil, P. H., Ed.; Marcel Dekker: New York. 33. Schroeder, C. W. "Modern Polypropylene Resins"; Fiber Producer Conference, Greeneville, S.C., 1981. 34. Chem. Age. October 1980. Chem Systems. 35. Modern Plastics, October 1980. 36. Modern Plastics, January 1983. 37. SRI International, World Photochemicals Program. 38. Modern Plastics, January 1984.


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