Chapter 1

Introduction

Introduction

1

1.1. Polyolefin

From the beginning of early civilization, human beings have been utilizing natural

polymers for clothes, housing and foods. Polymeric materials have contributed to society

for practical use even from the days prior to scientific developments. As chemistry

progressed, natural polymers were continuously modified to produce semi-synthetic

polymers, for example, cellulose acetate, viscose, rayon etc. The production of fibers from

regenerated cellulose and phenol formaldehyde started commercially in late 19th

century

which brought the symbolic breakthrough for synthetic polymers. 1, 2

Polymer science

started its journey with the loving care of a band of revolutionary physical chemists like

Staudingers, Mark, Meyer, Flory, Huggins and others in overseas and Palit in India. 3- 5

To achieve a higher standard of living, there have been increasing demand to

improve the function and performance of synthetic polymers. They include large volume

materials such as polyethylene (PE), polypropylene (PP) and poly(vinyl chloride) and

special materials such as ethylene–propylene elastomers EPR, EPDM, SBR and

polybutene-1 (PB-1).6-8

The success of polyolefin is logically explained as this polymer

family is found in the majority of industrial domains including films, packaging,

machinery parts, electrical insulators, inks and petroleum additives etc.9

Amongst polyolefins, polyethylene is the fastest growing commodity plastic

material which is reflected by the worldwide consumption of polyethylene (Figure 1.1).10-

13 According to Global Markets direct PE consumption is around 93.87 million tons in

2008, and it is estimated to reach 127.89 million tons in 2015.14

According to Global

Business Intelligence (GBI) research, Asia has emerged as the largest market for

polyethylene and the key driver of global demand. Asia Pacific region accounted for over

43% of the global polyethylene capacity in 2008, and is expected to account for nearly

50% of the planned capacity additions until 2015. Asia Pacific region will have the

maximum capacity additions of over 12 million tons during the period 2008-2015. In this

region, China and India will account for more than 8 million tons of future capacity

additions.14, 15

The largest polyolefin producers throughout the world are LyondellBasell

Industries, SINOPEC, Exxon Mobil Corp, Dow Chemical Company, SABIC, CNPC,

Ineos, Reliance, and TOTAL etc.16

Introduction

2

Figure 1.1: PE and PP growth

1.2. Significance of the Catalytic Systems

In 1898, Hans von Pechmann accidently synthesised the first synthetic linear

polyolefin probably polyethylene from diazomethane with heating.17

But the hydrocarbon

polymer very likely in solid form was obtained by Lind and Glockler by subjecting ethane

to a semi-corona discharge which resembles the present-day high pressure polyethylene

production.18

Unfortunately, they were not able to characterize the product as

polyethylene. In the early period of 1927 ethylene and its homologs were converted to oils

form by boron fluoride catalyst with cobalt as promoter. Friedrich and Marvel, in 1930,

reported the first low molecular weight polyethylene by an incidental polymerization of

ethylene sighting the production of a “non-gaseous” product.19

Also in 1930, Carothers et

al. published the synthesis of paraffin waxes from the reduction of decamethylene bromide

with sodium.20

In 1933, Eric Fawcett and Reginald Gibson (ICI) polymerized ethylene to PE as a

waxy solid form for the first time from the reaction of benzaldehyde and ethylene at 1400

bar and 170°C (Figure 1.2).21

The polymerization was initiated by trace amounts of

oxygen which was unknown to Fawcett and Gibson at that time. Unfortunately, repetition

of the experiment with ethylene gave only trifling amount of solid product due to the

occurring of violent explosion at high pressure. After extensive experiments the explosion

Introduction

3

Figure1.2: Apparatus used by Eric Fawcett and Reginald Gibson to discover

polyethylene.

was prevented by controlling the amount of oxygen in ethylene which acts as initiator. In

1935 Michael Perrin, Imperial Chemical Industries (ICI) chemist converted this accident

into a reproducible high-pressure synthesis for polyethylene that became the basis for

industrial LDPE production.22

His experimental condition yield 8 gm of highly ductile

polyethylene with melting temperature in the range of 110 °C. By 1936, ICI had

developed a larger volume compressor that made possible the production of polyethylene

commercially. Subsequently, high-pressure polyethylene was produced in the United

States since World War II, principally by the Bakelite Division of Union Carbide and

Carbon Chemicals Corporation and by E. I. DuPont under the license from Imperial

Chemical Industries. But the PE produced by free radical mechanism is highly branched

structure resulting into low density in the range of 0.910-0.935 g/c.c.

For obtaining a controllable polymerization, many catalysts and diluents systems

were studied. In addition to oxygen, many other catalysts were identified like hydrogen

peroxide, tert-butyl hydroperoxide, tert-butyl peroxide, methyl peroxide, ethyl peroxide,

acetyl peroxides, peracetic acid, persuccinic acid, diethyl peroxydicarbonate, di-tert-butyl

peroxydicarbonate, and tert-butyl perbenzoate, alkali metal persulfates etc. were

identified.23-33

Amongst these catalysts, only a few catalysts were active at moderate

temperature and pressure.

Introduction

4

After continuous attempts to synthesize PE at lower temperature and pressure led

to the discovery of the Phillips catalyst in the last half of 1951 by Robert Banks and John

Hogan working at Philips research laboratory in Bartlesville and Oklahoma.34-36

The

catalyst consists of calcined chromium oxide supported on silica-alumina (Scheme 1.1)

and other carriers.37- 39

Ethylene was polymerized to high density polyethylene at 88 °C

and pressure of 600 psi. The resulting polymer was initially known by the trade name as

Marlex which was quickly adopted for toy manufacturing. The melting point of the

resulting polymers such as Marlex 50 is 113-127 °C, the density is about 0.96 g/c.c., melt

flow index <1 and the degree of crystallinity is found to be 50 per cent greater than the

crystallinity of the conventional high pressure polyethylenes.40

The catalysts possess more

than one active site with each site having a distinct ratio of chain transfer to propagation

rates and comonomer reactivity ratios. Due to this multi-site feature, these catalysts yield

polymer with broad molecular weight distribution (PDI~15-30).41, 42

Scheme 1.1: Philips catalyst produced by reaction of chromium compound with silica

Academic researcher then became interested more on understanding the

mechanism of polymer chain growth and termination at the chromium center of Philips

catalyst. As illustrated in Scheme 1.2 the three general proposals came out after

continuous research over the years. 28, 29

The first mechanism was proposed by Cossee-

Arlman, which suggest the direct migration of the polymer chain onto coordinated

ethylene.45, 46

The second mechanism commonly known as Green-Rooney mechanism

involves a 1,2-hydrogen shift from the growing chain generates a metal-alkylidene

hydride.47, 48

Further reaction with ethylene produces a metallacyclobutane before alkyl-

hydride reductive elimination re-forms a linear alkyl chain. The third possibility was

supposed to occur via an extended metallacycle mechanism where ethylene insertion took

Introduction

5

place into growing metallacycles.49, 50

Amongst the three possibilities, Cosee-Arlman

mechanism for polymerization has been more or less established.

Around the same time in 1953, Karl Ziegler, at the University of Mülheim,

discovered heterogeneous catalyst based on titanium chloride (TiCl3·1/3 AlCl3) upon

activation with organoaluminium co-catalyst (AlEt2Cl) which can produce highly

crystalline polyethylene under mild conditions, atmospheric pressure and temperature (50

-100°C).51-53

Later, in 1954 Gulio Natta and co-workers discovered stereoregular

polymerization of propylene with a catalyst composed of TiCl3 and Et3Al. 54-56

The

CrP Cr

P PCr

CrP

H H

Cr

H

P

H

Cr

H

P

H

Cr

H P

HPCr

H H

Crn

Crn

Crn

(a) Cossee-Arlman ( Linear insertion)

(b) Green-Rooney (carbene hydride)

(c) Metallacycle

Scheme 1.2: Mechanistic proposal for ethylene polymerization with Philips Catalyst.

Where, P-polymer chain. (a) Cossee-Arlman mechanism (migratory insertion ethylene into the polymer

chain). (b) Green-Rooney mechanism (α-hydride shift and metallacyclobutane formation). (c) Metallacycle

mechanism (metallacyclopentane formation and growth by ethylene insertion into large-ring metallacycles).

Introduction

6

fascinating successive discovery of MgCl2 as a support with TiCl4 catalyst system

resulted in a two fold increase in the activity of the Ziegler-Natta catalyst which led to

extremely active heterogeneous ethylene polymerization catalysts.

The polyethylene produced by Ziegler-Natta catalyst at low pressure and relatively

low temperature was significantly different in physical properties from those produced at

higher temperatures and high pressure. The polymer is linear in structure with higher

density of 0.95 to 0.96 g/cm3 compared to the high pressure polyethylene about 0.92

g/cm3. Also this polymer was comparatively more rigid than high pressure polyethylene

and could handle boiling water. Due to this mild polymerization condition Ziegler-Natta

catalyst systems does not require expensive engineering as the high pressure one. In

comparison with Philips catalyst, Ziegler catalyst is costly and difficult to handle. And one

of the major drawback this catalytic system is the presence of multi-active sites due to

which it display different activities and selectivities for monomer insertion. This

phenomenon exercise poor control over polymer architecture, comonomer distribution and

resulting polymer with high polydispersity index. Ultimately, it influences the product

morphology, processability and properties of the polymer. However, heterogeneous

Ziegler-Natta catalysts were used extensively for polyethylene production.

In order to overcome those drawbacks it is necessary to prepare well-defined

active sites with uniform in composition and distribution and thereby generate “single-

site” homogeneous catalysts. In 1957, the first pioneering homogeneous polymerization

catalyst was prepared by Breslow and Natta within a few years of Ziegler’s discovery. 57-59

The catalyst consists of bis(cyclopentadienyl)titanium dichloride (Cp2TiCl2, Cp = η5-

cyclopentadienyl) and alkylaluminum chloride (AlR2Cl), the catalyst exhibited a low

polymerization activity ~104 g polyethylene (PE)/(mol Ti-h. atm). Even though they were

regarded as excellent models for understanding the mechanism by which heterogeneous

Ziegler-Natta catalysts are operated but these titanocene catalysts were of no commercial

interest due to their poor activity, tendency to decompose to inactive species and inability

to polymerize higher alpha-olefins like propylene etc.60

In 1976, Walter Kaminsky and Hansjörg Sinn introduced partially hydrolyzed

aluminum alkyls, in particular methylaluminoxane (MAO) as an efficient cocatalyst for

homogeneous single site catalysts (metallocenes: which is a sandwich complex with a

Introduction

7

transition metal coordinated to two cyclopentadienyl derivatives).61- 66

Generally, MAO is

an oligomer with [-O-Al(Me)-]n repeat units which may exist in a linear or cyclic form

with a molecular weight between 1000 and 1500 g/mol (Scheme 1.3).67- 69

The role of

MAO cocatalyst is believed to:70-75

i) alkylate the metallocene ring consequently forming an active complex,

ii) scavenge the impurities,

iii) stabilize the cationic center through an ion-pair interaction, and

iv) prevent bimetallic deactivation processes (Scheme 1.4 ).

Scheme 1.3: Possible structure of methylaluminoxane (MAO)

Cp'2Zr CH3 + Cp'2Zr CH2 ZrCp'2

2+

P CH2 ZrCp'2

P CH3+

P- polymer

Scheme 1.4: Bimetallic deactivation processes

Introduction

8

After the use of MAO as a cocatalyst, the catalytic activity was increased by

several orders of magnitude. Cp2TiC12/MAO has productivity of 9.3 x l06 g PE/ mol Ti/ h/

atm at 20 °C and Cp2ZrC12/MAO has the productivity of 9 x l07 g PE/ mol Zr/ h/ atm at

70 °C.76, 77

However, these catalysts were unable to produce stereospecific polypropylene

due to the symmetric characteristic of the active centre. Generally, these molecules

associated with C2v symmetry where the two Cp rings in the molecules are not parallel

(Scheme 1.5). The Cp2M fragment is bent back with the centroid-metal-centroid angle θ

about 140° due to an interaction with the other two σ bonding ligands. 78

Ti ClClHf CH3H3C

Cp2TiCl2 Cp2Hf(CH3)2

Scheme 1.5: Structure of two metallocene with C2ν symmetry

Brintzinger and co-workers in 1980 performed an initial follow-up studies on the

effect of ligand substitution on catalytic activity. They produced isotactic polyolefin with

homogenous single site catalysts that brought a new renissance to this economically

important area of chemistry.79-81

They synthesised some ansa-metallocene catalysts like

racemic ethylene-bridged bis(indenyl)zirconium dichloride, Et(Ind)2ZrC12 and racemic

ethylene-bridged bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, Et(H4Ind)2ZrC12

(Scheme1.6) which have both meso and racemic configiguration and produced

stereoselective polypropylene after activation with MAO with high productivity and high

density.79-84

A number of efficient metallocene catalysts developed by changing the

transition metals (Ti, Zr or Hf) and substituents on the Cp rings, as well as the bridging

Introduction

9

Zr ClCl Zr ClCl

Et(Ind)2ZrCl2Et(H4Ind)2ZrCl2

Scheme 1.6: Structure of the Brintzinger catalysts.

groups capable of producing highly stereo- and regio-regular isotactic polypropylene not

only in the laboratories but also in commercial scale.85-89

Among the substituted Cp ligands

methylcyclopentadienyl (MeCp), pentamethylcyclopentadienyl (Me5Cp), indenyl(Ind),

tetrahydroindenyl (H4Ind) and fluorenyl (Flu) ligands are most frequently used.90-104

Whereas ethylene (Et, -CH2CH2-), dimethylsilene (Me2Si, (CH3)2Si=), isopropylidene (iPr,

(CH3)2C=), and ethylidene (CH3CH=) are the common bridging groups. 105-113

It is believed that symmetry of the catalyst and the substituents on the

cyclopentadienyl ring are the main feature for stereospecific α-olefin polymerization.

Kaminsky et al. designed the basic structure of the C2-symmetric catalysts (ethylene-

bridged bis(tetrahydroindenyl) zirconium complex to produce isotactic polypropylene

(Scheme 1.7).114-116

[Me2Si(2-Me-4-naphthylindenyl)2]ZrCl2 and [Me2Si(2-Me-4-

phenylindenyl)2]ZrCl2 C2 symmetric ansa metallocene complexes are found to provide

highly isotactic polypropylene with high productivity.117

Ewen et al. also developed a new

class of catalyst known as Cs symmetry metallocene catalyst by introducing bulkier

fluorenyl moiety in the bis(cyclopentadienyl)-based structures (Scheme 1.7).118

Highly

syndiotactic polypropene was first obtained by this Cs symmetric metallocene catalyst.118-

123 Likewise, Chien reported first the synthesis of C1-symmetric complex, [1-(5-indenyl)-

1-(5-tetramethylcyclopentadienyl)ethane]TiCl2 which provide hemiisotactic elastic

polypropylenes with narrow molecular weight distribution (Scheme 1.7).124-127

Ewen et al.

synthesised a highly effective hemiisospecific metallocene catalyst [2-(9-5- fluorenyl)-2-

(3-methyl- 5- cyclopentadienyl) propane] zirconium dichloride and its hafnium

Introduction

10

analogue.118, 128

The hafnium analogue provides highest molecular weights (M̅w ~200000–

300000 g/mol) PP whereas the zirconium complex gives lower molecular weights (M̅w

~50 000 g/mol PP).

1.3. Metathesis olefin polymerization

In the mean time metathesis reactions were also taking most spectacular

improvements in the synthetic strategies for olefin production. In 1967, Calderon and co-

Scheme 1.7: Correlation of polymer microstructure with the catalyst symmetry

workers for the first time coined the term “olefin metathesis”, which became one of the

powerful methods for polymerization of α-olefin, cyclic compounds that include

carbocyclic, heterocyclic, and fused ring framework.129-134

Chauvin and Hérisson first

proposed the widely accepted mechanism of transition metal alkene metathesis.134-140

According to this mechanism, the coordination of an olefin to a metal carbene catalytic

species leads to the reversible formation of a metallacyclobutane (Scheme 1.8). In the last

stage of the catalytic cycle, this transition molecule is broken apart and gives a metathesis

product and the re-formation of the metal methylene molecule which is ready to act as

catalyst in another metathesis reaction. However, initially such reactions were limited due

to the insufficient catalytic performance. The considerable breakthrough brought by

Richard Schrock and Robert Grubbs in terms of catalyst discoveries opened new vistas in

Introduction

11

olefin metathesis.141-147

Generally, Grubbs' catalysts are a series of transition metal

[ruthenium (II) based] carbene complexes used as catalysts for olefin metathesis.140, 148

As

illustrated in (Scheme 1.9) there are two generations of the catalyst, 1st and 2

nd generation

of Grubbs catalyst; which are often used in organic synthesis to achieve olefin cross-

metathesis (CM), ring-opening metathesis polymerization (ROMP), acyclic diene

metathesis polymerization (ADMET), and ring-closing metathesis (RCM) etc. using

various kind of reactants (Scheme 1.10).134, 149-169

Grubbs' Catalysts are relatively less

sensitive to oxygen and water and therefore more functional group tolerant. Whereas,

M

M

R1

M

R2

M

R2

M

R1R2

R1

R1

R2

Scheme 1.8: General mechanism of olefin metathesis

Schrock catalysts are molybdenum(VI) and tungsten(VI) based (Scheme 1.11).170-175

Schrock catalysts are more active and are useful in the conversion of sterically demanding

substrates and monolithic polymeric materials.174, 176-179

Today’s state of the art of olefin

metathesis catalysts allow the synthesis of linear, cyclic and hyperbranched PE.157-189

By

modifying the ligands structure, the catalytic activity of both Grubbs and Schrock can be

enhanced and the development process is still going on.169,174, 175, 190-196

Introduction

12

Ru

P(Cy)3

PhCl

Cl

P(Cy)3

Ru

Ph

Cl

Cl

P(Cy)3

N N

H3C

CH3

H3C

CH3

H3C

CH3

(a) (b)

Scheme 1.9: Grubbs (a) 1st generation and (b) 2

nd generation catalyst

Scheme 1.10: Formation of the 1-methylidene and ethylidene complexes with 1st

generation Grubbs catalyst.

Ru

P(Cy)3

PhCl

Cl

P(Cy)3

Ph

Ru

P(Cy)3

Cl

Cl

P(Cy)3

Ru

P(Cy)3

Cl

Cl

P(Cy)3

Methylidene resting state

Ethylidene resting state

Introduction

13

Mo

N

CHO

O

H3C CH3

F3C

CF3

CH3

H3C

F3C

F3C

Scheme 1.11: Schrock catalyst

In spite of the rapid development of many catalytic systems for olefin

polymerization, Ziegler-Natta catalyst still dominates the production of olefin in

commercial scale due to some fascinating features: 52, 197

(i) the active centre is composed of a transition metal-carbon bond;

(ii) high molecular weight polyethylene is formed under mild condition;

(iii) high molecular weight polymer can be obtained from α-olefin, which is

otherwise impossible;

(iv) various stereorugular polymers are formed;

(v) polymerization can be diversely controlled by choice of cocatalyst and ligands.

1.4. A Brief Overview of Ziegler-Natta Catalyst

Karl Ziegler mentioned in his speech after receiving the Nobel Prize in 1963, “The

new development embarked on at the end of 1953 when I, together with Holzkamp, Breil,

and Martin, discovered within a few days in a nearly dramatic way that gaseous ethylene

was polymerized quickly to high-molecular plastics at low pressures of 5-100 atm and

even at normal pressure using simple producible catalysts.”198, 199

This elementary

invention based on fundamental investigations of the reactions between ethylene and

organometallic compounds, initiated many scientific studies in the field of catalysis using

organometallic compounds and provides outstanding relevance for the industrial synthesis

of polyolefins.

Introduction

14

Karl Ziegler devoted his enduring research interest to the investigation of the

emerging new class of metal alkyl compounds for C-C bond formation by means of

addition of alkali alkyls to olefins, styrene, and dienes etc.200, 201

At the end of 1952,

Ziegler and Holzkamp accidentally discovered the "nickel effect” which stemmed from

the observation that nickel contamination from an autoclave was recognized to prevent

ethylene propagation in the presence of aluminium alkyls and favour dimerization of

ethylene to produce exclusively 1-butene.198, 202-204

Subsequently, Ziegler launched a

systematic study with other transition metal compound similar to nickel combined with

aluminium alkyls on ethylene. In October 1953, Karl Ziegler and Heinz Breil eventually

found that a mixture of triethylaluminium (Et3Al) and zirconium acetylacetonate

polymerized ethylene into a white solid powder.198

Heinz Martin, student of Ziegler,

succeeded in polymerization of ethylene with a more effective catalyst system titanium

tetrachloride combined with triethylaluminium which allowed the polymerization to

proceed in a glass vessel (Figure 1.3) at atmospheric pressure and room temperature. 51- 54

Figure 1.3: Ziegler’s glass reactor for performing his low pressure ethylene

polymerization

On March 11, 1954 the group of Giulio Natta’s group tried to polymerize

propylene using various catalysts and eventually found that a catalyst composed of TiCl3

and Et3Al produced tacky product. Natta immediately recognized that the poly(propylene)

obtained is composed of different diastereoisomers which provide different physical

Introduction

15

properties. Natta distinguished between highly crystalline isotactic and syndiotactic and

amorphous atactic poly (propylene) by means of X-ray diffraction.54

Further, Natta

established the concepts of stereospecific polymerization which had far-reaching impact

on the progress of polymer science and technology.55, 56

The significance of this discovery

was recognized by the Nobel Prize Committee in 1963 and the committee awarded the

Nobel Prize to both Karl Ziegler and Giulio Natta for their eminent achievements. A.

Fredga commented while summarizing the work of Natta that- ‘‘Nature synthesizes many

stereoregular polymers, for example cellulose and rubber. This ability has so far been

thought to be a monopoly of Nature operating with biocatalysts known as enzymes. But

Professor Natta has broken this monopoly.’’205, 206

1.4.1. Mechanism of Polymerization with Ziegler-Natta catalyst

Amongst the various developed models and reaction mechanisms, Cossee and

Arlman’s comprehensive monometallic mechanism for Ziegler-Natta catalysis was

generally accepted and their mechanism was further supported by molecular orbital

calculations.45, 46, 207-209

According to Cossee polymerization of ethylene and 1-alkene

proceeded at the titanium centre. The reduced form of titanium is octahedral and contains

open coordination sites (□) and chloride ligands on crystallite edges. Initiation begins

after alkylation by TEA followed by formation of an active center, believed to be a

titanium alkyl (Scheme 1.12). The alkyl group migrates (rearranges) such that an open co-

ordination site moves to a crystallite edge position. The catalytic cycle starts with the side-

on π-complexation of ethylene which activates the C=C double bond. The reaction

proceeds in a concerted manner. The first step involves the insertion of the ethylene in the

Ti-alkyl bond by means of breaking of the C=C double bond. This leads to the formation a

C-Ti bond between the monomer molecule and the Ti site and a C-C bond between the

monomer and the growing chain. At the same time, the vacant site at Ti becomes free for

further complexation with another monomer molecule (Scheme 1.12).

Introduction

16

Ti C2H5

CH2=CH2

Ti

C2H5

Ti

C2H5

Ti

CH2CH2C2H5

Ti

Rp

π-complex

Transition state

n CH2=CH2

Rp = (-CH2CH2)n+1C2H5, a polymer chain

(C2H5)3 Al

Ti

Cl

Ti C2H5

Cl Al

C2H5

C2H5

δ+

δ−

-(C2H5)2 AlCl

Scheme 1.12: Alkylation and insertion of ethylene to active centre

Since Ti-C σ-bond is known to be unstable, so besides Cossee’s postulation a

different mechanism was postulated. It was suggested that the titanium alkyl become

stabilized by association with the aluminium alkyl. This postulation is known as the

"bimetallic mechanism" and essential features were originally proposed by Natta and other

workers in the early 1960s.210-214

In this bimetallic mechanism the basic steps are similar

to the Cossee-Arlman mechanism except the participation of the aluminium alkyl and the

key steps are illustrated in (Scheme 1.13). However, polymerization is still believed to

proceed by insertion of C2H4 into the Ti-C bond (rather than the Al-C bond).

Introduction

17

Ti

Cl

RCH2=CH2

Al Ti

Cl

R

Al

CH2=CH2

CH2

CH2

R

Ti

Cl

AlTi

Cl

CH3 CH2

R

Al

CH2=CH2

Scheme 1.13: Bimetallic mechanism

1.4.2. Polymerization by supported Ziegler-Natta catalyst with electron donors

The production of polyethylenes by polymerization of ethylene under normal

pressure (upto 5 MPa) and moderate temperatures (upto 900C) with Ziegler catalyst was

transferred into an industrial process in 1955 by Farbwerke Hoechst.215

In succession,

Montecatini Company started the industrial production of poly(propylene) at its Ferrara

plant in 1957.215

However, from an industrial point of view, the catalyst activities < 5 kg

PE/g (Ti) and 0.5-1 kg PP/g (cat.) and stereospecificities were so low which required

extensive purification by means of solvent extraction in order to removing colored and

corrosive catalyst residues as well as atactic poly(propylene).201, 216

The introduction of a

Lewis base (electron donor) into the antiquated catalyst system gave rise to the second-

generation catalysts with enhanced catalytic activity and stereospecificity. Yet most of the

titanium salt involved in the catalyst was left inactive as a polluting residue in the

polymer. An enormous research effort was devoted to develop a catalyst which could

fulfil all the chemical, physicochemical and environmental aspects. In the mid-1960s, ball-

milling of catalysts was introduced and resulted in increased surface area and higher

activity.217, 218

The major improvement occurred in the early 1970s with the application of

supported Ziegler-Natta catalyst.219-222

A variety of inorganic and organic compounds such

Introduction

18

as silica, alumina, carbon black and alkoxides of magnesium, manganese, iron, nickel,

cobalt, etc. were tested as supports.223-229

In British Petroleum process, the catalyst was

prepared by reducing TiCl4 with a co-catalyst (Et2AlCl) to precipitate as TiCl3 and TiCl3

was supported on silicon carbide, calcium phosphate, magnesium carbonate and sodium

carbonate. These supported catalysts performed maximum activities of 120 g PE/mmol

Ti.230

Sun Oil subsequently used γ-alumina as a support which gave an activity of 70g

PE/mmol Ti.227

Cabots’ achieved a comparable catalytic activity of 150 g PE/mmol Ti

with silica as a support.231-234

Solvay achieved a moderate success in 1963 with

Mg(OH)Cl support, which enhanced the catalytic activity 10 times higher (1100 g/

PE/mmol Ti) than the original non supported Ziegler catalyst.228, 235, 236

Their attempts

were to fix the catalyst precursor onto the solids with polar hydroxyl groups (Scheme

1.14).

Cl-M OH + M'Xn Cl-M OM'Xn-1+ HX

(M= Mg, Ca, Cd, Fe, co, Ni, Mn

M' = Ti, Sc, V etc. and Xn= halide or alkoxide)

Scheme 1.14: Fixation of transition metal compounds to solids with hydroxyl groups

In 1968, the sparking discovery of MgCl2-supported Ti based catalyst led to

the drastic improvement in productivity.237-240

Compared to conventional Ziegler-Natta

catalysts, the supported catalysts yield as high as 600-1000 kg of polymer/gm catalyst. In

the conventional process the product contains as much as 300-1700 ppm of residual metal.

Whereas this supported catalysts produce polyethylene with very low metal residue

(typically <5 ppm), obviating post-reactor treatment of polymer.241

With appropriate

electron donor in MgCl2 supported Ti catalyst, Montedison and Mitsui achieved high

activity and stereospecificity for propylene polymerization.224, 242

The outstanding success of MgCl2 is due to the same hexagonal crystal

structure as TiCl3, the nearly identical ionic radii of Mg2+

and Ti3+

and similar lattice

distances (Table1.1).237, 243

TiCl4 forms co-ordination bond on the surface of MgCl2 by

Introduction

19

donation of electron from MgCl2 to Ti4+

. The electronegativity of Mg2+

is smaller than

Ti4+

. The exchange of d electrons of Ti3+

to the π* orbital of the monomer (olefin) and π

electrons of the monomer to a vacant d orbital in Ti3+

, leading to effective accretion of

monomer insertion into the growing polymer chain.244

The coverage of the MgCl2 particle

surface with the titanium component and the conversion of this titanium component into

active sites after alkylation corresponding the Cossee /Arlman model are shown in

Table1.1: Crystallographic data for δ-MgCl2 and δ -TiCl3

Crystallographic Parametres:

δ = MgCl2 δ = TiCl3

hexagonal close packed layer structure of Cl-- ions

a = b = 3.53 Å a = b = 3.54 Å

c = 5.93 Å c = 5.86 Å

cation cordination: octahedral

Mg = Cl: 1.23 Å Ti = Cl : 1.25 Å

Mg2+ : 0.65 Å Ti4+ : 0.68 Å

Ti3+ : 0.76 Å

.

(Scheme 1.15).216, 243, 245

The reactive sites of MgCl2 are the co-ordinatively unsaturated

edges, in particular, the (1 0 0) surface with five coordinated Mg and the (1 1 0) surface

with four coordinated Mg. These two unsaturated surfaces differ in their binding abilities

due to the different coordinative saturation and steric environment. Along these two axis

(100) and (110) of MgCl2, TiCl4 can be absorbed. Thus the active sites are formed on the

surface of MgCl2. In the first step, one chlorine ligand at the titanium center is exchanged

with an alkyl group of the alkyl aluminium compound. In the next step, Ti4+

is reduced to

Ti3+

by splitting off an alkyl radical. This alkyl radical is deactivated by reaction with a

further alkyl radical. By these two reaction steps, the vacant site at the titanium center is

formed, which is necessary to complex and activate the ethylene molecule.

Introduction

20

In propylene polymerization, catalyst geometry is very important to ensure

that the methyl group in the propylene molecule is aligned in the same plane, producing

isotactic polypropylene chains. According to Busico’s model, the active sites formed on

the MgCl2 crystal surfaces; i.e. Ti3+

atoms located on the (100) cuts of MgCl2 produce

isotactic polymer, whereas isolated Ti3+

atoms on the (110) cuts produce atactic

polymer.246

It is generally accepted that a donor binds to the more reactive (110) surface

thereby affecting the stereospecificity by hindering the formation of aspecific

mononuclear Ti centers, whereas donor binding to the (100) surface likewise hinders the

formation of stereospecific dinuclear Ti centers according to Corradini’s model (Scheme

1.16).247-251

The introduction of internal ethyl benzoate (EB) predominantly adsorbs on

more acidic sites, the (110) faces, to prevent TiCI4 from forming non-stereospecific site

and converted some of the aspecific sites to highly stereospecific sites, thereby producing

polypropylene with high isotactic index (I.I.)=98%.246, 247

CH2

CH2

(100)(110)

(A)(B)

Step 1: Ti4+ Ti3+

Step2: Cl- alkyl Exchange

(A) Epitactic Fixation of TiCl4 on MgCl2 Surface

(B) Formation of active sites

Step3: Ethylene polymerization

Specific site

Non specific site

TiCl4Mg2+

Cl-

2TiCl4

Scheme 1.15: Surface topology of MgCl2-supported Titanium catalyst

Introduction

21

A great deal of effort with electron donor ethyl benzoate (EB) established the

platform of these PP production technologies.252, 253

In general, electron donors are

classified into two types, where one is an internal donor (ID) supported on the MgCl2

surface with TiCl4 during catalyst preparation and the other is an external donor (ED)

introduced with Et3Al at the start of propylene polymerization.254, 255

The addition of

(A)

Free specific site

Non specific siteblock by LB

Mg2+

Cl-

2TiCl4LB LB

LB

LB

LB LB

LB

LB

This is effect in isotacticity

Scheme 1.16: Competition between TiCl4 species and Lewis bases (LB) for the adsorption

on the lateral cuts of MgCl2

alkylaluminums (alkylation) results in the partial removal of the internal donor from the

catalyst surface.247

Therefore, external donors are needed to maintain high

stereoselectivity. The combination of dialkyl phthalate and alkoxysilane as internal and

external donor, respectively was found to be most efficient, providing 1000 times higher

activity than the TiCl3 catalysts and extremely high stereoregularity polypropylene(more

than 98% of isotactic index).256-270

However, when 1,3- diethers are used as internal

donors, they co-ordinate strongly with the (110) faces and cannot be removed by

alkylaluminums.271-278

As a consequence, Ziegler-Natta catalysts with excellent

isospecificity are obtained with diether internal donors that do not require any external

donor. The properties of the various generations of catalysts are reported in Table 1.2.

Introduction

22

Fifth generation of catalyst composed of diether is the most effective in productivity and

controlled isotacticity. Based on succinates as internal donors a new MgCl2-TiCl4 system

Table 1.2: Generations of Ziegler-Natta Catalysts for the Polymerization of Propylenea

Generation

(year)

Catalyst composition Productivity

(kgpp/g cat)

I.Ib

(%) mmmm

(%)

M̅w/M̅n

First

(1954)

δ-TiCl3.0.33AlCl3+AlEt2Cl

2–4

90–94

Second

1970

δ-TiCl3+AlEt2Cl

10–15

94–97

1968 MgCl2/ TiCl4 + AlR3 15 40 50-60

Third

1971

MgCl2/ TiCl4/benzoate +

AlR3/benzoate

15-30

95-97

90-94

8-10

Fourth

1980

MgCl2/ TiCl4/phthalate +

AlR3/silane

40-70

95-99

94-99

6.5-8

Fifth

1988

MgCl2/ TiCl4/diether +

AlR3

100-130

95-98

95-97

5-5.5

MgCl2/ TiCl4/diether +

AlR3/silane

70-100 98-99 97-99 4.5-5

Next

1999

MgCl2/ TiCl4/succinate +

AlR3/silane

40-70 95-99 95-99 10-15

aPolymerization conditions: liquid propylene, 70ºC, H2;

bIsotactic Index (I.I.)

Introduction

23

was also able to controll stereoregularity (either very high or low) of polymer with broad

MWD.279-283

Miro et al. studied different organosilicon electron donors with magnesium-

supported catalysts which provided polypropylene with reasonably broad MWDs,

desirable melt flow rates, low melting points and low decalin soluble fractions.284

Campbell and Chen reported that combination of esters or diesters of aromatic

dicarboxylic acids (ID) and aloxysilanes resulted in good catalyst activity and very good

process stability while maintaining the self-extinguishing property.285, 286

In general, most of the MgCl2-based catalysts, the content of ID depends on

the content of Ti and the ID/Ti molar ratio is normally between 0.77 and 0.82.287

The

amount of external donor, or Al/ED ratio, is an adjustable parameter to control the

stereospecificity of the promising catalyst for propylene polymerization. Recent

improvements in polypropylene catalysts can primarily be endorsed to the development of

new and more efficient electron donors.255, 288-292

The corresponding catalysts for

polyethylene in principle do not require any donor, even though electron donors can be

part of the catalyst system for the synthesis of special polymer grades. In particular, donors

tend to improve the quality of LLDPEs.293-298

However; the fixation of the active site at the

surface of MgCl2 does not necessarily means to have high mileage catalyst for olefin

polymerization. For the formation of high mileage catalyst its surface area must be very

high so as to maximize the amount and optimize the dispersion of active titanium sites and

hence the productivity of the resulting catalyst. Further, it is also essential to enable one to

shape the average particle size and particle size distribution.216

Amidst the different

preparative methods, the following routes are commonly knows to form high mileage

supported Ziegler-Natta catalyst:

• Dry milling of MgCl2 with TiCl4.253, 299-301

• Precipitation of C2H5OH soluble MgCl2 in hydrocarbon solvent and mixing

with TiCl4.300, 301

• Active MgCl2 is prepared by the reaction of dialkyl-Mg compounds with

aliphatic chlorohydrocarbon (eg. CHCl3) in a hydrocarbon diluents and

treating with TiCl4.303-305

Introduction

24

• Transformation of suspended Mg-dialkoxides with TiCl4 dissolved in

hydrocarbon followed by forming MgCl2-titanium catalyst and soluble Ti-

tetraalkoxides.306

1.4.3. Limitation of inorganic oxide supported Ziegler-Natta catalyst

Although due to several advantages of inorganic supported Ziegler-Natta

catalysts are used extensively, inorganic supports too possess certain ill effects.307, 308

One

of the major problems of these supported catalysts is that they liberate metals and

chlorides as contaminants into the polyolefin, which often alters the physical properties

and stability of the final polyolefin product. In addition, acidic inorganic supports have

reactive surfaces that can lead to catalyst deactivation. It is also essential either to calcine

the inorganic oxide support at high temperature or need chemical treatment with

appropriate reagents to remove physically adsorbed water and O2 from the surface since

they are well known in catalytic poisoning which can adversely diminish the catalytic

activity of the catalyst. Thus, a great care is required in handling and preparing inorganic

oxide supported catalysts. Moreover, inorganic oxide supports have a limited maximum

pore size which also can restrict the catalyst performance. Although large pore size

inorganic oxides are available, these materials may be friable and formed unwanted fine

particles by means of attrition.

To circumvent the above drawbacks commonly observed in inorganic oxide

supported catalysts, special attention has been devoted to immobilize the catalyst system

on organic material since it never releases any inorganic impurities into the synthesised

polymer. Polymer supported catalysts also provide the following advantages in olefin

polymerization.307- 311

• Easy to functionalize;

• No need of fastidious preparation, pre-treatment and dehydration

prior to use;

• Inert, easy to handle and non corrosive in nature;

• Adequate thermal and aero-stability;

• Available with different porosity, morphology and particle size.

Introduction

25

The presence of functional moiety in the polymer structure is the fundamental

to promote chemical bonds or well defined interactions between the catalyst precursor and

the polymer, so that the catalyst system does not release from the surface during alkylation

as well as polymerization.312

In the 1970s, Neckers and co-workers reported first example of a so-called

“polymer-protected reagent” where crosslinked polystyrene was used to form stable

complex with AlCl3.313

This catalyst is less sensitive to moisture and is stable for months

without significant change in its capacity. Neckers suggested that AlCl3 could be

stabilized by the interaction between its vacant orbital and π-electrons of benzene rings of

the hydrophobic polystyrene sheath used as a polymeric support. Polystyrene-AlCl3 (PS-

AlCl3) has found very effective in number of important applications. Thereafter,

functionalized polymers are frequently used as an alternative supports for copper, cobalt,

nickel and so many Lewis-acid catalysts.314- 320

In 1993, Ran demonstrated that the polymerization of isoprene with a

polystyrene- supported Ziegler-Natta (PS-TiCl4.Et2AlCl) catalyst gave a living-like

polymerization with productivity of 20 kg polyisoprene/ g Ti, in which both, monomer

conversion and molecular weight of the product increased linearly with polymerization

time.321

Jerico et al. has investigated ethylene polymerization with some chlorinated

organic polymer supported Ziegler-Natta catalyst.322

The catalysts showed productivity of

4 kg PE/mol Ti/ min/ bar and the viscosity average molecular weight of the PE is in

between 300-550 kg/mol. Mori and co-worker performed the similar study with a

modified-polypropylene supported Ziegler-Natta catalyst.308

They observed that yield

increased linearly as a function of polymerization time and found catalyst stability upto

100 h. Hsu et al. has prepared a Ziegler-Natta catalyst immobilized on a magnesium-

modified polymer support having carboxyl functional group which is highly active for

polymerization as well as copolymerization of olefins without substantial contamination

of the resulting products with chlorine or metal ions.323

Mteza et al. evaluated the effects of catalyst preparation condition on catalytic

activity for ethylene polymerization in slurry process with (polyethylene-gr-2-tert-butyl

amino ethyl methacrylate) supported Ziegler-Natta.324

Similarly, Sun et al. has synthesized

hybrid poly(ethylene-co-acrylic acid) supported Ziegler-Natta catalyst by different

Introduction

26

methods.325

Amongst the combinations, TiCl4/n-Bu2Mg.Et3Al/poly(ethylene-co-acrylic

acid) catalyst system performed excellent catalytic activity for ethylene polymerization.

Later on Gupta et al. anchored TiCl4 and Cp2TiCl2 on polyethersulfone (Scheme 1.17) as

a template to evaluate ethylene polymerization.326

These supported catalysts display

efficient catalytic productivity of 40-1600 kg PE/moleTi/h/bar and produced PE with

narrow polydispersity index (PDI~2.1-4.5).

S

O

O

Ti ClCl

(A)

S

O

O

TiCl

(B)

ClCl

Cl

Scheme 1.17: Polyethersulfone supported Titanium based catalyst

Ohnishi et al. has developed EPDM (ethene-propene-diene monomer)-

supported homogeneous Ziegler-Natta catalyst for the production of high molecular

weight polyethylene with narrow molecular weight distribution (PDI~ 2.2).327

Chung et al.

examined the kinetic of the gas phase ethylene polymerization over crosslinked

polystyrene supported (CH3)2Si[Ind]2ZrCl2.328

They noticed that the active sites follow the

first order deactivation and the activity is gradually decreased at high temperature resulted

from a strong dependency of the first-order decay rate constant on temperature. Kaur et al.

synthesized poly(methyl acrylate-co-1-octene) (PMO) support via Atom Transfer Radical

Polymerization (ATRP) and used it to immobilize TiCl4 with varying TiCl4/PMO weight

ratio.329

They claimed that TiCl4 was coordinated with potential Lewis base sites, such as

oxygen of carbonyl group and OCH3 group on poly(methyl acrylate- co-1-octene).

According to their finding, lower content of Ti (2.8 wt %) showed a higher activity

(productivity of 1.91 kg PE/g Ti/h and PDI~12.0) for ethylene polymerization.

Recently, Lijun et al. modified the surface of poly[styrene-co-(acrylic acid)]

(PSA) with different magnesium compounds and used to support TiCl4.330

According to

Introduction

27

XPS results, TiCl4 directly coordinated with the -COOH groups of PSA. It is noticed that

the chemical environments around Ti atoms and physical structures of the polymer-

supported catalysts are the governing criteria on catalytic performance. Jinhua et al.

produced porous polyethylene with nanofiber structure (diameter in the range of 20-80

nm) with CH3MgCl modified cyano-functionalised porous polymer supported Ziegler-

Natta catalyst (productivity of 100-200 g PE/mmol Ti/ h / atm) (Scheme 1.18).331

Scheme 1.18: The growth of nanofiber structure by porous polymer supported catalyst.

As a result various kinds of functional polymers have been employed as

support immobilizing the catalyst precursor for olefin polymerization.332-339

1.5. Polymerization Process

To polymerize ethylene in commercial scale utilizes four types of process:

high-pressure, solution-phase, gas-phase, and slurry-phase. Each process is able to

produce polyethylene with different physical properties.

1.5.1. High-Pressure Process

Amongst industrial processes for production of polyethylene, this process is

the oldest one for production of low-density polyethylene by free radical polymerization

under the most severe conditions. The polymerization is conducted in a thick-walled

autoclave or tubular reactor, typically employing at pressures of 1000-3000 bar and a

Introduction

28

temperature of 150-300 °C and is initiated by oxygen or more commonly peroxide.340

At

such high temperatures and pressure, polymerization of ethylene occurs in "solution" of

polymer in excess monomer without additional diluents. The product particles precipitate

from excess monomer when the reaction mixture becomes cool with exceptional clarity,

having no catalyst residues and low volatile organic contents (VOCs). Another advantage

of this process is the ability to copolymerize a variety of polar functional monomers to

produce interesting resins such as poly(ethylene-co-vinyl acetate) (EVA). Depending upon

the design of reactor LDPE resins contain both long- and short-chain branches. At the end

of 1960 Exxon Mobil has produced LDPE with high pressure processes.341

In high pressure process, safety is the key consideration to handle the

hazardous organic peroxide. Another severe concern is the possibility of ethylene

decomposition within the reactor which may generate catastrophic explosions.

1.5.2. Solution Process

In 1960, DuPont, Canada (now Nova) commercialized solution polymerization

process for polymerization of olefin with titanium and vanadium based Ziegler-Natta

catalysts.342

Later, Dow also developed highly successful solution processes for ethylene

polymerization.

Solution processes are carried out at temperature 130-250 ºC and pressures of

500-5000 psi sufficient to keep the polymer in solvent typically cyclohexane or C8

aliphatic hydrocarbon.340,341

Under such conditions, polymerization becomes

homogeneous since temperatures are well above the melting range of polyethylene. In

general the process is running in a continuous stirred tank reactor (CSTR). The main

advantages of this process are capable of producing polyethylene in the density range of

0.86-0.96 g/cm3, in particular, LLDPE, VLDPE, ultra-low-density polyethylene (ULDPE),

EPDM with wide range of molecular weight distribution and short residence times (1-10

min) lead to rapid grade transitions comparing to the other metal-catalyzed processes. The

significant snag of this process is high-energy consumption since after polymerization the

hot polymer solution is discharged from the reactor followed by vaporising and recycling

the solvent, hence a cost disadvantage.

Introduction

29

1.5.3. Gas phase polymerization

Gas phase ethylene polymerizations are typically conducted either a

mechanically stirred or fluidized bed as the polymerization zone at pressures of 20-30 bar

and temperatures of 80-110 °C.340-343

Ziegler-Natta catalysts are the predominant catalyst

for this process. Gas phase process was developed originally by Union Carbide (now

Dow) and later by Naphtachimie (now INEOS) which are commonly known as Unipol

and Innovene processes, respectively.344,345

The significant advantages are low-energy

consumption, no need of solvent and no mass transfer limitation.346,347

During

polymerization gas-solid phase exist only. Therefore, there is no solubility concern for

monomer, co-monomer and polymer during the polymerization. In this process PE, having

a density range of 0.91-0.97 g/cm3 can be produced. These advantages make convenient to

use in commercial production of wide range of polyethylene with different densities and

melt flow indexes (MFI).

1.5.4. Slurry / Suspension Process

In slurry process polymerization is typically conducted at temperatures around

70-90 °C and pressures of 2-24 bar with an aliphatic hydrocarbon (hexanes, propane,

isobutane etc.) as a diluent.348

It is essential that the diluents should be inert towards the

catalyst system. In 1955, Hoechst in Germany received the first licence for producing low

pressure polyethylene with Ziegler catalyst in slurry process at low pressure which was

improved over the years and known as Hostalen process.349

A simplified process flow

diagram for the Hostalen slurry process is shown in (Scheme 1.19).350

The Hostalen

process consists of two continuous stirred-tank reactors (CSTR). Depending on required

polymer grade the reactors are operated in parallel or in series. Cooling jackets are fitted

to remove the heat of polymerization otherwise, “hot” spots may form leading to a

softening of the polymer. Likewise the growths of particles are also suspended in the

“inert” solvent. Thus slurry polymerization process can be controlled by mass transfer

limitations by means of gas-solvent, solvent-particle and intra-particle phases. The

Hostalen process can be designed to produce either unimodal (broad or narrow molecular

weight distribution polymer) or bimodal polymer by using Ziegler-Natta catalysts. Slurry

process can be adopted in loop reactors (Phillips or horizontal stirred tank reactors) to

Introduction

30

produce high performance polyolefin.342

In comparison with gas phase polymerization

slurry process is relatively expensive. This process needs separation of product from the

slurry by the removal of solvent (C5-C8 hydrocarbon) via centrifugation or other

techniques and subsequently recycling the solvent. Even though the slurry process still

remain the significant demand in the market for olefin polymerization due to the

• very efficient heat removal, • wide co-monomer range,

• mild operating conditions and • relatively easier for handling.

Slurry processes are able to produce high density polyethylene with a density

range between 0.93-0.97 g/cm3.

Scheme 1.19: Slurry ethylene polymerization (Hostalen processes)

1.6. Nomenclature and application of polyethylene (PE)

The Society of the Plastics Industry (SPI) classified three main categories of

polyethylene based on the polymer architecture and the density of packing: high density

polyethylene (HDPE; few short or no branches), linear low density polyethylene (LLDPE;

many equal short branches) and low density polyethylene (LDPE; various branches on

branches).38, 342, 351

The generic architecture of LDPE, LLDPE, and HDPE are illustrated

Introduction

31

in (Scheme 1.20). However, recent advances in the area of catalysts further subdivided to

numerous subsets, most notably ultra-high molecular weight polyethylene (UHMWPE)

and very-low density polyethylene (VLDPE).352

The mechanical properties of PE depend

significantly on variables such as the extent and type of branching, the crystal structure

Scheme 1.20: Chain structures of polyethylene (a) HDPE (b) LLDPE and (c) HDPE

and the molecular weight (Table 1.3).353, 354

Polyethylene can be used in wide range of

applications starting from plastic bags to high strength fiber.

HDPEs are used in the area of high-performance pipes, since these materials

need to satisfy a number of severe specifications in terms of stiffness, impact strength, and

especially, short- and long-term ESCR and creep resistance (Figure 1.4). HDPE is also

used for cell liners in subtitle D sanitary landfills, wherein large sheets of HDPE are either

extrusion or wedge welded to form a homogeneous chemical-resistant barrier, with the

intention of preventing the pollution of soil and groundwater by the liquid constituents of

solid waste. In addition, HDPE is largely used wood plastic composites and composite

wood. Milk bottles and other hollow goods are manufactured with HDPE as well as in

packaging. Where as Ultra-high-molecular-weight polyethylene (UHMWPE) also known

as high-performance polyethylene (HPPE) are used in highly demanding applications due

to its outstanding physical and mechanical properties such as high abrasion resistance,

high impact strength, extremely low moisture absorption, good corrosion and chemical

Introduction

32

resistance, resistance to cyclic fatigue, and resistance to radiation. UHMWPE is applied in

artificial implant (Figure 1.5) materials eg. hip and knee joint prostheses.

Table 1.3: General properties of different polyethylene

Property LDPE LLDPE HDPE UHMWPE

Density (g/cm3) 0.91-0.93 0.90-0.92 0.94-0.97 0.93

Elongation at break (%) 100-700 200-1200 100-1000 300

Flexural modulus (MPa) 415-795 248-365 689-1654 _

Tensile Strength (MPa) 6.9-17.2 14-21 18-30 34.5

Melting Temperature (ºC) 106-112 100-126 130-136 132-135

Hardness (Shore D) 45-60 41-53 60-70 _

Figure 1.4: HDPE used in (a) container, (b) car shade and (c) pipe lines

UHMWPE fibers are also used in personal and vehicle armor, cut-resistant gloves, bow

strings, climbing equipment, fishing line, spear lines for spearguns, high-performance

sails, suspension lines on sport parachutes and paragliders, rigging in yachting, kites, and

kites lines for kites sports.

Introduction

33

Figure 1.5: UHMWPE used in the (a) artificial hip prosthesis and (b) sports equipments

LLDPEs are a family of poly-ethylene products obtained via copolymerization of

ethylene with α-olefin.297

LLDPEs have extensive applications such as making of plastic

bags and sheets (where it allows using lower thickness than comparable LDPE), plastic

wrap, stretch wrap, pouches, toys, covers, lids, pipes, buckets and containers, covering of

cables, geomembranes and mainly flexible tubing (Figure 1.6).

Figure 1.6: LLDPE used in (a) stretch wrap, (b) bags and (c) buckets

LDPE has more branching due to which its intermolecular forces (instantaneous-

dipole induced-dipole attraction) are weak.355

Its tensile strength is lower, and its resilience

Introduction

34

is higher. LDPE is mainly used in plastics bags. This grade of polymer also used for

manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic bags for

computer components, playground slides, plastic wraps and various molded laboratory

equipment. VLDPE is mainly used in food packaging due to high barrier property.

1.7. Objective of the present investigation

Inorganic oxide supported Ziegler- Natta catalysts have a significant demand in

the production of polyolefins. Unfortunately, they possessed certain drawbacks. They

introduce metal and chlorides as contaminants into the polyolefin which often alters the

physical properties and stability of polyolefins. To eliminate the drawbacks of inorganic

oxide supports, different polymers have been utilized as an alternative support for Ziegler-

Natta catalyst. In the present thesis, a considerable effort has been devoted to synthesize

some polymer supported Ziegler-Natta catalysts by immobilizing TiCl4 onto the backbone

of the functional polymer. All the synthesised polymer supported catalysts have been

utilized for ethylene polymerization at 1 atmospheric pressure and 40-50 ºC temperature.

The following objectives have been set for the thesis:

• Synthesis of polymer supports like polystyrene, poly(amic-acid), melamine

formaldehyde, poly(styrene-co-methyl methacrylate) and their characterization

with FT-IR, SEM, XRD and TGA.

• Immobilization of TiCl4 onto the backbone of solid polymer particles and their

characterization with XPS, UV-visible spectroscopy, FT-IR, SEM-EDX, XRD and

TGA.

• Evaluation of catalytic activity of polymer supported-TiCl4 adducts for ethylene

polymerization.

• Optimization of the Ti content of the supported catalysts.

• Characterization of the resulting polyethylene with GPC, MFI, FT-IR and DSC.

Introduction

35

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Introduction

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