Recent advances in polyolefin technolog

Jinliang Qiao,*ab Meifang Guo,ab Liangshi Wang,ab DonWenbo Songab and Yiqun Liuab

Received 28th October 2010, Accepted 23rd February 2011

DOI: 10.1039/c0py00352b

Polyolefin resins are the major plastics consumed in the world and are

works. This review will discuss how polyolefin technology has been d

particular attention to new products, catalysts, polymerization proce

world as well as in China.

hazardousness, dangerousness, emission and waste. Both new

fin developed based on new

ess, new additive and polymer

e development of our society.

olyolefin, new PE pipe grades

d many other materials; the

Dynamic Article LinksC

pipe in earthquake. In fact, no PE pipes were broken in that

Kobe earthquake. The PE pipe is not only suitable for gas and

cold water supply, but also feasible for hot water application.

PE-RT pipe, a non-crosslinked PE pipe, grows very fast in recent

years to replace crosslinked PE pipes for hot water trans-

portation.

Several PE manufacturers can produce PE-RT resins, such as

Dow chemical company, Lyondellbasell and SK Corporation.

New generation PE-RT with specifically designed molecular

structures and controlled side-chain distribution, produced by

different polymerization processes, has better high-temperature

long term hydrostatic strength. For example, DOWLEX 2388 is

an ethylene-octene copolymer produced by Dows constrained

geometry catalyst and solution process. PE-RT pipe has many

advantages, such as non-crosslinking, high flexibility, excellent

weldability, and long-term application at elevated temperature.

PE-RT is a kind of environment friendly material with high

including flexible PVC, EVA (ethylene-vinyl acetate copolymer),

lighter weight and lower cost. Compared with other polyolefin

based elastomers including TPV, polyolefin elastomer (POE),

polyolefin plastomer (POP) and amorphous poly-alpha-olefin

(APAO), OBCs also have advantages, such as better compres-

sion set and improved processibility.

1.3 Transparent PP with low soluble fraction

The transparent PP film and container have been used more and

more widely in food contact applications, which often require

heating with food by microwave oven. Compared with glass or

other transparent materials, transparent PP can help save energy

during the formation process, transportation and application.

However, transparent PP usually contains a high level soluble

fraction, which is detrimental to human health when the soluble

components dissolve into food. Several kinds of transparent PP

with a low soluble fraction have been developed in recent years.

qian

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Liangshi Wang; Xiaofan Zhang; Meifang Guo; Luperformance and low cost.

1.2 Soft PP and olefin elastomer

To replace PVC (polyvinyl chloride), SBS (styrene-butadiene-

styrene block copolymer), SIS (styrene-isoprene-styrene block

copolymer), SEBS (hydrogenated styrene-butadiene-styrene

copolymer), TPVs (thermoplastic vulcanizates), etc., several soft

PPs and olefin elastomers including low modulus impact PP,

stereoblock PP4 and olefin block copolymers (OBCs)5 etc. have

been developed in recent years, and among them OBCs with the

trade name INFUSE are the most attractive.

The INFUSE OBCs were developed and manufactured by

Dow Chemical Company using a shuttling polymerization

process, which will be introduced in detail in section 2.1 of this

review. OBCs applications include flexible molded goods, profile

extruded products, hoses and tubes, elastic fibers and films,

foams, adhesives and tapes. OBCs comply with all requirements

for food contact as set forth by FDA and EU. OBCs maintain

excellent flexibility at much higher temperature than traditional

olefin elastomers and have exceptional processibility, a similar

permanent set to SEBS and similar compression set to TPV at

70 C. Compared with traditional thermoplastic elastomers1612 | Polym. Chem., 2011, 2, 16111623g Yu; Dongbing Liu; Yiqun Liu and Wenbo SongThe random PP copolymer made by metallocene catalyst has

a much lower soluble content compared with traditional random

PP copolymer because of its uniform co-monomer distribution.

It can be used for both food contacting films and food

containers.

Propylene/butene-1 random copolymer usually contains less

soluble fraction than traditional propylene/ethylene random

copolymer. SINOPEC has commercialized propylene/butene-1

random copolymers by using new additives and its high iso-

tacticity catalyst recently. The propylene/butene-1 random

copolymers, both BOPP (biaxially orientated polypropylene)

film and transparent container grades with less soluble content,

are extremely suitable for Chinese food packing since Chinese

food usually contains oil. The propylene/butene-1 random

copolymers are not only suitable for Chinese food packaging, but

also can benefit Chinese PP manufacturers since butene-1 is even

cheaper than ethylene in China. The PP manufacturers in the

refinery with lower cost monomer can also easily produce

propylene/butene-1 random copolymers. Therefore, propylene/

butene-1 random copolymers could be a kind of high perfor-

mance polypropylene with low cost in China.

BOPP film is widely used for food packaging; therefore, BOPP

resin with low soluble content is required by the food packing

industry. However, high speed BOPP resin must contain more

than 4% soluble fraction according to traditional theory. BasedThis journal is The Royal Society of Chemistry 2011

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program,6 SINOPEC has developed a technology to produce

high speed BOPP resin with soluble fraction of less than 2%,

which is extremely suitable for food packaging.

1.4 Low VOC impact PP

Impact PP grades with high melt flow index (MFI) are usually

produced via a controlled-rheology process, where PP with low

MFI is produced in a reactor first and then peroxide is used to

reduce PPs molecular weight. This process inevitably produces

a lot of VOCs in the final PP resin. VOCs can give plastic

products unpleasant smell like that in new cars. In order to

reduce VOCs, the key point is to produce high MFI impact PP in

the reactor directly with the help of external-donors. Among the

processes developed is SINOPECs unique asymmetric external

donor (ASD) process,7 which will be introduced in section 3.2 of

this review.

1.5 Phthalate free PP

Phthalate has been found to be harmful to health, e.g. reducing

male semen; therefore, materials containing phthalate have been

restricted around the world. Unfortunately, PP catalysis

commonly uses phthalate as an internal donor. The use of

phthalate containing PP is likely to be prohibited in the near

future, especially in food and toy industries.

Several phthalate free PP catalysts have been commercialized

in recent years and make phthalate free PP available, which will

be reviewed in section 2.2 hereunder. For example, Lyondellba-

sell and SINOPEC can produce various PP grades without

phthalate based on their phthalate free PP catalysts.

1.6 Reactor-made high performance PP

A large amount of modified PP or PP compounds are used in

automobile industry and household appliances because virgin PP

resins could not meet the requirements of these industries in the

past. Secondary processing apparently means more energy use

and more pollution; therefore, it is necessary to develop high

performance PP resins directly from the reactor to replace

modified PP. In recent years, reactor-made high melt strength PP

and high performance impact PP have been developed and made

a market success.

High melt strength PP (HMSPP) has been used in PP foaming,

thermoforming and many other applications. However, HMSPP

manufacturing is a costly process. The first commercial HMSPP

was produced by irradiation process and then followed by

reactive extrusion process. In recent years, reactor-made HMSPP

is available by different technologies, such as Dows metallocene

catalyst technology, Lyondellbasells Spherizone process and

SINOPECs ASD process. With those new technologies,

HMSPP will replace more other materials. For example, foamed

PP made of HMSPP will replace more and more foamed poly-

styrene.

The automobile industry consumes a lot of PP compounds,

such as in bumpers. Now the PP manufacturers can supply

reactor-made PP resin that can satisfy the automobile industrys

requirement for bumpers. It only needs a newmould to overcome

the shrinkage difference between PP resins and PP compounds.This journal is The Royal Society of Chemistry 2011SINOPEC has developed a special impact PP resin for car

bumpers and successfully replaced some PP compounds in

China.

1.7 Functional polyolefin with lower cost

Functional polyolefins account for only a small market share,

one reason for this is due to their high cost. For example, anti-

bacterial polyolefin can benefit human health, but it is not very

popular in the market because of the expensive anti-bacterial

additives used. In order to reduce the cost, SINOPEC has

developed a compound additive technology platform for high

efficiency additives including a-PP nucleating agent, b-PP

nucleating agent, LLDPE nucleating agent, laser marking

pigment, anti-bacterial agent, etc. With the help of this tech-

nology, some high performance polyolefins with lower cost have

been commercialized, which will be introduced in section 4 of this

review.

2. Catalyst development

The history of polyolefin is actually the history of catalyst

development. The catalyst, from ZieglerNatta catalyst and

Phillips catalyst to single-side catalyst, allows us to control the

polyolefin purity, isotacticity and isotacticity distribution. It was

the breakthroughs in catalyst technology that eliminated the

process for the removal of the atactic fraction and catalyst resi-

dues to make the polymerization process simpler and cleaner. It

was also the breakthroughs in catalyst technology that expanded

the product properties envelope of polyolefins to make poly-

olefins market share increase continuously.

2.1 PE catalysts

After LDPE, a non-catalyzed polyolefin developed by the

Imperial Chemical Industries (ICI) in the 1930s, HDPE, LLDPE

and PP became major plastics with the development of Ziegler

Natta catalysts8 and chromium catalysts9 in the 1950s. After

more than 50 years of development, now there are four types of

PE catalysts, ZN catalysts, chrome catalysts, metallocene

single-site catalysts and non-metallocene single site catalysts.

2.1.1 ZieglerNatta catalysts. Modern PE industry was

started from a breakthrough of high activity catalyst made by

Mitsui Chemical and Montecatini in 1968. This famous catalyst

was TiCl4 supported on MgCl2 carrier and its activities were

more than 100 times higher than traditional TiCl3 catalysts.10

This high activity catalyst not only simplified the polyethylene

polymerization process by deleting the de-ash process, but also

improved the performance of the polyethylene resin. Afterwards,

ZN catalyst development was focused on catalyst performance

improvement and catalyst suitability for the polymerization

process, such as modifying the morphology of the catalyst

particle to narrow particle size distribution and achieve spherical

morphology, increasing the sensitivity of H2 response, decreasing

the fine content of PE particles, increasing bulk density of PE

powder, etc.

An emulsion process for ZN catalyst preparation was

developed by Borealis recently. The process was originally

developed for PP catalysis and then found to be also suitable forPolym. Chem., 2011, 2, 16111623 | 1613

metallocene catalyst that can increase activity of a supported

catalyst by more than 50% compared with traditional silica

supported catalysts.30

Borealis developed an emulsion-based single-site catalyst

preparation process as described in Fig. 1.31 This process enables

catalyst particles to have an inherently perfect spherical shape

and unique intra and inter-particle homogeneity. The catalyst

has higher catalyst activities and its morphology is shown in

Fig. 2. The catalyst particles are very compact and have a low

surface area.

Single reactor bimodal HDPE technology is another PE

catalyst hot research field. Univation has developed the

PRODIGY Catalyst for bimodal HDPE in a single gas phase

reactor.32 The PRODIGY Bimodal Catalyst system has two

single site catalysts, a metallocene catalyst and a post-metal-

locene catalyst for HMW and LMW components. Another

representative single site catalyst, as reviewed in 1998 by

McKnight and in 2003 by Gibson, is an ansa-cyclopentadienyl-

amido group IV catalyst called constrained geometry catalyst

(CGC).33 In the 1990s, Dow Chemical Company commercialized

the CGC technology to produce polyolefin plastomers and

elastomers with long-chain branches in a solution process.

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area, perfect spherical particle shape and a narrow particle size

distribution. The polymer particles produced with this catalyst

are compact and perfectly spherical with a narrow particle size

distribution, reduced porosity, high bulk density and intra-

particle homogeneity.11 This catalyst also leads to a polymer with

advanced properties, such as a low level of fines etc.12,13

N-series PP catalyst (BASF Lynx 1000 series based on the

same patent technology) developed by SINOPEC BRICI delivers

PP with higher isotacticity and better stability. Recently SINO-

PEC BRICI extended the N-catalyst technology to PE catalyst as

BCE and BCS catalysts for slurry and gas-phase processes. The

BCE catalyst is a new generation high activity PE catalyst for

slurry process.1416 The catalyst can be applied to CX, Hostalen,

Innovene and Phillips loop processes to produce HDPE, espe-

cially high added value grade, such as PE-100+ pipe grade and

PE powder for CPE (chloride polyethylene) grades. The BCE

catalyst has narrower particle size distribution; therefore, PE

resin produced with the BCE catalyst has a lower fine content

and higher polymer bulk density. The BCE catalyst also has

better copolymerization ability, which can decrease the content

of soluble low molecular weight PE. The PE production process

with BCE catalyst consumes less energy, less steam and has

a long operation cycle with easy and stable operation. The BCS

catalyst is a slurry injection catalyst for gas phase process to

produce LLDPE resin. The BCS catalyst has such advantages as

high activity, excellent co-polymerization, good flowability, good

particle size distribution of both catalyst and PE, and low fine

content.17

2.1.2 Chromium catalysts. The Cr/SiO2 catalysts, also called

Phillips catalysts, are still important catalysts for HDPE with

broad molecular distribution. The Phillips catalysts have

attracted a great deal of academic and industrial interest.

However, the structures of active sites remain controversial until

now.18,19 Even recently McDaniel from Chevron-Phillips

Chemical Co. still published papers on the activation of this

catalyst.2023 The PE industry always has the dream of producing

HDPE with Cr catalyst in reduced condensing mode in a gas

phase process; therefore, many researchers have been continu-

ously striving to increase the activity of Cr catalyst in gas phase

polymerization process.

2.1.3 Metallocene catalysts. In 1980, Kaminsky reported that

zirconocene/MAO (methylaluminoxane) could be used for olefin

polymerization with high activity at the IUPAC Polymer

Congress in Firenze.24,25 Afterwards, metallocene catalysts, as

single site catalysts, became great focuses of industrial and

academic research. The catalyst supporting technology is always

one of main hot research fields because only the supported

metallocene catalysts can be directly used in the current gas phase

or slurry processes. Several papers have reviewed the advance on

supporting metallocene catalysts in detail.26,27

From the industrial point of view, increasing the supported

catalysts activity and reducing catalyst cost are still the most

important issues. Recently Albemarle developed a technology,

ActivCat technology that can increase the productivity of

conventional MAO/silica metallocene catalysts by 200%.28,29

SINOPEC BRICI developed a MgCl2 modified silica carrier for1614 | Polym. Chem., 2011, 2, 161116232.1.4 Non-metallocene single-site catalysts. Brookhart devel-

oped the Ni, Pd, diimine catalyst system for olefin polymeriza-

tion in 1995,34 which was also called the late transition metal

catalyst. Afterwards, many other non-metallocene single site

catalysts were developed, such as Grubbs catalysts,35

FeBis(imino)pyridine catalysts,36 FI catalysts37 etc. Gibson has

reviewed the non-metallocene single-site catalyst in detail.33 So

far only a few non-metallocene single-site catalysts were

commercialized.

The pyridyl amine-base catalyst was developed by Dow

Chemical Company in conjunction with Symyx high throughput

technologies in 2004. The catalyst as shown in Fig. 3 is a non-

metallocene single site catalyst. It allows copolymerization of

Fig. 1 The preparation process for emulsion-based single-site catalyst.

Fig. 2 SEM image of the single site catalyst prepared by the emulsion-

based process.This journal is The Royal Society of Chemistry 2011

propylene with various olefin comonomers over a broad range of

compositions in an isotactic fashion and with high molecular

weight. Dow commercialized the propylene/ethylene copolymer

under the trademark VERSIFY. The VERSIFY plastomers and

elastomers have a very narrow molecular weight distribution,

a broad and unique composition distribution and unique regio-

errors.3

Chain shuttling catalyst technology is another of Dows

innovations on single site catalysts. Chain shuttling is defined as

the passing of a growing polymer chain between catalyst sites,

such that portions of a single polymer molecule are synthesized

by at least two different catalysts. The chain shuttling agent

(CSA) is a component that facilitates this transfer, such as

a metal alkyl complex. The chain shuttling mechanism can be

described in Fig. 4.5

Cat1 (solid circles) and Cat2 (solid triangles) represent cata-

lysts with high and lowmonomer selectivity respectively, whereas

the CSA (solid squares) facilitates the chain shuttling reaction.

Cat1 produces a segment of hard polymer with low comonomer

content. Shuttling occurs when this segment is exchanged with

the CSA bearing a soft copolymer with higher comonomer

content. Further chain growth at Cat1 then extends the soft

copolymer chain with a hard segment, thus giving a block

copolymer. Pyridyl amine-based catalyst is used as Cat2 to

properties differ a lot when different kinds of compound are used

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catalyst (known as FI catalyst) is used as Cat 1 for the segment of

hard polymer.

In 2006, Dow announced ethyleneoctene INFUSE olefinblock copolymers (OBCs), which are produced using the chain

shuttling catalyst technology. This technology employs two

catalysts, one forming high-density PE with higher melting points

and the second forming an elastomer with high incorporation of

octene in the ethylene chain. A chain shuttling agent, diethyl zinc,

forces an exchange of catalysts as the polymer chain grows leading

Fig. 3 Pyridyl amine-based catalysts.

Fig. 4 Depiction of the likely chain shuttling mechanism in a single

reactor, dual-catalyst approach.This journal is The Royal Society of Chemistry 2011as the internal donor. At present, the catalysts using 1,3-diether,

succinate or 1,3-diol ester as the internal donor have been

commercialized or are in the stage of application promotion.42

The catalysts with these three types of internal donors are all

phthalate free and so are the polypropylenes made with these

catalysts.

1 1,3-Diethers as internal donors. In 1989, a PP catalyst with1,3-diether as the internal donor was developed by the Himontto block polymers that combine the attributes of high-density PE

and the olefin elastomer. Compared to ethyleneoctene random

copolymers made bymetallocene catalysts, the block architecture

of OBCs imparts a higher crystallization rate and higher melting

temperature while maintaining a lower glass transition tempera-

ture and a more highly order crystalline morphology. Because of

these structural differences, the physical and mechanical proper-

ties of OBCs are different from random copolymers, resulting in

better performance in many applications.

2.2 PP catalysts

Since the first isotactic PP formed with TiCl3-AlR3 catalyst was

developed in 1954,38 PP catalysts have been the main research

focus for both the PP industry and academia. There are three

major series of PP catalyst systems now: traditional Ziegler

Natta catalysts, metallocene catalysts and non-metallocene

single-site catalysts.

2.2.1 ZieglerNatta catalysts. The traditional ZieglerNatta

catalyst is still the most widely used catalyst system in the poly-

propylene industry. In the development history of the Ziegler

Natta catalyst there are five generations of catalysts and the

electron donors have played an important role.39 As early as the

first generation of the catalyst, it was found that addition of

a third component would make a huge impact on the olefin

polymerization behavior and polymer properties. This third

component was actually an electron donor, also known as the

Lewis base. The Lewis base that is added in the preparation of

solid catalyst is called an internal donor, while the Lewis base

added in the olefin polymerization process is called an external

donor. The nature of the electron donor has significant influence

on active sites and catalyst properties. Therefore, PP catalyst

research is always focused on more efficient routes for catalyst

preparation and more efficient combinations of electron

donors.40

2.2.1.1 Internal donors. In the early 1980s, a MgCl2 sup-

ported catalyst system with a new internal donor, alkylphthalate,

was developed. This new catalyst could afford much higher

catalytic activity and isotacticity, so that de-ashing and removal

of the atactic polymer could be avoided.41 This catalyst is called

the fourth-generation ZN catalyst, so far it is still widely used in

the polypropylene industry. Since then, many further studies on

electron donors have been done.Many internal donors have been

developed, such as 1,3-dione, isocyanate, 1,3-diether, malonic

ester, succinate, 1,3-diol ester, glutaric acid ester, diamine, 1,4-

diol, 1,5-diol ester, phthalate esters, cycloalkyl esters and the use

of a variety of binary electron donor complex etc. The catalyticPolym. Chem., 2011, 2, 16111623 | 1615

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extremely high activity and isotacticity without the need for any

external donor, which breaks the restrictions of the last two

generations of catalyst that the PP catalyst must contain both

internal and external donors.43 It could be considered as the fifth

generation of PP ZN catalyst. Compared with the conventional

catalyst, the diether catalyst has higher activity, higher hydrogen

sensitivity, and narrower molecular weight distribution. More-

over, the obtained polymer exhibits high isotacticity and low

oligomer content. Diether catalyst has been successfully applied

to Montell Spheripol process. At present, Basell has developed

a series of PP catalyst containing diether compound as the

internal donor. PPs obtained with these catalysts have narrow

molecular weight distribution, which is suitable for melt spinning

process. Due to the fact that the ZN catalyst with 1,3-diether as

an internal donor has excellent hydrogen sensitivity, it is also

suitable for the multi-reactor polymerization process, which can

broaden the molecular weight distribution of polypropylene in

existing operating conditions.

2 Succinates as the internal donors. Succinate as internaldonor was also developed by Basell. In 2003, Basell began to

commercialize their new ZN polypropylene catalysts with

succinate as internal electron donor.43 The catalytic activity

increased and the polymer molecular weight distribution became

wider compared with the fourth generation catalyst using the

phthalate. The stiffness and processing performance of the

polypropylene resins made by this series of catalysts are greatly

improved. The new type of catalyst can be applied to most of the

PP production processes, including bulk, gas phase and slurry

processes.

3 1,3-Diol esters as the internal donors. In 2002, a new seriesof PP catalysts were developed by SINOPEC BRICI with 1,3-

diol esters as the internal donors.44,45 Such catalysts are charac-

terized by a high catalytic activity and better stereospecificity and

the obtained polymers have a wide molecular weight distribu-

tion. The hydrogen sensitivity can be varied by changing the type

and position of substituent of the 1,3-diol ester compound. In

addition, the PP performance has also been greatly enhanced.

The industrial application shows that the catalyst with 1,3-diol

esters has unique advantages and PP made with this catalyst can

be used in many applications.

2.2.1.2 External donors. The external donors also have great

influence on polypropylene catalyst activity, hydrogen sensitivity

and stereospecificity. It is necessary that the internal and external

donors have proper match to each other.46 At present, the most

commonly used catalyst in the PP industry is the fourth gener-

ation catalyst, containing phthalate as the internal electron

donor and the organic siloxane compound as external donor. In

the development of organic siloxane, many studies have shown

the great effect of the structures of alkyl and alkoxy on catalyst

performance.4648

Proto and Sacchi found that the performance of organic

siloxane was affected by the alkoxy number, size, and groups

connected with the silicon atoms.48,49 According to their study,

small alkoxy groups, such as methoxy and ethoxy were ideal for

suitable organic siloxane. For the non-alkoxy groups, the effect1616 | Polym. Chem., 2011, 2, 16111623of organic siloxane size on polypropylene isotacticity was in

sequence: methyl < butyl < isobutyl phenyl < isopropyl.50Currently the most commonly used external donors in PP

industry are methyl cyclohexyl dimethoxy silane (CHMDMS),

diisobutyl dimethoxy silane (DIBDMS), dicyclopentyl dime-

thoxy silane (DCPDMS) and diisopropyl dimethoxy silane

(DIPDMS). The effect on the stereospecificity was in the

sequence: DCPDMS > DIPDMS > CHMDMS > DIBDMS; the

effect on the hydrogen sensitivity was in sequence: DIBDMS >

CHMDMS > DIPDMS > DCPDMS.47 Among the known

organic siloxanes, the catalyst using DCPDMS as an external

donor showed the highest stereospecificity, but the worst

hydrogen sensitivity. Therefore, when PP with a broad molecular

weight distribution is produced by using two external donors,

one must be fixed on DCPDMS, another is usually an external

donor with high hydrogen response and moderate stereospeci-

ficity properties.50,51

In recent years, scientists also have tried to develop new types

of external electron donors. A series of PP catalysts using amino-

silanes as external donors have attracted much attention.52,53 The

catalyst with amino silane can broaden the polypropylene

molecular weight distribution without reducing its isotacticity

compared with the organic siloxane catalyst. It is also reported

that the polypropylene produced by the catalyst containing

a ring-amino-silane as external donor not only has a high crys-

tallinity, but also a wide molecular weight distribution.54 When

diethyl amino-silane is used as external donor, the catalyst

exhibits high activity, high stereospecificity and good hydrogen

response.55

Kemp developed an external donor, the calixarene, which is

a macrocyclic oligomer formed by condensation reaction of

phenol and formaldehyde.56,57 Using calixarene as external

donor, the catalyst stereoregularity could be improved signifi-

cantly while the catalytic activity changed slightly. When alkyl

group substituted calixarene was used as an external donor, the

PP isotacticity could be further enhanced while the catalytic

activity decreased. In addition, if calixarene substituted siloxane

was used as external donor, the catalyst stereoselectivity signifi-

cantly increased while the activity changed little.

Himont also reported a kind of catalyst using 1,3-diether as an

external donor and phthalate as the internal donor. This catalyst

system has similar activity and hydrogen response to the catalyst

using 1,3-diether as an internal donor.58

2.2.1.3 Catalyst preparation process. PP catalyst preparation

process can be simply divided into two categories: one is

preparing MgCl2 support first and then reacting the support with

TiCl4 and the electron donor, such as Basells spherical catalyst;

the other is dissolving a magnesium compound first and then co-

precipitating MgCl2 with titanium in the presence of TiCl4, such

as the Mitsui Companys TK catalyst. Catalyst preparation

process is also important for catalyst performance. Although the

final components of the solid catalyst made from different

processes are almost same, their microstructures can be quite

different, such as catalyst active center number and distribution,

so that the performance of catalysts can differ a lot.

In recent years, some new catalyst preparation processes have

been developed and among them the most noteworthy is the

emulsion process developed by the Borealis Company.59This journal is The Royal Society of Chemistry 2011

Catalysts prepared by this method have good morphology and

narrow particle size distribution. PP produced by these catalysts

has narrow particle size distribution, low fine content, and high

bulk density. Although the catalysts prepared by this method

have a small surface area and low porosity, the catalytic activity

is still very high. This result introduces a new PP catalyst research

direction that is completely different from the traditional one. In

order to enhance catalyst activity, traditionally catalyst scientists

always concentrate on increasing the catalyst surface area and

porosity. Now the PP catalyst with small surface area and low

porosity can also have high activity.

thoroughly studied catalysts (Fig. 7). When R1 is methyl

(complex 2), the molecular weight of polypropylene can be

significantly increased. When R2 is naphthyl, propylene poly-

merization activity of catalyst 2 is as high as 8.8 108 g PP/molM h, and polymer molecular weight is 920000, with a melting

point of 161 C, isotactic pentad ratio (mmmm) of 99.1%.63 Sofar 2-methyl-4-aryl-indenyl-based catalysts developed by the

Spaleck research team are the most successful metallocene

catalysts for producing isotactic polypropylene.

In recent years, one of the main research focuses on metal-

locene catalysts is new supports to further reduce the amount of

MAO and further improve the metallocene catalyst activity.64

Grace has developed a support technology named IOLA, which

was prepared by spray drying a reactant of SiO2 and clay.65

IOLA is a micro spherical particle with very rough surface,

plenty of caves and pores inside. These micro particles were

ionized clay particles enabling the Lewis acid uniformly

dispersed throughout the particles and interacted easily in the

catalyst. In the pre-polymerization stage, IOLA acts as an acti-

vator for the catalyst, and in polymerization stage, it acts as

a support for the catalyst. Thus IOLA has dual functions as both

support and activator, eliminating the need for expensive MAO

or borane as a cocatalyst, dramatically reducing the production

cost of single-site catalysts.

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comprised of the central metal, the ligand containing cyclo-

pentadienyl (Cp) group and the activator (also known as co-

catalyst). Ligand structure plays a key role in catalyst perfor-

mance and only the ligand with appropriate substitution

combined with the most appropriate transition metal atoms can

form an efficient metallocene catalyst. Metallocene catalyst is

a kind of single-site catalyst. It offers precise control of not only

molecular weight, molecular weight distribution and crystal

structure but also to the comonomers position in the polymer

chain. The catalyst structure is very important. A large number

of studies show that polypropylene with different stereoregu-

larity including random, syndiotactic and isotactic and with

different copolymer composition can be made by using different

metallocene catalysts with different ligand structure.

In 1995, Coates and Waymouth60 synthesized a non-bridged

metallocene catalyst (2-Ph-Ind)2ZrCl2 with both meso and

racemic structures that can be converted to each other (Fig. 5). It

was found that the raceme mainly resulted in preparing isotactic

polypropylene, while the meso structure made random poly-

propylene segments and the segment length was determined by

the conversion rate between raceme and meso and monomer

insertion rate. By changing the polymerization conditions, PP

with various structures can be prepared. By changing the

substituents on the phenyl group of the ligand, the research team

developed a number of other metallocene PP catalysts with

similar properties.61

Ewen and coworkers found that the CS-symmetrical metal-

locene catalyst, as shown in Fig. 6, can be used for producing

highly syndiotactic PP, which is a significant breakthrough in the

development of syndiotactic polypropylene.62 This finding not

only produced a new kind of plastic, but also provided proof for

the double active site polymerization mechanism of the metal-

locene catalyst.

The zirconocene catalysts rac-Et(Ind)2ZrCl21 and the silicon-

bridged metallocene2 (R1 and R2 are hydrogen) are the two most

Fig. 5 The structure of rac- and meso-(2-Ph-Ind) 2ZrCl2.This journal is The Royal Society of Chemistry 2011Compared with metallocene polyethylene catalysts, the

development of metallocene polypropylene catalysts is relatively

slow. However, the metallocene polypropylene has shown its

advantages; therefore, the metallocene polypropylene catalysts

are expected to develop as good as metallocene polyethylene

catalysts in the future.

2.2.3. Non-metallocene single-site catalysts. This is another

kind of single site catalyst which does not contain cyclo-

pentadienyl, indenyl or a fluorine-based ligand. The ligand in this

Fig. 6 CS-symmetrical metallocene catalysts.

Fig. 7 Ethyl- and dimethyl silicone-bridged metallocene catalysts.Polym. Chem., 2011, 2, 16111623 | 1617

In the past 10 years, the main PE processes almost remain

unchanged. However, some progress in modification of existing

The LLDPE process was mainly focused on further enlarging

capacity, bimodal LLDPE technologies, controlling comonomer

distribution and easy-processing metallocene LLDPE. To reduce

the cost of the production, the capacity of Unipol gas-phase

polyethylene reactor can reach 600,000 tons per year. Because of

the increase in the reactor diameter, heat transfer must be

strengthened in engineering, so are the materials transfer, the

predicting of reactor agglomeration and the technology of

reducing the transition product.72The Spherilene technologywith

a dual-reactor recently took advantages of the Lupotech G tech-

nology to simplify the catalyst system and optimize exhaust gas

removal system. TheWestlake Company developed a technology

to control comonomer distribution in polyethylene chain by

adding some special compound during polymerization, which can

improve the performance of LLDPE resin.73 The Evolue process

now can use metallocene catalyst and a dual-reactor to produce

bimodal metallocene polyethylene. NOVA Chemicals have

adopted the Sclairtech dual-reactor and non-metallocene single-

(advanced process control) technology has also been applied

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HDPE processes, the main advances were enlarging the device

capacity, using multi-reactors to manufacture multi-modal PE

products and simplifying the processes. For example, the

Chevron Phillips company had improved its loop process in

following aspects: (1) use larger scale reactor to realize the

economic benefits; (2) reduce the number of devices; (3) improve

the equipments efficiency and capacity; (4) simplify the instal-

lation, such as using aself-supporting reactor with fewer steel

structures; (5) improve the catalyst feed system; (6) improve

process control technology. The investment cost of the improved

devices was reduced by 50% compared to the old one designed in

1990. In order to produce bimodal and multi-modal HDPE

products smoothly, the improvement of Hostalen process was

mainly focused on the reactor cooling system, gas emission

system and de-waxing separation part.

In the LDPE process, the main progress lies in three areas:

improving product quality, increasing productivity and reducing

costs. For example, the capacity of single-tube CTR technology

has been increased to 400,000 tons per year. In high-pressure

autoclave LDPE field, LyondellBasell has developed the Lupo-

tech A process that has a special monomer and initiator injection

system to control the reaction pressure and temperature distri-

bution, so as to obtain the higher conversion rate. This process

can be used to produce adhesives and sealants that contain up to

40% vinyl acetate.kind of catalyst is alkyl or aryl containing N, O, S, or P atoms.

Performance of the non-metallocene catalyst can reach or even

exceed the metallocene catalyst.66 Some non-metallocene poly-

ethylene catalysts have been industrialized, while non-metal-

locene polypropylene catalysts are still in the laboratory stage.

The most representative three kinds of non-metallocene PP

catalysts are the diimine nickel/palladium catalysts, the pyridine

diimine iron/cobalt catalysts and the salicylaldiminato titanium/

zirconium catalysts.

In 1998, the Fujita group of the Mitsui Chemicals Company

developed a novel type of salicylaldiminato group IV transition

metal complexes, known as FI catalysts, which showed excellent

catalytic performance on olefin polymerization.67 Such catalysts

showed high polymerization activity for ethylene polymeriza-

tion. But for propylene polymerization, the stereoselectivity was

highly depended on the central metal and cocatalyst. With MAO

as the cocatalyst the salicylaldiminato zirconium catalyst is very

difficult to make high stereoselectivity polypropylene. However,

under certain polymerization conditions, the salicylaldehyde

titanium catalyst can prepare high regularity syndiotactic poly-

propylene.68,69 It is also found that with iBu3Al/Ph3CB (C6F5)4 as

cocatalyst, high molecular weight atactic polypropylene could be

prepared when using the salicylaldiminato titanium catalyst,

while moderate to high molecular weight isotactic polypropylene

can be prepared by zirconium or hafnium salicylaldiminato

catalyst with the same cocatalyst.70,71

3. Polymerization process

3.1 PE process1618 | Polym. Chem., 2011, 2, 16111623universally.77,78

Fig. 8 Some typical current polypropylene processes.site catalyst to produce high-performance LLDPE products.74

3.2 PP process

Since the PP industry started in 1957, the advances in catalyst

technology and the demand for improving product performance

have always been the two main driving forces to promote the

development of propylene polymerization processes.2 The typical

current PP processes are shown in Fig. 8.

Since the loop reactor has lots of advantages like high space-

time yield, full use of reactor volume, simple structure, less

investment in construction, excellent heat and mass transfer and

seldom fouling in the reactor etc., it is regarded as the best reactor

for propylene homo-polymerization. This leads the loop reactor

process to be the most important manufacturing process for

polypropylene now. Actually, the autoclave process has been

abandoned.1,75,76

In order to improve the economics of plant operation, the scale

of polypropylene plants have become larger and larger, and the

largest production line is now over 450 kt/a. In order to meet the

requirement for high quality polypropylene products, APCThis journal is The Royal Society of Chemistry 2011

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. View Article Online(BRICI) in 2006. The aim of ASD technology is to improve the

overall performance of broad molecular weight distribution PP.

Through regulating the amount and/or species of external donor

in different polymerization stages, the catalyst performance, such

as stereoregulating effect, hydrogen sensitivity and co-polymer-

ization behavior, can be controlled in different polymerization

stages by using ASD technology. The technology has been widely

applied in a series of PP products, such as the high performance

BOPP resin, PPH pipes, high melt strength polypropylene and

high melt index impact polypropylene with low VOC.

4. Additives

Additives are indispensable components for polyolefin resins,

which can improve the resins processibility and other important

end-use properties. There are tens of different kinds of additives

used in polyolefin resins, in which nucleating agent is one of the

most noteworthy additives. Some progress of PP nucleating

agent will be introduced in this section.

4.1 PP nucleating agent

Nucleating agent is an important additive for semi-crystalline

polyolefin, especially for polypropylene. Some important phys-

ical and mechanical properties of PP resin can be greatly

improved when low levels (generally between about 0.01 and 0.5

wt%) of commercial high-performance nucleating agent are

added in the resin formula. The research concerning nucleating

agent is very active and there are hundreds of patents filed in this

area. Some review articles or books have been published in the

last decade on various aspects of polymer nucleating agents.8386

4.1.1 Ultra-fine or nano-sized melt insensitive nucleators.

Commercial PP nucleating agents can be divided into melt

sensitive and melt insensitive ones. Both types need adequatereactor (MZCR), which contains two different regions, i.e. Riser

and Downcomer. Compared with the multi-reactor technology,

the residence time of the particles circulating around in MZCR is

much shorter than that in the multi-reactor, so that the particles

uniformity is improved significantly.

Flexible control of catalyst properties is another way to ach-

ieve high performance polypropylene. It can be reached by

Asymmetric External Donor (ASD) technology developed by

SINOPEC Beijing Research Institute of Chemical IndustryFurther improving polypropylene performance, such as poly-

mer flexibility, rigidity and melt strength, is an important target

in the PP industry. Among current polypropylene processes,

some processes are designed for this purpose, such as the Cata-

lloy process developed by BASELL.79,80 The Catalloy process is

based on reactor particle technology (RGT), in which broader

MWD, higher isotactic index, random copolymer with up to

15 wt% comonomer, multi-phase polyolefin alloy containing

more than 70 wt% rubber can be produced. The most important

advantage of this copolymerization technology is that the low

modulus PP resins can be produced.

The commercial Spherizone process started in 2003 by the

Basell Company was the most important breakthrough in PP

production processes in the last 10 years.81,82 The core technology

of the Spherizone process lies in the multi-zone circulatingThis journal is The Royal Society of Chemistry 2011dispersion prior to compounding, especially for melt insensitive

nucleators. The size of nucleating agent has substantial influence

on the properties of PP. The larger the particle size, the lower the

nucleation efficiency. Smaller nucleator particles usually generate

smaller spherulites, higher mechanical properties and better

transparency. Many researchers try to make nucleators with

smaller particle size in order to improve the dispersibility of

nucleators and their nucleating efficiency.

Asahi Denka Corp.87 reported using pulverization apparatus

to produce finely pulverized phosphoric acid aromatic ester

metal salt in a European patent application. By controlling

pulverization time, the average major-axis length of nucleator

can be adjusted between 0.15 microns and the resulting nucle-

ator has enhanced dispersibility. Their examples show that the

nucleator particle size can reach 0.3 microns after jet-milling

30 min and ball-milling for another 2.5 h. Particle size below

0.1 micron is not preferred due to higher pulverization cost and

lowering of fluidity.

Xinzhong et al.88 disclose a method for producing ultrafine

organic phosphate nucleating agent. Said method includes the

following steps: dissolving organic phosphate nucleating agent in

organic solvent, adding surfactant, adding water to the above-

mentioned solution under stirring, then collecting the described

ultrafine organic phosphate nucleating agent from the reaction

product. Its average particle size is less than 0.3 micrometres.

Libster et al.89,90 disclosed a novel dispersion method to

improve the nucleation efficiency of a water soluble nucleator,

bicycloheptane dicarboxylate salt, commercially known as HPN-

68 fromMilliken. Their concept is based on using microemulsion

technology as a transport vehicle. The nucleating agent must be

dispersed in a microemulsion to decrease its size from micro to

nanoscale, which was then introduced to the target PP using

a mixer. By the end of the melt mixing, when the water phase had

evaporated, only the nucleator and the surfactant remained in

the matrix. SEM results showed a four-fold decrease in the PP

spherulite size due to the improved dispersion of HPN-68 within

the matrix via microemulsion compared to conventional nucle-

ator incorporation. DSC results show that only 50 ppm HPN-68

is sufficient to achieve the highest TC, similar to the one obtained

by adding 300 ppm dispersed nucleator powder, i.e. 20% nucle-

ating agent is required to achieve the same nucleating effect.

4.1.2 In situ nucleating technologies. The current nucleating

technologies are mostly post-reactor technologies, i.e. nucleating

agents are incorporated into polyolefin resin after the resin

powder has been produced from the reactor. The dispersion of

nucleating agent during the melt pelletizing process is critical to

ensure good nucleating effect and resin properties for these

technologies. Another category of nucleating technology can be

described as in situ technology, i.e., nucleating agent is combined

with polymerization catalyst or can be transformed after catal-

ysis process. This kind of in situ technology can theoretically

make very uniform dispersion of nucleating agent in resin

matrices. However the polymerization activity of ZieglerNatta

catalysts is susceptible to various functionalities related to the

nucleating agent. Most research concerning in situ nucleating

technology is accordingly focused on single-site catalysts, which

can still function well under environment of multiple functional

groups.Polym. Chem., 2011, 2, 16111623 | 1619

Xinzhong et al.91 disclosed a non-metallocene compound for

catalyzing and nucleating polyolefin. This non-metallocene

compound (as shown in Fig. 9) can be supported on magnesium

halide to catalyze olefin polymerization. After catalysis, the

compound is deactivated to become the nucleating agent for

crystallizing polyolefin to raise the crystallization performance,

mechanical performance and optical performance of polyolefin

greatly.

Yi et al.92 developed a method of exploiting a nucleation agent

as support for metallocene catalyst. The metallocene compound

can be well supported on 1,3,2,4-dimethyl benzylidene sorbitol

(DMBS), a sorbitol based nucleating agent. Propylene poly-

merization with the supported catalyst resulted in i-PP polymers

with granular morphology. The role of a catalyst support ensures

Fig. 9 The chemical structure of non-metallocene catalysts.

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and (d) in regular homo-PP matrix; (e) and (h) in heco-PP matrix; (g) and (f

1620 | Polym. Chem., 2011, 2, 16111623pound in different PPmatrices. (a) and (b) in high isotactic PPmatrix; (c)

) in raco-PP matrix.This journal is The Royal Society of Chemistry 2011

compound nucleating agent can achieve the same or even better

nucleating effect as compared to pure NA-11. This technology

Even though the direct observation of nucleator in PP matrix

is difficult due to its low concentration, some indirect experi-

mental results as shown in the PLM micrographs (Fig. 11)

indicate that the crystal size is smaller and the nuclei density is

higher in PP sample nucleated with compound nucleator. These

results can be preliminarily used as evidence to prove that ENPs

can promote the dispersion of nucleator in PP matrix and

enhance the nucleating efficiency accordingly.

5. Polyolefin development in china

China is now a major polyolefin consumer and producer in the

world. Mainland China consumed about 28 million tons and

produced about 16 million tons of polyolefins in 2009. Chinas

polyolefin industry uses monomers from 3 sources, ethylene

plant, refinery and MTO/MTP plant. Based on coal, the MTO/

MTP-polyolefin industry has grown very fast in China recently.

There are 1.52 million tons/a polyolefin capacity based on MTO/

MTP at present and more capacity is still in construction in

China. It is difficult for polyolefin manufacturers to avoid fierce

competition in China because polyolefin capacity increases much

faster than consumption does in China and nearby countries

recently. To meet the challenge, major polyolefin producers such

as SINOPEC have targeted development and production of both

sustainable products with environmental and safety benefits and

speciality high performance polyolefin.

The polyolefin is so important in China that even the funda-

mental research of polyolefin has been supported by 973

program for 10 years from 1999 to 2010. 973 program, the

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. View Article Onlinehas been successfully applied to some high impact heco-PP

grades and high crystalline homo-PP grades in SINOPEC in

recent years. And more than 100,000 tons of high performance

PPs with lower cost have been produced by using this compound

nucleating agent in recent years.a good dispersion of the nucleation agent in the formed i-PP

matrix. The i-PP/DMBS composition exhibits a much higher

crystallization temperature (TC) than the neat i-PP (129.5 vs.

113.3 C), as well as much finer spherulites, a typical represen-tation of crystallization with the assistance of nucleation agent.

4.2 Improving efficiency of polyolefin additives with

elastomeric nanoparticles

Apart from a few polyolefin additives such as antioxidants etc.,

which can be molten and well mixed with the resin matrix via

melt compounding with polyolefins, most additives need to be

well dispersed in polyolefin matrix so as to function well at low

addition levels. Some powerful additives are usually expensive

and the improvement of their efficiency means the reduction of

additive loading and reduction of polyolefin resin cost, which is

thus very attractive for resin producers and converters.

Rigid nanoparticles such as nanosilica, layered silicate etc. are

well known and have been used as polymer fillers for many years.

The elastomeric nanoparticles (ENPs) mentioned here are flex-

ible powder made from rubber latex through Narpow tech-nology invented by SINOPEC BRICI a decade ago. These ENPs

are used as toughening agent for thermoplastic9396 and ther-

mosetting resins.9799 The Narpow technology uses rubber latexand irradiation sensitive as raw material and fully vulcanizes the

latex particles through irradiation and finally spray-dries the

vulcanized latex to obtain ENP products.100,101 It can be inferred

from this manufacturing process that the surface part of ENP has

a higher cross-linking degree than the interior due to a higher

concentration of irradiation sensitivity near the surface part as

well as more reactions with excited molecules and ions in water

produced by the irradiation. Such a unique structure assures that

ENPs maintain good elastic properties and at the same time the

agglomerates of ENPs can still be easily broken apart and well

dispersed in a plastic matrix.

It has been found in recent years that ENP can significantly

improve the efficiency of some plastic additives, which means

a similar or better performance can be achieved at lower loading

of additive when ENPs are compounded with the corresponding

additive. The compound nucleating agent introduced in this

review is one of the examples that ENPs can improve the

efficiency of PP additives.102

The compound nucleating agents can be obtained by mixing

commercial nucleating agents, such as sodium 2,20-methylene-bis-(4, 6-di-t-butylphenylene) phosphate (NA-11) and styrene-

butadiene rubber ENPs. The nucleating effects of pure NA-11

and the compound were compared in different PP matrices as

shown in Fig. 10. The crystallization temperature, flexural

modulus and heat distortion temperature curves all show that the

compound nucleating agent has better properties even at lower

loading. Typically ca. 25% of NA-11 concentration in theThis journal is The Royal Society of Chemistry 2011Fig. 11 PLM micrographs of homo-PP modified by 0.4& NA-11 (a)pure NA-11 (b) NA-11/ENP compound.Polym. Chem., 2011, 2, 16111623 | 1621

nn

e

s

c

sors, researchers

t

n

25 W. Kaminsky, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 39113921.

Chem. Rev., 2005, 105, 40734147.

c

.0

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. View Article OnlineBOPP biaxially orientated polypropylene

MFI melt flow index

ASD asymmetric external donor

HMSPP high melt strength polypropylene

ICI Imperial Chemical Industries

Z-N ZieglerNatta

CPE chloride polyethylene

MAO methylaluminoxane

IUPAC International Union of Pure and Applied

Chemistry

HMW high molecular weight

LMW low molecular weight

CGC constrained geometry catalyst

FI salicylaldiminato-type catalyst

CSA chain shuttling agent

CHMDMS methyl cyclohexyl dimethoxy silane

DIBDMS diisobutyl dimethoxy silane

DCPDMS dicyclopentyl dimethoxy silane

DIPDMS diisopropyl dimethoxy silane

Cp cyclopentadienylPOP

APAO1622 | Polym. Chepolyolefin plastomer

amorphous poly-alpha-olefinPOE polyolefin elastomerEU European UnionFDA Food and Drug AdministrationOBCs olefin block copolymersTPVs thermoplastic vulcanizatesSEBS poly(styrene-b-ethylene butylene-b-styrene)SIS poly(styrene-b-isoprene-b-styrene)SBS poly(styrene-b-butadiene-b-styrene)PVC poly(vinyl chloride)PE-RT polyethylene of raised temperature resistancePE polyethyleneVOC volatile organic compoundsPP polypropyleneLLDPE linear low density polyethyleneHDPE high density polyethyleneLDPE low density polyethylene6. Abbreviatio sdevelop in the fu ure.going mouldy. The polyolefin industry in China will continue torequirements for washing machine to prevent the barrel fromdifferent macromolecules, anti-static, printability, and specialtechnology of polyolefins, such as co-monomer distribution onindustry, are stillfrom Chinese Academy of Sciences and

carrying on studies on the basic science andThe polymer s ientists in China, including university profes-introduced in thi review.olefin has had gr at achievements and some of them have beenolefin copolymer and impact PP. The 973 program on poly-polymer science, such as basic polymer science in random poly-Academy of Scie ces and SINOPEC working together on basicmillion USD to support the scientists from universities, ChineseScience and Tech ology. The program was granted more than 10keystone basic research program organized by the Ministry ofNational Basic Research Program, is Chinas on-going nationalm., 2011, 2, 1611162334 L. K. Johnson,1995, 117, 6414ThisC. M. Killian and M. Brookhart, J. Am. Chem. Soc.,6415.283315.33 A. L. McKnigh2598; V. C. Git and R. M. Waymouth, Chem. Rev., 1998, 98, 2587bson and S. K. Spitzmesser, Chem. Rev., 2003, 103,32 J. H. Oskam, Proceedings of PEPP 2008 conference, Zurich, , 2008.

Mater. Eng., 2 05, 290, 250255.31 M. Bartke, M Oksman, M. Mustonen and P. Denifl, Macromol.29 A. Motto, Pro30 China Petroleueedings of PEPP 2007 conference, Zurich, , 2007.m & Chemical Corp., CN Pat., 200710176589, 2007.28 C. Knight, Proceedings of PEPP 2008 conference, Zurich, , 2008.26 G. G. Hlatky, Chem. Rev., 2000, 100, 13471376.27 J. R. Severn, J. C. Chadwick, R. Duchateau and N. Friederichs,APC advanced process control

RGT reactor particle technology

MWD molecular weight distribution

MZCR multi-zone circulating reactor

SEM scanning electron microscope

DMBS 1,3,2,4-dimethyl benzylidene sorbitol

ENP elastomeric nano particles

USD United States dollar

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