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DOI: 10.1002/adma.200800603

Self-Similar Growth of Polyolefin AlloyParticles in a Single Granule Multi-Catalyst Reactor

By Jiang Du, Hui Niu, Jin-Yong Dong,* Xia Dong, and

Charles C. Han*

In this news article, a new kind of in-reactor alloying process (Chinese multi-catalyst reactorgranule technology, CMRGT) is introduced. It provides the possibility to produce series of newpolyolefin materials that can be used in automotive and appliances parts by the substitution ofsome traditional engineering plastics. A characteristic ‘fractal (self-similar) growth and pore-filling mechanism’ for the CMRGT-produced alloy particles is found. It reveals the fundamentalscientific principles that govern the chemistry and physics in the formation mechanism of suchnovel olefinic alloys. This mechanism allows a detailed control of the dispersion and structure ofthese polyolefin alloys from a nanometer to micrometer size range, and possesses great potentialto fulfil the requirements for a new generation of recyclable automotive materials.

1. Introduction

The world today is under a transition from a high resource

and energy consumption society towards a resource and energy

sustainable one. More and more efforts are focused on natural

resource protection and pollution reduction, and advanced

materials play an important role in fulfilling these new

challenges. New materials today, and more in the future,

should have high performance, high functionality, and should

also be low cost and environmentally friendly. Polyolefin

materials, which are already the most widely used materials in

the plastics industry today, have the potential to meet these

requirements. For example, more and more toughened

injection grades of polyisoprene/polyolefin elastomer blends/

alloys are used in automobiles and appliances in replacement

[*] Prof. C. C. Han, Dr. J. Du, X. DongState Key Laboratory of Polymer Physics and ChemistryJoint Laboratory of Polymer Science and MaterialsInstitute of Chemistry, Chinese Academy of SciencesBeijing 100190 (P.R. China)E-mail: c.c.han@iccas.ac.cn

Prof. J. Dong, Dr. H. NiuCAS Key Laboratory of Engineering PlasticsJoint Laboratory of Polymer Science and MaterialsInstitute of Chemistry, Chinese Academy of SciencesBeijing 100190 (P.R. China)E-mail: jydong@iccas.ac.cn

� 2008 WILEY-VCH Verlag Gmb

of traditional high-performance or engineering plastics for

their lower cost, good properties, and recyclability.

Over the years, significant progress has been made towards

new polyolefin materials and a number of improved/innovative

propylene-based products have been developed that clearly

outperform the previously available olefinic materials. In

particular, a family of in-reactor prepared polymer alloys have

been synthesized, on the basis of reactor granule technology

(RGT) defined as:[1] ‘the controlled, reproducible polymeriza-

tion of olefinic monomers on an active magnesium chloride

supported catalyst, to give a growing, spherical polymer

granule that provides a porous reaction bed within which other

monomers can be introduced and polymerized to form a

polyolefin alloy’.

RGT is considered to be a fundamental milestone in

polyolefin technology development,[1–4] and has led to a

number of innovative, advanced, and versatile industrial

polymerization processes: Spheripol and Catalloy by Montell,

Unipol by Dow, and Novoven by BASF. The polymer particles

themselves act as the reactors in which polymerization occurs,

and by changing the monomers, it is possible to obtain other

polymers intimately dispersed within the mass of the solid

granule of the matrix. The most important fact is that it is now

possible to achieve an ideal mixing of different and even very

different components inside the same granule, which over-

comes the difficulties of sub-micrometer size mixing by

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conventional blending technology. Thus, it enormously

enhances the degree of freedom in achieving new materials.

Since the spherical morphology of the catalyst particle could be

replicated in the final polymer as the particle growth takes

place during the polymerization,[5–9] it is possible to eliminate

the pelletization step, which is expensive and damages the

integrity and quality of the polymer.

Figure 1. SEM images of the initial catalyst particles (a) and the productpolymer spheres (b).

2. From RGT to CMRGT

The RGT is generally carried out using the same catalyst in

various stages: the product obtained in the previous stage is

sent directly to the next stage without altering the nature of the

catalyst. Therefore, the characteristics of the products obtained

in the different individual stages are not always optimized.[3]

For example, when heterophase polymer alloys are prepared in

multistage processes using titanium catalysts, the properties of

the rubbery copolymers produced in the second stage are

usually poor. In fact, it is known that titanium catalysts

produce copolymers that contain relatively long sequences of

the same monomer unit and the segment distribution is far

from random (or statistical). Consequently, the elastomeric

properties of the products are poor.

Recently, the metallocene catalysts have attracted much

attention, as they allow the synthesis of homo- and copolymers

endowed with a narrow molecular-weight distribution, a

narrow distribution of stereoregularity, and a narrow chemi-

cal-composition distribution. Furthermore, they allow control

over the stereoregularity in 1-olefin polymerization and the

comonomer placement in copolymerization. Consequently,

these molecular features turn into new or improved physical-

mechanical properties of the final products.[10,11] Thus, a

modified RGT has been exploited to take advantage of both

the excellent tacticity control of Ti-based catalysts and the

outstanding copolymerization ability offered by metallocenes.

A new process has been devised and named ‘multicatalyst

reactor granule technology’ (MRGT).[12,13] In the first stage, an

olefinic homopolymer is prepared with the spherical Ti-based

catalysts, followed by a treatment stage in which the catalyst

used previously is deactivated, and in the third stage, activated

metallocene catalysts are introduced into the porous homo-

polymer spheres through an in situ impregnation and drying

process. Finally, an elastomeric copolymer, catalyzed by the

metallocene catalyst, is produced in the presence of the

homopolymer spheres.

Although the MRGT combines the advantages of different

catalysts, the complex processes make it difficult to scale up for

industrial production. Moreover, the metallocene catalyst

could not be separated well in the porous polymer particles

obtained in the first stage, and the superfluous copolymer

produced by the metallocene catalyst on the polymer particle

surface would tend to increase the tackiness of the solid

polymeric particles, which results in fouling of the reactor.

An alternative process has now been developed at the

Institute of Chemistry of the Chinese Academy of Sciences

Adv. Mater. 2008, 20, 2914–2917 � 2008 WILEY-VCH Verl

(ICCAS), and we shall call this process ‘Chinese multi-catalyst

reactor granule technology’ (CMRGT). A wide range of

olefinic polymer blend/alloy compositions, as well as the

compositions of the elastomeric components, can be produced

conveniently in only two stages by using a compound catalyst

system (Ti/Zr hybrid catalysts).[14–16] The process is summar-

ized simply as follows: in the first stage, isotactic poly(propy-

lene) (iPP) is prepared selectively by the Ziegler–Natta

component of the Ziegler–Natta/metallocene hybrid catalyst,

while the metallocene component has been blocked by an

inhibitor. In the second stage, one or more olefins are

polymerized in the presence of the product obtained from

the first stage, after the reactivation of the metallocene

component by a reactivator. The spherical morphology is

perfectly maintained during these processes (Fig. 1). This

realistic CMRGT process provides a great application

potential for polyolefin in-reactor alloys.[14,15]

3. Self-similar Growth in CMRGT

The CMRGT in-reactor blending/alloying process provides

a new kind of initial state (nascent state) of the polyolefin

alloy that cannot be obtained by conventional blending

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Figure 2. Proposed model of the in-reactor prepared iPP/PEOc polymer alloy particle.

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processes.[15,16] Because of this special nas-

cent structure, a fine dispersion of elastomer

droplets at a length scale of 100 nm or below

can easily be obtained during material

processing and parts production.[15] It pro-

vides a cost-effective, flexible, and better way

to control the structure, dispersion, and finally

the properties of elastomer-toughened iPP

alloys in application.

Some studies have been reported on the

in-reactor prepared alloys,[5–9,13] and some

models have been proposed.[5,6,8] However,

the detailed experimental evidence is far

from sufficient to support a detailed mechan-

ism. Perhaps, the reason for this is that

most published reports are based on single-

catalyst (Ziegler–Natta) technology,[5–9] and

very few studies are concerned with the

MRGT,[13] while no report has been pub-

lished on the CMRGT.

We have performed an extensive analysis

of the nascent structure of the alloy particle

produced by CMRGT and found a very

interesting fractal (self-similar) growth pro-

cess and hierarchical morphology. The

proposed model is summarized in Figure 2.

During the first homopolymerization stage,

Figure 3. TEM images of the ultrathin section of an iPP/PEOc alloyparticle. a) A typical scene within the alloy particle. b) TEM image of asingle microparticle.

the propylene monomers diffuse easily and evenly into the

interior of the porous catalyst particles (Fig. 1a) and

polymerize at the active centers, which are homogeneously

dispersed within the catalyst particles. The porous catalyst

particles are then progressively expanded and burst into

fragments as a result of the growing polymers. These fragments

are held together by the intermingling of the polymer chains.

As polymerization proceeds, the catalyst particles undergo

further fragmentation into smaller units. The subsequent

polymerization then takes place over these subfragments.

These processes repeat over and over again, to form a self-

similar (or self-replicating) growth process. The catalyst

fragments disperse randomly in the polymer particles. Each

fragment is surrounded by a shell of poly(propylene), which

evolves into a sub-micrometer-sized microparticle (Fig. 3).

These iPP microparticles comprise even smaller nanoparticles,

with diameters of tens of nanometers, and present a porous

morphology (Fig. 4d). The iPP microparticles tend to cluster

into larger structures (subparticles, about 10–20mm in

diameter, Fig. 4c), and again, the subparticles agglomerate

together, tomake up the whole iPP particle (Fig. 4a and b). The

interstitial spaces between the subparticles, microparticles, and

nanoparticles contribute to the porosity of the whole particle.

In the second stage, the poly(ethylene-co-octene) (PEOc)

copolymers preferentially form on the active metallocene sites

of the catalyst fragments inside the porous iPP homopolymer

microparticles, and tend to be squeezed out and flow from the

densely packed crystalline regions to the interstices and the

micropores that are located around the iPP nano- and

www.advmat.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 2914–2917

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Figure 4. SEM images of the iPP/PEOc particles after removing the elastomer phase by solventextraction. a,b) The granule particle is composed of many secondary globules (subparticles). c) Thesubparticle is in turn made up of some smaller microparticles. d) The micropasmaller nanoparticles and present a porous morphology.

microparticles, and then to the macropores between the

subparticles, and finally form a continuous network of a rubber

phase inside the particle. The rubbery copolymers could

eventually bloom to the surface of the polymer particle, which

gives the particle a relatively smooth appearance (Fig. 1b). In

this respect, it should be taken into account that the glass

transition temperature of the elastomeric phase of the PEOc

alloy used in this report is low and the copolymerization

reaction is generally carried out well above this tempera-

ture.[16]

During heating and processing, a phase inversion should

naturally happen in the alloy particles, leaving the minority

elastomer component as the discrete phase (droplets). By

simply changing the annealing time, we could continuously

adjust the size of the elastomer droplets (for example between

0.1–10mm) with uniform spatial dispersion in the iPP

spherulites, and optimize the impact performance.[15]

4. Summary and Outlook

We have determined a characteristic ‘fractal (self-similar)

growth and pore-filling process’ for the CMRGT-produced

polyolefin alloy particles. It reveals the fundamental scientific

principles that govern the chemistry and physics of the

Adv. Mater. 2008, 20, 2914–2917 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, We

formation of such a self-similar fractal

structure of these olefinic alloys. Indeed,

this fractal growth provides a fine disper-

sion, special structures, and superior

properties of the in-reactor prepared

polyolefin alloys, which has been pursued

by scientists and engineers for many years.

The CMRGT has not only provided a

new technology for producing iPP-based

alloys with different elastomer toughen-

ing characteristics, but also provides a low

cost, in-reactor manufacturing possibility

to produce series of injection-grade

recyclable thermoplastics and thermo-

plastic elastomers. These application

potentials actually come from the inter-

esting fractal (self-similar) growth

mechanism in a two-step process on a

catalyst-carrying porous granule particle.

This iPP alloy has great potential for

application in various automotive and

appliance parts in the near future.

Published online: July 9, 2008

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