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6.4. Synthesis Of Polymeric Materials

At present, the U.S. is likely the main competitor of
References
Europe in the field of processing of ceramic materials, but
1. Fundamental Research Needs in Ceramics, USA NSF Workshop Report,
with research topics more driven by technological appli-
Y.M. Chiang and K. Jakus, April 1999.
cability than by pure basic scientific curiosity. It is evident
2. Joint Research Consortium of Synergy Ceramics, Fine Ceramics Research
Association, Japan, 1999.
that there is a drastic need of fundamental research in
ceramic processing in Europe which is poor in comparison
to U.S.
TERIALS SYNTHESIS AND PROCESSING
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6.4. Synthesis of Polymeric Materials
P. J. Lutz | Institut Charles Sadron, 67083 Strasbourg, France
207
6.4.1. Scope of Polymer Synthesis
these polymerization processes with respect to the nature
of the polymerization procedure and its implication on
Synthetic polymers are utilized increasingly in our daily
the control of the molar mass and its distribution, the func-
life and manifold industrial applications have contributed
tionality, on the composition for copolymers and on the
to their expansion. Chemical industry devotes a great part
structural parameters. Selected examples will be present-
of its activity to these polymeric materials. They are em-
ed and discussed more in detail: controlled free radical
ployed as substitutes for and /or in combination with met-
polymerization, coordination polymerization and the spe-
als, glass, ceramics, wood and paper. The worldwide evo-
cial case of free radical polymerization in emulsion in-
lution over recent years of the consumption of polymeric
cluding the recent developments on the polymerization
materials is given in Fig. 6.7 together with the repartition
in nano-structured medium. Special attention will be giv-
of the different polymeric materials. The purpose of the
en to the recent breakthroughs in theses polymerization
present report is to discuss the approaches to design sev-
processes. The second part concerns some conclusions
eral polymeric materials over a large range of properties.
and recommendations concerning the future orientations.
The major part is concerned with a general discussion on
87%
12%
1%
86
North America
Europe
73
Japan
43
Standard Polymers [polyolefins (47%) and others (40%)]
Engineering Polymers
1980
1995
2000
Speciality Polymers
Fig. 6.7. Evolution of plastics consumption (kg/person) in North America, Europe and Japan [1] (a). Worldwide market for plastics in 1996 [1,2] (b).
6.4.2. State of the Art of Polymer Synthesis Research 1
Transfer Radical Polymerization (ATRP). They were first
applied to homogeneous polymerization processes. Their
(a) Controlled polymerization processes
extension to water represents a major challenge for many
ALLFORSCHUNG STUTTGART
Anionic polymerization carried out under proper condi-
polymer chemists. That point will be discussed in the sec-
tions with efficient metal-organic initiators was consid-
tion polymerization processes in nano structured medium.
ered until recently to be the most powerful way to design a
Nitroxide such as those derived from TEMPO (2,2,6,6-
large scope of polymers of controlled molar mass, func-
tetramethylpiperidinyl) are used for the reversible trap-
tionality, composition and/or topology. Anionic polymer-
ping of growing radicals. At a temperature below 100°C
ization has several intrinsic limitations: It requires strict
the resulting alkoxyamine is stable whereas at higher tem-
experimental conditions and is only applicable to a limited
perature the C-O bond undergoes homolitic cleavage thus
number of monomers. Therefore the number of materials
allowing again the propagation. These systems have been
obtained by anionic polymerization processes is rather re-
shown to be efficient for controlled polymerization of sty-
MAX-PLANCK-INSTITUT FÜR MET
stricted. The specific case of the synthesis via anionic ster-
rene and substituted styrenes. They are less efficient for
eospecific polymerization of elastomeric materials based
acrylic ester monomers. On the contrary phosphonylated
208
on polydienes has to be mentioned. Therefore anionic poly-
nitroxides introduced by Tordo et al (France) in collabora-
merization remains the method of choice to control the
tion with Elf-Atochem are well suited for the controlled
topology of polymers. Two recent breakthroughs in the do-
free radical polymerization of both classes of monomers.
main have to be outlined. They concern the good control
Similar work has been performed worldwide by several
of the anionic polymerization at high monomer concentra-
other groups.
tions and the controlled anionic polymerization of
meth(acrylic) monomers even at room temperature. These
For the ATRP polymerization, copper complexes or ruthe-
two results obtained in the frame of collaborations with
nium salts were developed by Matyjaszewski (U.S.) [6]
European industrial companies have opened new perspec-
and Sawamoto (Japan) [7] to polymerize vinylic monomers
tives for the preparation of well-defined materials. On the
in a controlled way. ATRP is based on the reversible trans-
contrary, polymerization processes based on free radical
fer of halogen atoms between dormant alkyl halides and
polymerization are extensively used for the preparation of
transition metal catalysts by redox chemistry. High molar
a large scope of materials in industrial productions. Free
mass polymers and well-defined block-copolymers could
radical polymerization is easy to process, can be applied in
be obtained. The major limitation to the extend the pro-
dilute solution, in suspension or emulsion and even in the
cess to industrial applications is due to the presence in the
bulk. Free radical polymerization is well know to be appli-
material of inorganic salts or complex ligands.
cable to a much larger number of monomers. In most cas-
es, that polymerization process leads to high molar masses
Reversible Addition Fragmentation Transfer (RAFT) poly-
and does not allow to the control of structural parameters.
merization represents an alternative to the two controlled
This is attributed to the short live time of growing sites
free radical polymerization processes just described and
leading to irreversible termination reactions. Block copol-
offers interesting perspectives.
ymer synthesis is almost impossible. Many efforts have
been devoted recently to obtain a better control of structu-
(b) Polymerization and copolymerization of olefins by
ral parameters in free radical polymerization. New genera-
coordination catalysts
tions of initiators have been introduced in U.S., Japan and
Polymeric materials based on polyethylene (PE) and poly-
also in Europe. These systems called controlled radical
propylene (PP) and their related polymerization processes
polymerization [3] based on a reversible termination reac-
have attracted increasing interest over the last 50 years.
tion or a reversible chain transfer reaction controlling the
They more and more replace other polymers as they com-
activation deactivation cycles have been subject of increas-
bine low prices with tunable properties and good recycling
ing interest in the last five years. An equilibrium takes place
behaviour. They can be designed for numerous applica-
between the macromolecular radical and a dormant cova-
tions such as packaging, textile fibres, automotive parts
lent counter part. Pioneering work has been done by
and even as biomaterials after appropriate modification.
Rizzardo [4] and later by Georges [5]. Two systems have
emerged: Nitroxide mediated polymerization and Atom
The first decisive step was the discovery by Ziegler-Natta
in the 50th of the transition metal catalyzed polymerization
of olefins opening new perspectives in the domain of com-
1) A more detailed discussion on the different topics presented in that
report is given in the following reference:
modity and specialty polymers. Continuous progresses
Materials Science and Technology, A Comprehensive Treatment,
have been made with the aim of improving their efficien-
Volume Synthesis of Polymers, Ed. A.D. Schlüter, Wiley-VCH
Weinheim-New-York (1999)
cy. In the last 20 years new generations of the heterogene-
ous Ziegler-Natta based catalysts have been introduced in
important progress in controlled synthesis of functional
Europe, Japan and in the US. Important cost reductions
polymers carrying hydrophobic and or ionic sites. The spe-
could be obtained with the 3rd and 4th generation of Zie-
cific features of the processes – small size of the droplets
gler-Natta catalysts based on titanium trichloride/magne-
and large overall and large interfacial aria – result in a
sium chloride in the presence two Lewis bases. These cat-
unique microenvironment which can be taken advantage
alysts are highly stereospecific and very active.
of the product to produce novel materials with widely dif-
ferent properties designed for a large scope of applications
In the 80’s the introduction by Kaminsky [2](Germany) of
[12]. The polymerization in aqueous emulsion or suspen-
the homogeneous catalysis based on metallocenes and acti-
sion to design materials directly in water for various appli-
vated by methylaluminoxane was expected to revolution the
cations such as painting/coating. Increasing interest has
domain of organometallic chemistry, polymer synthesis and
been devoted to such (co-polymerization) reactions in
processing. The driving force for the use of these catalysts
water who constitutes an ideal medium for at least two rea-
TERIALS SYNTHESIS AND PROCESSING
MA
is the existence of single-site feature enabling the precise
sons: environment and safety. Presently such arguments
control of molecular parameters: narrow molar mass distri-
are very important in most countries. Beside the classical
bution, chemical composition and microstructure [8,9].
free radical polymerization, it has been shown that ATRP
209
New tailor-made polymers could be obtained and some
is also applicable in emulsion. In addition it has to be men-
products based on metallocene catalysts are now on the
tioned that amphiphilic block copolymers were designed
marked including some types of isotactic polypropylenes.
by ATRP or nitroxide mediated polymerization as stabiliz-
ers in the free radical polymerization emulsion of styrene.
The discovery in industrial companies in Europe and later
in the U.S. of the late transition metal complexes less oxo-
These reactions are yet limited to polymers where no tac-
philic compounds and therefore less sensitive to polar me-
ticity control is required. Only recently work has been done
dia. These catalysts have generated considerable interest
to develop efficient catalytic systems for the polymeriza-
for the polymerization and copolymerization of olefins with
tion of olefins in water. Pioneer work on coordination poly-
polar and especially acrylic monomers [10,11]. In the
merization in water has been done in the group of Lyon in
1995s Brookhart and co-workers have developed a new
collaboration with Elf Atochem. Emulsion polymerization
generation of palladium and nickel (late transition metals)
of ethylene polymerization using bimolecular P,O-chelat-
catalysts aimed either at the polymerization of olefins or at
ed ligands based on nickel yielded in aqueous dispersed
the copolymerization of olefins with some acrylates. The
medium latexes of HDPE. Similar work has been done in
incorporation of functional groups into hydrocarbon poly-
Germany to prepare with palladium catalysts branched
mers constitutes an interesting approach for modifying the
poly(ethylenes).
chemical and physical properties of these polymers, such
as permeability, compatibility, dyeability, adhesion, solid
state morphology, and rheology. The same generation of
catalysts has been shown to provide access to poly(olefins)
6.4.3. Future Visions and Expected Breakthroughs
of controlled topology.
In the present report the most important polymerization
(c) Polymerization in nanostructured media2
processes have been presented and their contribution to
Free radical polymerization in colloidal dispersions such
the preparation of materials covering a large range of prop-
as emulsions has been increasingly used over more than
erties discussed.
50 years to design in industrial processes numerous inter-
mediates for paints adhesives polishes and coating mate-
The preparation of well-defined linear homo or block co-
rials. That polymerization process has been subject of
polymers including functional polymers via controlled free
constant interest and even, if some problems are still open,
radical polymerization is now well possible for an extended
particles of controlled size and functionality are now easily
number of monomers and therefore provides access to var-
accessible. In the 80s the concept of microemulsion poly-
ious materials. The stereospecific controlled free radical
merization has been introduced. That polymerization in
polymerization remains a major challenge and work should
nano-structured media (micelles, microemulsions) based
be done along that line. The combination of controlled free
on classical free radical polymerization represents a
radical polymerization procedures with a polymerization in
emulsion (or in nano-structured medium) has been shown
to access materials of controlled structural parameters
2) A detailed overview of the present situation in the domain of
directly in water. That domain should be examined
polymerization in dispersed medium is given in the following
reference : Macromol. Symp. 150, 2000.
thoroughly in the near future.
The possibility to design polyolefins with controlled
in supercritical carbon dioxide constitutes a great deal.
branching or containing controlled amounts of polar
The increasing contribution of various polymerization
monomers by low pressure polymerization processes rep-
processes to the synthesis of polymeric biomaterials of
ALLFORSCHUNG STUTTGART
resents also attractive perspectives for the low cost indus-
controlled characteristics has to be highlighted.
trial preparation of materials. The search for new families
of non metallocene catalysts with enlarged possibilities
should be supported.
References
1. P. Barghoorn; U. Stebani; M. Balsam, Acta Polymer, 49 (1998) 266.
The possibility to polymerize olefins in water has been
2. W. Kaminsky, J. Chem. Soc., Dalton Trans., 1413 (1998)
3. K. Matyjaszewski; “Controlled Radical Polymerization”, ACS Symp. Ser.
demonstrated. Further work in that direction should be
Washington, D.C, 685, (1998).
strongly supported in Europe. The heterogeneization of
4. E. Rizzardo, Chem. Aust. 54 (1987) 32.
metallocenes catalysts represents also an important chal-
5. M.K. Georges, R.P.N. Veregin, P.M. Kazmaier, G.K. Hamer, Macromole-
cules 26 (1993) 2987.
MAX-PLANCK-INSTITUT FÜR MET
lenge.
6. J. Qiu, K.Matyjaszewski, Acta Polym. 48 (1997) 169.
7. M. Sawamoto, M. Kamigaito, Trips 4 (1996) 371.
210
Many other possibilities or polymerization conditions /re-
8. K. Soga, T. Shiono, Prog. Polym. Sci., 22 (1997) 1503.
9. A.E. Hamielec, J.B.P. Soares, Prog. Polym. Sci., 21 (1996) 651.
actions exist or are under investigation for the preparation
10. G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. Int. Ed, 38, (1999
of polymeric materials. Among these, the chemistry in the
428.
molten state has to be mentioned, this concerns the chem-
11. S.D.Ittel, L.K. Johnson, M. Brookhart, Chem. Rev., 100 (2000) 1169.
12. F. Candau, Polymerization in Microemulsions, Handbook of Microemul-
ical modification of polymers and their synthesis starting
sion Science and Technology, Marcel Dekker Inc, New-York (1999).
from monomers. Along the same line the polymerization
6.5. Metal-Matrix Composites: Challenges and Opportunities
A. Mortensen | Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland
T.W. Clyne | University of Cambridge CB2 3QZ, United Kingdom
6.5.1. Scope and Definitions
1. The “composite” approach to metallurgical processing
is the only pathway for the production of entire classes
A metal matrix composite (MMC) combines into a single
of metallic materials. Only in this way can aluminium,
material a metallic base with a reinforcing constituent,
copper, or magnesium be combined with significant
which is usually non-metallic and is commonly a ceramic.
volume fractions of carbide, oxide or nitride phases
By definition, MMCs are produced by means of processes
because, unlike iron, the solubility of carbon, nitrogen
other than conventional metal alloying. Like their poly-
or oxygen in the molten metal is (with the exception of
mer matrix counterparts, these composites are often pro-
O in Cu) far too low.
duced by combining two pre-existing constituents (e.g.
a metal and a ceramic fibre). Processes commonly used
2. The approach facilitates significant alterations in the
include powder metallurgy, diffusion bonding, liquid phase
physical properties of metallic materials. Composites
sintering, squeeze-infiltration and stir-casting. Alterna-
offer scope for exceeding the specific elastic modulus
tively, the typically high reactivity of metals at processing
value of about 26 J kg-1, which is exhibited by all the main
temperatures can be exploited to form the reinforcement
engineering metals. Composites also offer the only path-
and/or the matrix in situ, i.e. by chemical reaction within a
way for producing materials with tailored physical prop-
precursor of the composite.
erty combinations: an example is that of low thermal
expansivity combined with high thermal conductivity,
There are several reasons why MMCs have generated con-
a combination of importance for electronic packaging.
siderable interest within the materials community for near-
ly 30 years:

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