Microstructures and Properties of Materials
Module Code: B11MS1
Responsible Person: Dr V. Arrighi
Dr V. Arrighi, Dr A. Kraft.
Week 10 Index
Theme of Week 10 : Trends in Polymer Synthesis.
The synthesis of many standard polymers by radical or step-growth polymerisation or crosslinking
reactions has already been covered in Chemistry of Materials in Year 3 and will not be repeated
here. For a summary of common polymer-forming reactions, see Background Material. We will
instead concentrate on some of the more important recent developments in polymer synthesis that
allow polymers to be made with precisely controlled chain length (molar mass) and architecture
(tacticity). These are the topics that will be covered in Week 10. Click on each page to navigate or
choose the printable pdf version of this week's lecture material.
 Anionic Polymerisation
 Living Radical Polymerisation
 Ziegler-Natta Polymerisation
 Catalysts for Polymerisation of Ethylene and Propylene
 Mechanism of Ziegler-Natta Polymerisation
 Metallocene Catalysts
 Supramolecular Polymers
 Week 10 Tutorial and Revision List
Further reading
Chapter 3.16 - 3.23, 4.10 - 4.15, 7.1 - 7.6, and 7.14 - J.M.G. Cowie and V. Arrighi, Polymers:
Chemistry and Physics of Modern Materials, 3rd edition, CRC Press (2007).
Anionic Polymerisation
Alkenes bearing anion-stabilising substituents (e.g., phenyl, vinyl, ester, nitro, cyano
groups) can react with carbanions to form relatively stable anionic intermediates. The
addition of anionic nucleophiles is reminiscent of Michael additions in Organic
Chemistry particularly when acrylates or methacrylates are used as monomers for
anionic polymerisation. Propagation can lead to polymerisation.
A classical example is the polymerisation of styrene with butyllithium as initiator.
Addition of BuLi to styrene monomer gives 1-phenylhexyllithium, a benzylic carbanion
that reacts with another styrene molecule in the next step and so on.
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Another common initiator is the radical anion of naphthalene, sodium naphthalide. Its
cheapness makes it acceptable for industrial applications. The radical anion of
naphthalene is stable in certain non-polar solvents. On reaction with styrene, electron
transfer occurs, leading to the formation of naphthalene and a styrene-derived radical
anion. Two radical anions dimerise to form a dianion, which can then propagate in two
directions.
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Metals, such as sodium or lithium, can also act as initiators. The efficiency of an
electron transfer to the monomer is, however, rather low. Sodium (Na) has been used
historically for the production of BuNa S, a synthetic rubber made by anionic
polymerisation of butadiene (Bu) and styrene (S). The two monomers were present in
solution at the same time, thus producing a statistical copolymer. (Nowadays this
process has been completely replaced by the more easily accomplished radical
polymerisation of the two monomers in an aqueous emulsion that is stabilised by the
addition of surfactants.)
The initiation step in an anionic polymerisation is fast compared to propagation. As a
result of the high reactivity of carbanions, oxygen and moisture or any other protic or
carbanion-sensitive impurities have to be rigorously excluded. If care is taken during an
anionic polymerisation, termination is virtually absent and, for this reason, anionic
polymerisation is also called a 'living' polymerisation. Whereas in radical
polymerisations two growing polymer chains can easily react with each other (by
recombination or disproportionation) and terminate chain growth, such a step is not
possible in ionic polymerisation because of electrostatic repulsion of equally charged
chain ends. Chain transfer is just as unlikely, since this would involve the transfer of a
hydride (H–) from a growing chain to a monomer molecule or the elimination of lithium
hydride (LiH) — both reactions are energetically unfavourable. The polymerisation is
'living', but not immortal. It is eventually quenched by reaction with a terminating agent,
either deliberately or accidentally.
Carbanions are highly sensitive against moisture, and anionic polymerisations have to be
carried out under nitrogen in thoroughly dried solvents and glassware. Even then, there
is almost always some residual moisture left with which the initiator will react first.
Below you see a typical set-up for an anionic polymerisation. The Schlenk tube contains
a solution of 2,3-dimethylbutadiene (the monomer) in cyclohexane. The initiator is secbutyl lithium. It is added dropwise with a gastight syringe through a septum cap to the
initially colourless monomer solution. Note the red colour of the sec-butyl lithium
solution which is due to a small amount of 1,1-diphenylhexyl lithium, a resonancestabilised carbanion that does not polymerise but makes it easier to see when all the
moisture in the monomer solution has finally reacted. Once all the water has been
"titrated" away, a light yellow colour persists. Now, the calculated amount of initiator
solution is added and the anionic polymerisation can finally start.
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You might wish to have a look at the videoclip as it shows how it looks like when
residual moisture in the monomer solution is titrated in a Schlenk tube.
(1086 kB)
(4658 kB)
This video can be viewed with Windows Media Player. If you are unable to view the
video with existing software, try to download Microsoft Windows Media Player.
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There are three important points about anionic polymerisations to remember.
1) A closer look at the kinetics of anionic polymerisation reveals that conversion and
degree of polymerisation increase linearly with time. If all the monomer is allowed to
react, the degree of polymerisation xn in an anionic polymerisation is adjustable and
depends just on two concentrations. It is identical to the monomer (M)-to-initiator (I)
ratio if an initiator such as BuLi is used in THF as solvent.
Anionic polymerisations are therefore ideal for producing standards for gel-permeation
chromatography (GPC). Owing to the extreme care needed in their preparation, GPC
standards made by anionic polymerisation are quite expensive, and 1 g of a typical
polystyrene standard costs about £40.
Questions:
1) What will be xn if the radical anion of naphthalene is used for initiating an anionic
polymerisation?
2) In a radical polymerisation, xn is proportional to the kinetic chain length. How does it depend on
the concentrations of monomer and initiator? click for hint
2) Anionic polymerisations produce polymers with a very narrow molar mass
distribution.
All polymers grow at approximately the same rate, and the polydispersity (M w/Mn)
approaches 1.0. Well, almost. In the best case it can be as low as 1.02, but this requires
Schlenk line or glove box techniques, usually involving a thoroughly flame-dried and
sealed all-glass apparatus. Even without such specialised equipment, polydispersities of
about 1.1 can be obtained if only a little care is taken.
For comparison, the polydispersity of a typical condensation polymerisation is 2.0. The
dispersity of a radical polymerisation is, in theory, also 2.0 (or 1.5 if termination occurs
exclusively through disproportionation). In practice, when radical polymerisations are
carried out at high concentrations (in bulk), polydispersities are often much larger owing
to chain transfer reactions and the formation of branched polymer chains. On the other
hand, when a polymer is reprecipitated into a non-solvent, the resulting removal of lower
molecular weight material and fractionation leads to a narrowing of the molar mass
distribution (to about 1.5).
Please note that radical polymerisations give rise to a Schulz-Flory distribution of the
molar mass, which is very broad and has a characteristic tail towards higher molar
masses. In contrast, the molar mass distribution that is achieved by an anionic
polymerisation is a much narrower and symmetrical Poisson distribution.
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Question::
Give an example of a polymer with polydispersity of exactly 1.00.
To see how the polydispersity affects the molecular weight distribution curve, click here.
Open the Excel spreadsheet where you can adjust polydispersity (PDI) and/or degree of
polymerisation (xn) using the scroll bars. Notice how the distribution curve shifts on
varying xn and broadens with increasing PDI.
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3) Although the polymerisation stops when all monomer has been consumed, the
polymers still have a 'living' chain end. It is therefore possible to add a variety of
electrophiles that can introduce a functional end group:
,-Disubstituted polymers with defined functional end groups at either end are useful
for the preparation of network polymers. They often serve as macromonomers in a
condensation polymerisation. Quite a number of low-molar-mass polymers with defined
end groups at both ends are commercially available; they are called telechelic polymers
or telomers.
Provided that the polymeric carbanion is sufficiently reactive, it can induce
polymerisation of other monomers as well. A growing polystyrene anion PS– can, for
example, induce the polymerisation of methyl methacrylate (MMA). The result will be a
block copolymer, in this case a diblock copolymer consisting of a block of polystyrene
and a block of PMMA, both joined end-to-end. The increasing importance and interest in
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block copolymers arises from their unique properties in solution and in the solid state.
As a consequence of their molecular structure, blocks are usually incompatible and have
a tendency to segregate. For example, a poly(ethylene oxide)–poly(styrene) diblock
copolymer will dissolve in polar as well as non-polar solvents. Polar solvents are
compatible with poly(ethylene oxide) only, whereas non-polar solvents will solvate both
blocks. The polymer can behave like an amphiphile (that is, a macromolecular
surfactant) in water, which is only compatible with the poly(ethylene oxide) segment.
Similarly, separate domains tend to form in the solid. Phase-separated polymers often
show unusual properties. A poly(styrene-block-butadiene-block-styrene) triblock
copolymer with a polybutadiene segment to which polystyrene blocks are attached at
either end is a typical thermoplastic elastomer.
Question:
3) How would you synthesise a poly(styrene-block-butadiene-block-styrene) triblock copolymer
(SBS), which you encountered before in the discussion of thermoplastic elastomers?
Despite its restrictions, anionic polymerisations have been carried out on an industrial
scale. Few applications require precise molar masses (namely, GPC standards), although
this might change as a much more convenient controlled polymerisation technique (see
ATRP in the next chapter) has been introduced about 10 years ago. However, anionic
polymerisation is an excellent method for the preparation of block copolymers, and it is
one of the few polymerisation techniques that allow a good control over the polymer
structure.
The kinetics of an anionic polymerisation depend both on the cation and the solvent.
Several equilibria have to be taken into account. Species with covalent carbon-metal
bonds (RLi) are in equilibrium with contact ion pairs (R–Li+), which again may be
solvated by a good solvent. Although contact ion pairs can contribute to propagation, the
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reaction rate is quite slow. Free carbanions (R–) are much more reactive but their
concentration is very low in THF. The kinetics in non-polar solvents (hexane, diesel oil)
is even more complicated. Whereas BuLi exists as a dimer in THF at –40 °C, it forms a
hexameric aggregate in benzene or cyclohexane. Only non-aggregated ion pairs can
propagate. While the polymerisation may still occur without termination in non-polar
solvents, the variety of propagating species broadens molar mass distribution
significantly.
Controlled Radical Polymerisation
Radical polymerisations are important for the mass production of a variety of
commodity polymers. However, their molar mass distributions are difficult to control
because of fast and irreversible termination steps. Several ways have been suggested
during the last years to make radical polymerisations quasi-'living', that is to eliminate
the termination step, if not completely, then at least almost so. Under certain conditions a
radical polymerisation can thus provide a polymer with a predetermined molar mass and
a very narrow molar mass distribution, as well as lead to a block copolymer after one
monomer has been replaced by another. Reducing the likelyhood of a termination step
means that recombination and disproportionation reactions between two growing chains
(R•) must be avoided. The basic idea is to make the steady-state concentration of active
radicals very low in order to minimise unwanted chain termination. Although this does
not exclude chain termination or chain transfer from happening at all, at least their
propability becomes much, much lower than in an ordinary radical polymerisation. The
trick is to use a 'dormant' radical species that makes it possible to keep radical
concentrations [R•] at such low levels so that, since the rate of termination scales with
the square of [R•], chain termination events take place very rarely. The outcome is called
a controlled radical polymerisation.
Questions :
1) What is the rate of termination in a radical polymerisation? click for hint
2) What is the typical lifetime of a growing radical in a radical polymerisation that is not
'controlled'?
Three important routes make use of this 'dormant' radical approach. The first method is
called nitroxide-mediated polymerisation. It was developed by M. K. Georges and C. J.
Hawker and uses TEMPO — a relatively stable organic nitroxyl radical that acts as a
radical trap and readily combines with radicals (other than TEMPO molecules) in
solution, such as the chain end of a growing polymer. The initial product with styrene is
a simple compound that can be isolated in up to 42% yield after chromatography.
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The TEMPO adduct is stable at temperatures below 100 °C and can serve as initiator for
polymerisation. When heated with excess styrene to 130 °C, the weakest bond is cleaved
again, producing TEMPO and a benzyl radical which then adds onto one or more styrene
monomers. Chain propagation rapidly stops as soon as TEMPO recombines with the
growing polymer chain. Again and again, the TEMPO end group splits off and allows
more monomer molecules to be inserted before recombination takes place once more.
Radical chains grow slowly. Owing to the low radical concentration, termination is
suppressed and thus polydispersity reaches levels as low as 1.1–1.3, not as good as in a
'living' anionic polymerisation but still considerably better than in ordinary free radical
polymerisations.
A slightly different approach has been suggested by K. Matyaszewski who made use of a
Cu(I)/Cu(II) redox system to generate the dormant radical species. For practical reasons
the copper catalyst is stabilised by a bipyridine (bipy) or a related bidentate ligand, some
times with additional substituents R that improve solubility. A complex of CuCl (which
for a change should not be too pure) and bipy reacts with an initiator that looks like an
HCl adduct onto the monomer.
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Abstraction of a chlorine atom by the copper catalyst produces a benzyl radical. At the
same time, Cu(I) is oxidised to Cu(II). The radical then adds to styrene. Soon afterwards,
the growing polymer reacts with the chloro-Cu(II) species. Transfer of a Cl• atom yields
a small polymer with a benzylic chloride as end group and the original Cu(I) bipy
complex. Because of this continuous transfer of halogen atoms, the polymerisation is
named atom-transfer radical polymerisation or ATRP.
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There is a dynamic equilibrium between growing and dormant polymer chains. Most of
the time the equilibrium is on the side of the dormant chains, and chain propagation
occurs only occasionally. Once again, the probability for bimolecular chain termination
is minimised and a polystyrene with a narrow polydispersity (ca. 1.05) is obtained. Like
in a 'living' polymerisation, the molar mass rises linearly with polymerisation time.
The polymerisation of styrene requires heating to 130 °C under inert atmosphere when a
CuCl/chloro initiator system is chosen. Despite the high temperature, thermal initiation
is less than 1%. Nowadays, other chelating ligands such as the macrocylic Cylam B, the
multidentate Me6Tren and the commercially available N,N,N',N",N"pentamethyldiethylenetriamine (PMDETA) are increasingly used and, because they
make the copper(I) catalyst much more reactive, the polymerisation can proceed at lower
temperature (20 – 60 °C).
The ability to control the molar mass comes, however, at a price. ATRP polymerisations
tend to be rather slow. It takes about 4 hours to add 100 styrene units onto a growing
polymer chain and reach an Mn of about 10000 g/mol. Polymerisation of methyl
methacrylate (MMA) is considerably faster and can be carried out at a lower temperature
with an MMA-like -bromoisobutyrate initiator. Note that a compromise is needed since
polydispersity increases again if the polymerisation becomes too fast.
Even block copolymers can be prepared as long as the two monomers are not too
different, such as with MMA and a related alkyl methacrylate. Nitroxide-mediated
polymerisation, ATRP and Reversible Addition Fragmentation Chain Transfer (RAFT)
polymerisation (another method that requires the addition of a dithioester as a chaintransfer agent, see below) are all controlled radical polymerisations that open the
possibility to make block copolymers of defined length and composition without the
hassle of an anionic polymerisation. These three controlled radical polymerisations are
being increasingly used nowadays to graft polymer chains in a controlled manner from a
polymer backbone (leading to polymer brushes) or to graft polymer chains from surfaces
such as glass, silicon or silica particles.
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Ziegler-Natta Polymerisation
In 1952, Karl Ziegler found that ethylene H2C=CH2 can be polymerised with certain
organometallic catalysts at room temperature and ambient pressure. This polymerisation
produces a linear polyethylene (HDPE).
At the time, the only known way for polymerising ethylene required high pressure
(1400–3500 bar) and temperature (130–330 °C). The high-pressure polymerisation of
ethylene was originally developed by ICI. It is initiated by the presence of a small
amount of oxygen that, although not your typical radical initiator, produces peroxide
intermediates which decompose under the reaction conditions and start a radical
polymerisation. The polyethylene obtained by this process has lots of small and large
branches (owing to intra- and intermolecular chain transfer reactions) with a lower
density than a linear polyethylene (LDPE).
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Question:
The critical point of ethylene is observed at 9.9 °C/50 bar. What does this mean when you compare
it with the polymerisation conditions?
In 2003, the amount of polymers produced in the United States alone were as follows (in
thousands of metric tons):
At a rough estimate, the amounts of polymer produced in Europe and Asia together are
similar to those given above (Figures from Chemical & Engineering News, 11 July 2005,
pp. 67–76).
Catalysts for Polymerisation of Ethylene and Propylene
A typical Ziegler catalyst system consists of
• a group I to III metal alkyl, such as AlEt3, AlEt2Cl, Al(OEt)Et2, AlH(iBu)2, and
• a group IV to VIII transition metal halide, such as TiCl4, TiCl3, Ti(OBu)4, VOCl3, VCl4,
ZrCl4, NiCl2, WCl6, MnCl2, mixed together in
• a thoroughly dried non-polar solvent, such as heptane or benzene. It is, however, more
economic to use the monomer itself (e.g. propene) as the solvent or no solvent at all (a
gas phase polymerisation is preferred for ethylene).
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Most commonly used are triethylaluminium (AlEt3) and titanium tetrachloride (TiCl4).
When these two liquids are combined, a solid precipitates that catalyses the
polymerisation of ethylene. After polymerisation, residual catalyst is destroyed by the
addition of methanol containing a small amount of HCl.
A year after Ziegler's discovery, Giulio Natta found that a related catalyst made from
TiCl3 and AlEt3 polymerises propylene (propene), CH3-CH=CH2, to produce a highly
isotactic polypropylene (PP). Isotactic PP has become one of the most important
thermoplastics since, and it has recently overtaken poly(vinyl chloride) in production
figures, second only after polyethylene.
Ziegler-Natta polymerisation provides the only way for polymerising propylene. The
first-generation catalysts produced isotactic PP together with a few percent of atactic PP.
Atactic PP is a virtually useless waxy solid. It is also much more soluble than isotactic
PP and can therefore be easily extracted with hot heptane.
In contrast, isotactic PP is a crystalline polymer with materials properties that make it far
more attractive for a wide range of applications. The latest generations of heterogeneous
Ziegler catalysts use additional modifiers and are highly stereoselective so that there is
no longer any need to extract atactic PP impurities. Other recent developments include
the formulation of single-site catalysts on solid supports (frequently MgCl2 or silica).
Mechanism of Ziegler-Natta Polymerisation
The exact nature of the catalyst and the details of the mechanism has long been disputed.
One of the more favoured mechanisms that explains the stereochemistry of the polymer
produced was proposed by Cossee. The Cossee mechanism suggests that Ti(III) is
initially alkylated by reaction with AlEt3. Reactions are confined to the crystal surface. A
vacant (cationic) site is generated to which the monomer, propene, coordinates as a complex. In the next step, propene inserts into the existing Ti–C bond. The polymer
chain then migrates back to its original position and regenerates the vacant site before
the catalytic cycle starts again. The growing polymer chain remains attached to the
transition metal and degrees of polymerisation can become very high. In practice, a
small amount of hydrogen is frequently added to the monomer; it acts as a chain transfer
agent, thereby limiting the molar mass of the final polymer.
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Question:
What is a chain transfer agent?
In the case of propene, the orientation of the incoming monomer is always the same. The
smaller =CH2 group points towards the lattice. For steric reasons the -CH3 group prefers
to avoid congestion at the crystal surface; it orients away from the crystal. This
determines the configuration of the monomer during the complexing stage and remains
the same throughout the polymerisation. A highly stereoregular (isotactic) PP is
produced. We will see at another example later on that it is the stereochemistry of the
catalyst that ultimately determines the stereochemistry of the polymer.
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Metallocene Catalysts
During the last two decades, research by Brintzinger, Kaminsky, Ewen, Brookhart and
many other polymer chemists as well as organometallic chemists has led to the
development of soluble (= homogeneous) polymerisation catalysts. A major
breakthrough in polyolefin synthesis was made when certain metallocenes were
identified as highly active, single-site, soluble catalysts for the polymerisation of
ethylene and, more importantly, the stereoregular polymerisation of propylene.
Metallocenes are 'sandwich' complexes of transition metals with two cyclopentadienyl
ligands. As a reminder: cyclopentadiene is quite acidic, and deprotonation with even
such a weak base as NaOH gives the cyclopentadienyl anion, an aromatic -system with
6 electrons.
The existence of two stable oxidation states for titanium, Ti(IV) and Ti(III), always leads
to problems. That is why the homologue zirconium (and less often hafnium) is generally
preferred in metallocene catalysts since Zr(IV) is stable and accidental reduction of the
catalyst can be avoided.
Efficient polymerisation catalysts require a more sterically demanding cyclopentadienyl
ligand (indenyl, for example). Two such ligands are linked by a –CH2– or –Si(CH3)2–
bridge in between, which keeps the two cyclopentadienyl/indenyl groups from rotating
so that several stereochemical isomers are possible. The meso form and the racemate
(rac) can be separated by fractional crystallisation. The bridged complex is called an
ansa-metallocene. In addition, the bridge controls the 'bite' angle which determines the
reactivity of the catalyst. Additional substituents influence both the rate of
polymerisation and stereocontrol. Catalysts have to carefully optimised to minimise side
reactions that lead to chain transfer (usually -hydride elimination) and thus limit the
molar mass.
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Question:
1) What is the difference between the two rac complexes, the one in the 3-D model and the one
drawn above?
Zirconocene catalysts have to be activated. This is generally done by adding a large
(around 1000-fold) excess of methylaluminoxane (MAO), an inorganic polymer
obtained by partial hydrolysis of AlMe3. MAO reacts with the zirconocene dichloride, as
well as any inadvertent water that may be present. The cationic and chiral intermediate
obtained by alkylation of the zirconocene with MAO polymerises both propylene and
ethylene. Propylene polymerisation is more interesting (from an academic and
commercial point of view) since the zirconocene complex has a pronounced effect over
the stereoregularity of the polypropylene.
Propene forms a -complex with the cationic metallocene. The steric bulk of the top
indenyl ligand is responsible that propylene forms a -complex in which the methyl
group consequently (almost) always points away from the indenyl ligand. In the next
step, propylene inserts into the existing Zr–carbon bond and regenerates a vacant site.
Another propylene monomer then coordinates to the cationic zirconium, with the
monomer's methyl group now pointing up, again away from the interfering indenyl rest
(this time underneath it). The cycle repeats itself and the polymer chain grows.
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It is important to realise that the stereochemistry of the polymer does not depend on the
stereochemistry at the end of the polymer chain (chain-end control) but rather on the
difference in bulkiness of the ligands on the metal (enantiomorphic site control).
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The two alternatives can be differentiated by a closer look at the mistakes that happen
when a single monomer unit is incorporated the wrong way into the growing polymer
chain. By analysing a high-field 13C NMR spectrum of the polymer up to ten signals are
found for the methyl groups of a polypropylene (called 'pentade analysis'). A typical
mistake in isotactic PP made by catalyst 1 will look like (a) rather than (b). It is therefore
the stereochemistry at the metal catalyst site (NOT the chain end of the polymer) that
controls the stereochemistry of the polymer chain.
The nice thing about metallocene catalysts is that you can make, for example,
syndiotactic polypropylene by changing to a catalyst with Cs symmetry, such as catalyst
2. This is no longer a chiral metallocene. Owing to its symmetry plane, this catalyst is
actually used in the meso form. So, meso-2 makes syndiotactic PP. With catalyst 2, the
methyl group of a coordinated propylene will always point up and away from the bulky
fluorene, which differs from the bis(indenyl) catalyst at every second monomer addition.
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The properties of isotactic, syndiotactic and atactic polypropylene differ in many
respects:
Property
density [g/cm3]
melting point [°C]
solubility in hydrocarbons
at 20 °C
yield strength
Isotactic
0.92–0.94
165
Syndiotactic
0.89–0.91
135
Atactic
0.85–0.90
–
none
medium
high
high
medium
very low
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Now, what happens if the bridge in an ansa-metallocene is omitted? In this case, the
cyclopentadienyl rings will usually be able to rotate freely and fast. Introduction of
bulky substituents can, however, hamper rotation and slow it down considerably. It may
be suspected that such a zirconocene catalyst invariably yields an atactic polypropylene,
yet this does not always happen. When, for example, bis(2-phenylindenyl)zirconium
dichloride is activated with methylaluminoxane, the polymerisation catalyst switches
between two coordination geometries (3) and (3'). However, the phenyl substituent on
the indenyl ligand slows down the rate of ligand rotation so that isomerisation between
(3) and (3') becomes much slower than monomer insertion; it nevertheless happens
many times during the time needed to build one polymer chain. Note that the catalyst 3
like catalyst 1 is chiral and produces a block of isotactic polypropylene. On the other
hand, catalyst 3' is achiral (it has still a symmetry plane left) and generates a block of
atactic polypropylene. The oscillating stereocontrol hence leads to a polypropylene with
alternating isotactic–atactic stereoblocks.
Incidentally, the resulting polymer is a thermoplastic elastomer. It owes its elasticity to
atactic blocks and its resistance to fracture to the crystallised blocks of isotactic PP.
In recent years new types of non-metallocene catalysts have been discovered that are all
able to polymerise ethylene and/or 1-alkenes with (almost) equal ease. The most
prominent are metallocene analogues where one of the cyclopentadienyl ligands is
missing. They are called ansa-monocyclopentadienyl-amido or 'half-sandwich' Ti(IV)
catalysts or constrained geometry catalysts. These catalysts are quite good at
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copolymerising ethylene with a 1-alkene (such as 1-octene), which provides an
interesting route for making LLDPE.
The Brookhart diimido catalysts work instead with late (group 10) transition metals,
such as Ni(II) and Pd(II). These catalysts give rise to unique microstructures. Given the
right metal and ligand, polymerisations sometimes become even 'living'. Other catalysts
show a propensity to work also with functionalised vinyl monomers such as acrylic
esters, which are 'deadly' to metallocenes or Ziegler catalysts. Typical for half-sandwich
and Brookhart catalysts is the use of a non-coordinating and bulky (and expensive)
tetraarylborate counteranion, which is usually fluorinated to improve solubility.
Finding a good catalyst that is cheap, produces high-molar-mass polymer with little
defects, gives good incorporation of a comonomer, and excellent control over the
microstructure is not an easy task. A lot of effort goes into catalyst research because of
the commercial importance of polyolefins. The properties of polyolefins can be adjusted
through tacticity, crystallinity, comonomer content, size of branches and degree of
branching. Moreover, polyolefins are cheap, which is always a commercial incentive,
and they pose few(er) problems when it comes to recycling or waste disposal. It is
therefore not surprising that polyolefins are starting to replace many other polymers.
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Supramolecular Polymers
Supramolecular interactions between polymer chains play an important rôle in Polymer
Chemistry and are frequently responsible for special materials properties. To take an
example, the strength of nylon fibres is due to hydrogen bonds between polymer chains.
The number of hydrogen bonds is maximised when the polymer chains are stretchaligned and annealed. It is therefore not surprising that nylon fibres are always drawn to
optimise their strength. These inter-chain H-bonds become even more dominant in
aromatic polyamides, such as Kevlar, the material used for making body armour (bulletproof vests).
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Body armour requires high modulus fibre materials
such as Kevlar or ultra-high-molecular-weight polyethylene (UHMWPE)
Hydrogen bonds can also link monomer units together. When a polymer is formed
through reversible hydrogen bonds rather than covalent bonds, it is called a
supramolecular polymer. One particularly good example has been reported by E. W.
Meijer and is based on the strong association of 2-ureido-4-pyrimidones. These
compounds form strong hydrogen-bonded dimers in the solid and in many solvents. The
total of four hydrogen bonds and the self-complementary array of hydrogen bond donors
and acceptors ensures that the equilibrium is almost completely on the side of the dimer.
The intramolecular H-bond has the beneficial effect that the urea group is coplanar with
the adjacent heterocyclic pyrimidone ring.
A compound with two such groups similarly hydrogen-bonds to other molecules at each
end and forms a long (polymeric) chain. Strong association leads to a polymer with a
number-average molar mass of over 105 g/mol in chloroform at room temperature. The
exact value of the polymerisation degree will, however, depend critically on
temperature, concentration and the amount of monofunctional impurities present in
solution. Just as in an ordinary step-growth polymerisation a high polymerisation degree
is only achieved when a large number of monomer molecules are linked together.
Question
What is Carothers equation? click for hint
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