1
Coordinative Chain Transfer Polymerization
Andreia Valente,
i
,1,2,3,4
André Mortreux,
1,2,3,4
Marc Visseaux,
1,2,3,4
Philippe Zinck*
1,2,3,4
1 Univ Lille Nord de France, F-5900 Lille, France
2 ENSCL, UCCS, CCM, F-59652 Villeneuve d’Ascq, France
3 USTL, UCCS, CCM, F-59655 Villeneuve d’Ascq, France
4 CNRS, UMR8181, F-59652 Villeneuve d’Ascq, France
1. Introduction ............................................................................................................... 2
2. Coordinative chain transfer polymerization of single monomers ............................. 5
2.1. Ethylene ............................................................................................................. 5
2.2. Propylene ........................................................................................................... 9
2.3. Higher 1-alkenes .............................................................................................. 13
2.4. Styrene ............................................................................................................. 14
2.5. Conjugated dienes ............................................................................................ 17
3. Statistical Coordinative Chain Transfer co-Polymerizations (CCTcoP)................. 23
4. Chain Shuttling Polymerization (CSP) ................................................................... 31
4.1. CSP combined to Chain Walking Polymerization ........................................... 31
4.2. CSP between enantiopure catalysts in a racemic mixture ............................... 31
4.3. Chain shuttling copolymerizations .................................................................. 32
i
Current adress: Centro de Ciências Moleculares e Materiais, Faculdade de Ciências da Universidade de
Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
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1. Introduction
The development of single-site catalysts has led to great potentialities for fine-
tuning the microstructure of polyolefins and polydienes. Both regioselectivity and
stereoselectivity of the polymerization can be controlled, together with the growth of
well-defined branches onto the macromolecular backbone. Various types of
microstructures and architectures are now reachable, particularly under conditions
where a living polymerization is occurring, i.e. one molecule of catalyst leads to the
growth of only one macromolecular chain (Scheme 1a). In order notably to reduce the
consumption of highly expensive transition metal based catalysts and to control the
molecular weight of the polymer, strategies enabling the growth of several
macromolecular chains per catalyst, such as Coordinative Chain Transfer
Polymerization (CCTP), have been developed.
1-8
CCTP involves the use of a single
transition metal based catalyst and a chain transfer agent (CTA), usually in the form of a
main group metal alkyl. In this case, the growing macromolecular chain is able to
transfer from the catalyst (active species) to the chain transfer agent, which is usually
considered as a dormant species in the course of the polymerization, via transalkylation.
CCTP is thus a degenerative group transfer polymerization, i.e. a process involving a
dynamical equilibrium between propagating and dormant species (Scheme 1b). In
contrast with a classical living polymerization where each molecule of catalyst affords
the growth of a single polymer chain (Scheme 1a), chain transfer from the catalyst to the
CTA allows the growth of several polymer chains per catalyst molecule (Scheme 1b).
The transfer has to be rapid vs. propagation, reversible, and other chain termination
pathways, such as βH abstraction, must not occur, or occur in a negligible way. Narrow
molecular weight distributions are obtained, and the macromolecular chains are end-
capped with the chain transfer metal, enabling further functionalization based on the
3
chemistry of the main group metal. This can be viewed, as proposed by Gibson
2
and
Kempe
5
, as a metal complex catalyzed Aufbaureaktion.
9
If the chain transfer
efficiency
10
is high, i.e. most of the alkyl groups are involved in the transmetalation, the
polymer appears to be growing on the main-group metal alkyl. These latter reactions
involving fast and reversible transfer together with high transfer efficiencies and the
non-occurrence of other chain termination pathways (i.e. CCTP with quantitative
transfer efficiency vs. CTA) are conceptually close to controlled radical
polymerization.
11
They were given the name of Catalyzed Chain Growth (CCG).
2
Note
that this should not be confused with catalytic chain growth referring to metal-catalyzed
olefin insertion into a growing alkyl chain. CCG reactions are interesting in terms of
atom economy but also for the synthesis of block copolymers starting from the resulting
polymer. For block copolymer synthesis, the chain transfer efficiency must be
quantitative because, upon addition of the second monomer, the residual main group
metal alkyls can lead to the growth of a homopolymer chain of the second monomer,
resulting in a blend of block copolymer and homopolymer requiring purification work-
up in order to recover the block copolymer pure.
In addition to the transfer efficiency, the ratio between the chain transfer agent and
the catalyst is also a parameter of importance. Indeed, the higher this ratio and the
higher the transfer efficiency, the higher the number of chains that can be grown per
expensive catalyst molecules. This can be considered as a catalyst economy.
CCTP and CCG reactions thus afford a controlled polymerization in the absence of
termination pathways other than chain transfer to the CTA which is of particular interest
for monomers such as ethylene that are scarcely polymerizable via anionic or radical
polymerizations. The coordination of the monomer on the transition metal in the course
of the polymerization gives access to a stereoselective polymerization. This is of
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particular interest for monomers, such as olefins and dienes (e.g. 1,4-trans
stereoselective polymerization of 1,3 conjugated dienes, syndiospecific polymerization
of propylene, styrene, etc…), as the stereoselective character cannot be achieved by
classical living polymerizations (i.e. anionic and controlled radical polymerizations).
Early CCTP studies were motivated by the beneficial effect that can be expected in
terms of catalyst economy, control over the molecular weight and end-functionalization
of the resulting polymer. Studies conducted in the recent years were in turn oriented
toward catalytic systems for CCTP able to finely control the microstructure and the
architecture of the polymer, in order to access to original macromolecular enchainments
exhibiting new properties. In a cutting-edge study, Arriola and coworkers reported the
straightforward synthesis of a new class of thermoplastic elastomers.
3
Multi-block
ethylene/1-octene copolymers with sequential crystallizable (low 1-octene content) and
non-crystallizable (high 1-octene content) statistical copolymer segments were
synthesized via chain shuttling copolymerization (Scheme 2). Such an original
microstructure results from the simultaneous presence in the reactive medium of chains
growing on two different catalysts and of a chain transfer or chain shuttling agent. The
co-monomer reactivity ratios are different for the two catalysts, leading to ethylene-rich
and 1-octene-rich segments. The chains are able to growth in a sequential way on the
two different catalysts via transfer to the chain shuttling agent. This concept referred to
as Chain Shuttling Polymerization (CSP) has been highlighted in numerous perspective
articles.
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The present review will be devoted to CCTP with a focus on the factors enabling
decreased occurrence of termination pathways other than transfer to the CTA such as
βH abstraction and on the new concepts that can be derived for the control of the
microstructure and the architecture of the resulting polymers. End-functionalizations
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will not be dealt with, and interested readers are invited to consult other reviews on this
subject.
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We will discuss the CCTP of single monomers, including ethylene, propylene
and higher 1-alkenes, styrene and conjugated dienes. A particular emphasis will be
given to the microstructure. As the CCTP of ethylene has been reviewed in 2007,
5
we
will focus on the work published after this period and briefly present the main earlier
studies. We will further present the application of CCTP to statistical copolymerizations
and the resulting new concepts that have been derived.
14
The last part of this article will
be devoted to chain shuttling polymerizations, including a brief discussion of the
properties of the resulting polymers showing unprecedented microstructure and
architecture.
2. Coordinative chain transfer polymerization of single monomers
2.1. Ethylene
The CCTP of ethylene has been reviewed in 2007.
5
We thus present in this
section a brief overview of the field, together with the studies published in the period
2007-2012. The different pre-catalysts used for ethylene CCTP are presented in Figure
1. Chain growth on a main group metal was first shown to proceed via a stepwise
insertion of ethylene into the Al-C bonds at high temperature and pressure.
9
Catalyzed
versions of this reaction were proposed under smoother conditions with transition
metals and rare earths using aluminum,
15-27
zinc
2;28-31
and magnesium alkyls
1
as chain
transfer agents.
The polyethylene chain transfer reaction using a main group metal and a
transition metal based catalyst have been described in the patent literature to occur with
AlEt
3
as the chain transfer agent and activated hafnocenes
15
leading to Schulz-Flory
distributions of aluminium alkyl chains. Actinidocenes
16
gave in turn rise to Poisson
