1 Introduction
The Ziegler and Natta discovery of catalysts for ethene polymerization (Ziegler 1953)
and stereospecific propene polymerizations (Natta 1954), awarded with the 1963 Nobel
Prize in Chemistry, generated searches for catalysts and cocatalysts for the polymerization of other olefins, of which the most interesting are the cyclic olefins. In the early
1960s, cycloolefins such as norbornene could be polymerized by using heterogeneous
systems based on titanium, tungsten, or molybdenum halides and strong Lewis acidic
cocatalysts [1–4]. The unsaturated 1,3-dimethylenecyclopentane repeating units in the
polymer chain suggested that polymerization occurred via ring opening. Indeed,
conventional heterogeneous Ziegler–Natta catalysts generally yield cycloolefin
polymers containing both addition and ring-opening metathesis polymerized
(ROMP) units [3]. Searches for catalysts for ROMP [5, 6] led to the development of
commercial products based on polyalkenamers from cyclopentene, cyclooctene,
norbornene, and dicyclopentadiene and more recently to “well-defined” catalysts
based on a range of metals including titanium, tantalum, tungsten, and molybdenum
thanks to advances in this area, especially by Grubbs and Schrock (awarded with the
2005 Nobel Prize in Chemistry jointly with Chauvin) [7–12].
After Kaminsky, Brintzinger, and Ewen discovered homogeneous metallocene/
methylaluminoxane (MAO) catalysts for stereospecific α-olefin polymerization (for
reviews on olefin polymerization, see [13–21]), the first report [22, 23] on addition
cycloolefin polymerization without ROMP appeared. This stimulated a great interest
in these polymers and in catalysts for cycloolefin polymerization (Fig. 1).
Cycloolefins such as cyclopentene, cyclooctene, and norbornene can be polymerized
via addition (Fig. 2). Polycycloolefins by metallocenes are difficult to process due to
their high melting points and their low solubility in common organic solvents.
However, metallocenes allow the synthesis of cyclic olefin copolymers (COC),
especially of cyclopentene and norbornene with ethene or propene, which represent
a new class of thermoplastic amorphous materials (Scheme 1) [24, 25].
There is tremendous interest in cycloolefin homo-[26] and copolymers [24, 25]
because of the easy availability of the monomers and interesting polymer properties.
Amorphous cycloolefin copolymers, namely norbornene with ethene, with excellent
transparency, high refractive index, and variable glass transition temperatures are
industrially produced (e.g., Topas). Polymers with cycloaliphatic repeating units
display good thermomechanical properties, high optical clarity, and low dielectric
constants and are suited for microelectronic and optical applications. Soluble
saturated cycloaliphatic homopolymers can now be obtained using late transition
metal catalysts (palladium, nickel, and cobalt) in high yield.
An overview of the state of the art on cycloolefin copolymerization is given here.
The driving force influencing the reactivity in cycloolefin addition polymerization
is both the ring strain of the cycloolefin and the non-planarity of the reacting double
bond. The possibility or not of undergoing β-hydrogen elimination, which leads to
isomerization and chain termination, has consequences on polymer structure and
molar mass [24, 25].
118
L. Boggioni and I. Tritto
The Ziegler and Natta discovery of catalysts for ethene polymerization (Ziegler 1953)
and stereospecific propene polymerizations (Natta 1954), awarded with the 1963 Nobel
Prize in Chemistry, generated searches for catalysts and cocatalysts for the polymerization of other olefins, of which the most interesting are the cyclic olefins. In the early
1960s, cycloolefins such as norbornene could be polymerized by using heterogeneous
systems based on titanium, tungsten, or molybdenum halides and strong Lewis acidic
cocatalysts [1–4]. The unsaturated 1,3-dimethylenecyclopentane repeating units in the
polymer chain suggested that polymerization occurred via ring opening. Indeed,
conventional heterogeneous Ziegler–Natta catalysts generally yield cycloolefin
polymers containing both addition and ring-opening metathesis polymerized
(ROMP) units [3]. Searches for catalysts for ROMP [5, 6] led to the development of
commercial products based on polyalkenamers from cyclopentene, cyclooctene,
norbornene, and dicyclopentadiene and more recently to “well-defined” catalysts
based on a range of metals including titanium, tantalum, tungsten, and molybdenum
thanks to advances in this area, especially by Grubbs and Schrock (awarded with the
2005 Nobel Prize in Chemistry jointly with Chauvin) [7–12].
After Kaminsky, Brintzinger, and Ewen discovered homogeneous metallocene/
methylaluminoxane (MAO) catalysts for stereospecific α-olefin polymerization (for
reviews on olefin polymerization, see [13–21]), the first report [22, 23] on addition
cycloolefin polymerization without ROMP appeared. This stimulated a great interest
in these polymers and in catalysts for cycloolefin polymerization (Fig. 1).
Cycloolefins such as cyclopentene, cyclooctene, and norbornene can be polymerized
via addition (Fig. 2). Polycycloolefins by metallocenes are difficult to process due to
their high melting points and their low solubility in common organic solvents.
However, metallocenes allow the synthesis of cyclic olefin copolymers (COC),
especially of cyclopentene and norbornene with ethene or propene, which represent
a new class of thermoplastic amorphous materials (Scheme 1) [24, 25].
There is tremendous interest in cycloolefin homo-[26] and copolymers [24, 25]
because of the easy availability of the monomers and interesting polymer properties.
Amorphous cycloolefin copolymers, namely norbornene with ethene, with excellent
transparency, high refractive index, and variable glass transition temperatures are
industrially produced (e.g., Topas). Polymers with cycloaliphatic repeating units
display good thermomechanical properties, high optical clarity, and low dielectric
constants and are suited for microelectronic and optical applications. Soluble
saturated cycloaliphatic homopolymers can now be obtained using late transition
metal catalysts (palladium, nickel, and cobalt) in high yield.
An overview of the state of the art on cycloolefin copolymerization is given here.
The driving force influencing the reactivity in cycloolefin addition polymerization
is both the ring strain of the cycloolefin and the non-planarity of the reacting double
bond. The possibility or not of undergoing β-hydrogen elimination, which leads to
isomerization and chain termination, has consequences on polymer structure and
molar mass [24, 25].
118
L. Boggioni and I. Tritto