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1
Metathesis Polymers
Polymers using the ring opening metathesis polymerization (ROMP) technique were first obtained at 1960 by Eleuterio (1,2). The patents deal with the polymerization of bicyclo[2.2.1]heptene-2, i.e., norbornene using a molybdenum catalyst dispersed on alumina.
The polymer was found to contain double bonds in trans and cis-configuration in considerable amounts. The mechanism of polymerization has been described as shown in Figure 1.1.
Metal-catalyzed olefin metathesis had an enormous impact on organic synthesis in general. Extensive research on mechanistic aspects (3,4) and the development of catalysts has been performed, which culminated in the award of the Nobel Prize for Chemistry in 2005 to Chauvin, Grubbs and Schrock.
Figure 1.1: Metathesis Polymerization of Norbornene and Cyclopentene
Table 1.1: Monomers for Metathesis Polymerization
| Monomers | References |
| Cyclopentene | |
| 1,5-Cyclooctadiene | |
| Norbornene | (1,2) |
| 1,4-Dihydro-1,4-methanonaphthalene | |
| Norbornene 2-ethylhexyl carboxylate | (5) |
| Norbornene isobornyl carboxylate | (5) |
| Norbornene phenoxyethyl carboxylate | (5) |
| Dodecylenedinorbornene dicarboxyimide | (5) |
| exo, exo-N,N′-Propylene-di-(norbomene-5,6-dicarboxyimide | (5) |
| 8-Methyltetracyclo[4.4.0.12.8.17.10]dodeca-3-ene | (6) |
| Dicyclopentadiene | (6) |
1.1 Monomers
Cyclopentene is readily available as a byproduct in the ethylene production. Norbornene 2-ethylhexyl carboxylate is obtained by the Diels-Alder reaction of 2-ethylhexyl acrylate with cyclopentadiene (5). Norbornene isobornyl carboxylate, norbornene phenoxyethyl carboxylate, and other related monomers are synthesized according to the same route. Polymers obtained from these esters exhibit excellent properties in terms of controlling the crosslinking density, the associated product modulus, and the glass transition temperature (Tg), thus allowing tailoring the properties of elastomers, plastics and composites. Other suitable monomers are summarized in Table 1.1 and sketched in Figure 1.2.
1.2 Polymerization and Fabrication
The monomers dealt with can be polymerized by various mechanisms, not only by ROMP. For example, a rapid polymerization of norbornadiene occurs using a homogeneous catalytic system consisting of nickel acetylacetonate or a nickel-phosphine complex, such as nickel bis-(tri-n-butylphosphine) dichloride (NiCl2 (TBP)2) or nickel bis-(tricyclohexylphosphine) dichloride (NiCl2 (TBP)2). Nickel acetylacetonate as catalyst is known to initiate rather a classical vinyl polymerization (7). The classical vinyl polymerization of cyclic monomer deserves much less attention in the literature, nevertheless there is a big variety of catalysts described (7).
Figure 1.2: Monomers used for Metathesis Polymers
Figure 1.3: Difference Between Vinyl Polymerization and Ring Opening Metathesis Polymerization (7)
By the way, the intended use of this polymer is as a solid high energy fuel (8). The difference between ordinary vinyl polymerization and ring opening metathesis polymerization is shown in Figure 1.3.
1.2.1 Metathesis Reaction
The metathesis reaction consists of a movement of double bonds between different molecules, as shown in Figure 1.4. Thus, the metathesis reaction can be addressed as a transalkylideneation reaction. The cleavage of the carbon-carbon double bonds was established using isotopic labelled compounds that were subjected to ozonolysis after reaction (9).
Clearly, if the radicals R1 and R4 are connected via a carbon chain, a longer chain will be formed, resulting consecutively in the formation of macromolecular structures. For this reason, this type of polymerization is also called ring opening polymerization. The polymeric structures contain double bonds in the main chain. This allows classical vulcanization processes with sulphur. Since the reaction is reversible, the metathesis process has been used to synthesize degradable polymers with vinyl groups in the backbone. In this way, the structure of crosslinked rubbers has been elucidated.
Figure 1.4: Scheme of Metathesis Reaction
Table 1.2: Types of Metathesis Reactions (10)
| Term | Acronym |
| Ring opening metatheses polymerization | ROMP |
| Living ring opening metatheses polymerization | LROMP (11,12) |
| Ring closing metathesis | RCM |
| Acyclic diene metathesis polymerization | ADMET |
| Ring opening metathesis | ROM |
| Cross-metathesis | CM or XMET |
The mechanism of metathesis is used in several variants, either to polymerize, degrade, etc. The various reaction types are summarized in Table 1.2. The metathesis reaction is catalyzed by metalcarbene complexes. The mechanism, exemplified with cyclopentene is shown in Figure 1.5. In the first step, the complex reacts with a monomer to regenerate the carbon metal double bond. This double bond is able to react further with another monomer thus increasing the size of the molecule.
If the metathesis polymerization is performed in solution, the preferred solvents are methylene chloride or chlorobenzene. Preferably, the solvent is aprotic in order to avoid ionic side reactions. The molecular weight is controlled by the addition of an acyclic olefin, such as 1-butene (13).
The polymerization reaction can be quenched by the addition of alcohols, amines or carboxylic acids, such as ethanol, tert-butyl phenol, diethylamine, acetic acid. The polymerization reaction is an equilibrium reaction. The relevant equilibria are
Figure 1.5: Initial Steps of the Metathesis Polymerization
Monomer-polymer equilibrium, in more general sense,
Equilibrium between polymers of different chain length,
Ring-chain equilibrium, and
Cis-trans-equilibrium.
The free enthalpy of polymerization (ΔGp) is sufficiently negative for rings of a size of 3, 4, 8, and larger to have the equilibrium on the side of the polymer. However, for rings of a size of 5, 6, and 7 - because of the low ring tension - the free enthalpy of polymerization can be even positive. For example, ΔG0,P for the formation of the cis-polymer of cyclohexene, ΔG0,P = +6.2 kJ mol−1 and for trans-polymer of cyclohexene, ΔG0,P = +7.3 kJ mol−1 (14). However, at cryogenic temperatures, ΔGP decreases and oligomers can be formed.
The polymer contains a fraction of high molecular linear chains and a cyclic oligomeric fraction. If initially the monomer concentration is below the equilibrium value for a linear polymer, essentially no polymer is formed, but only cyclic oligomers. At higher concentration, both a linear polymer and a cyclic oligomer is formed.
The ratio of the amounts of cis-linkages to trans-linkages depends on the nature of the catalyst. A tungsten or molybdenum catalyst, respectively, can be prepared by heating tungsten trioxide with phosphorus pentachloride in o-dichlorobenzene up to 120° under vigorous stirring. The solution changes from colorless to deep red and a considerable amount of precipitate is left behind at the bottom of the reaction vessel. The soluble chloride is used for the further steps.
Table 1.3: Monomers for ROMP Polymerization (15)
| Monomer! | Ratea | b |
| Cyclopentenec | 1,590 | 2.05d |
| Bicyclo[2.2.1]heptene-2c | 1,365 | 1.88d |
| 5-Cyano-5-methyl-bicyclo[2.2.1]heptene-2 | 1,365 | 1.22e |
| 3,6-Methylene-1,2,3,6-tetrahydro-cis-phthalic anhydride | 1,283 | 0.97e |
| 2,3-Diethoxycarbonyl-bicyclo[2.2.1]hepta-2,5-diene | 1,264 | 1.17e |
| 1,5-Cyclooctadienec | 1,202 | 1.98d |
| N-Phenyl-3,6-methylene-1,2,3,6-tetrahydro-cis-phthalimide | 1,182 | 1.05d |
| N-Butyl-3,6-methylene-1,2,3,6-tetrahydro-cis-phthalimide | 1,121 | 1.07e |
| 5,6-Dimethoxycarbonyl-bicyclo[2.2.1]heptene-2 | 1,039 | 0.70e |
| 5-(4-Quinolyl)-bicyclo[2.2.1]heptene-2 | 998 | 0.81e |
| 5-Acetoxy-bicyclo[2.2.1]heptene-2 | 978 | 0.85e |
| 5-Methoxymethylbicyclo[2.2.1]heptene-2 | 978 | 0.69e |
| N,N-Diethyl-bicyclo[2.2.1]heptene-2-carbonamide | 937 | 0.94e |
| 1,4-Dihydro-1,4-methanonaphthalene | 897 | 0.78d |
| 5-Chloromethyl-bicyclo[2.2.1]heptene-2 | 876 | 0.80d |
| 5-(2-Pyridyl)-bicyclo[2.2.1]heptene-2 | 876 | 0.81e |
| 5,5-Dichloro-bicyclo[2.2.1]heptene-2 | 815 | 1.11d |
The actual polymerization takes place in an a...
