Thermodynamics of ROMP

Table 11.2 Thermodynamics of ROMP of cycloalkenes A//°, A5°, and AG° for conversion of liquid monomer into solid amorphous polymer at 25°C (Lebedev 1992, 1994 see also Rranz 1972 Lebedev 1977a,b Cherednichenko 1979 Hocks 1975a)...

The ring strain is not the only driving force for the ROMP, and the thermodynamics of ROMP in terms of the Gibbs-Helmholtz equation AG° = — TAS ... 

Introduction of proper pendant groups in monomer unit of addition polynor-bornene is the way to get polynorbornenes with desired properties. However, the appearance of substituents in norbornene molecules resulted in a decrease of their activity as monomers [220, 223]. In the case of ROMP, this effect is softened to some extent by substantial thermodynamic driving force in the process appearing as a result of opening highly strained bicyclic norbornene skeleton (see Table 1). AP is not such a thermodynamically favorable process. Therefore, in this case the introduction of side substituents, especially bulky or functional groups, led to a dramatic decrease in activity of norbornene derivatives [224-227]. Probably, this fact is one of the reasons for the limited number of publications devoted to AP of silyl substituted norbornenes. 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

The stereochemistry of the C=C bond in the polymer chains that result from ADMET of dienes of the type H2C=CH-(CH2) -CH=CH2 tends to be mostly trans in contrast to the result from ROMP of simple cycloalkenes, where trans C=C bond content may not be the predominant stereochemistry. For example, ADMET polymerization of 1,5-hexadiene gave a linear polymer with a trans C=C bond content of over 70% (catalyzed by Schrock catalyst 35), which is close to the value expected on the basis of thermodynamics.59 Earlier (equation 11.21), we saw that a similar polyalkenamer results from ROMP of methylcyclobutene (catalyzed by (CO)5W=CPh2) this time the stereochemistry of the C=C bond was 93% cis.60... 

Some thermodynamic data for the ROMP of cycloalkenes are summarized in Table 11.2. For cyclohexene the thermodynamic fimctions of the polymer must be estimated from the data for related polymers. Note that AC° is negative for the polymerization of all monomers save cyclohexene. 

In the ROMP of norbomadiene and the dicyclopentadienes, the opening of the second double bond is thermodynamically less favourable than the opening of the first double bond, but nevertheless occurs if the concentration of the repeat units is somewhat above 0.2 M (see Sections 13.4.1.1 and 13.4.3). 

When attempting the ROMP of any new monomer, it should be remembered that the most favourable thermodynamic conditions are high [M] (which makes AS less negative), low T (if A// is negative), and high P if A K is negative). 

Cross Metathesis. Of the three major types of olefin metathesis, cross metathesis (CM) has been the most challenging to selectively control. CM is an intermolecular reaction between two olefins that releases ethylene gas, among a statistical mixture of combinatorial products it is a thermodynamically controlled reaction, where impurities arising from homodimerization are common. In addition, unlike ROMP, which relieves ring strain, and RCM, which forms stable 5- and 6-membered rings, CM has no enthalpic driving force (71). 

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