Substituent Effects
As previously stated, ROMP catalysis is dependent on ring strain. Therefore, the best substrates are bi- and tri-cyclic rings; however, these reactions can lead to numerous products. The addition of substituents to the ring system can result in more complex or more functional polymer products. Unfortunately, substituents on the ring can react deleteriously with some of the most common catalysts. The first Grubbs’ catalyst is poisoned by nitrile or amine groups. Many common molybdenum or tungsten metathetical catalysts are affected by oxygenate or nitrogenous groups. Thus alternative catalysts, such as ruthenium carbene complexes that are not affected by these functional groups are being researched.
The position of the substituent in the ring complex has a correlation to the poisoning effect on the catalyst. However, in cases where it is non-poisoning, it also plays a role in determining the reactivity of the substrate. Substituents cannot be placed on the carbon with the double bond or the reaction will not take place. Slugovc, et al. tested the effect of numerous functional groups on the ROMP reaction using the ‘Super-Grubbs’ catalyst, (H2IMes)(PCy3)(Cl)2Ru=CHPh. The experimental results show that the addition of common substituents to the reaction mixture can be used to tune the molecular weight range of the polymer produced.
Depending on the catalyst, some substituents can increase the rate of reaction. Norbornene epoxides increase the rate of reaction when a ruthenium trichloride/alcohol mixture is used as the catalyst. Basset, et al. contribute this rate increase to the production of a metallooxacyclobutane complex that, upon metathetic opening, gives the active ruthenium carbene complex directly. It stands to reason that other functional groups that can react with a similar mechanistic pathway will also increase the rate of reaction.
Precision control of polymer stereo- and regiochemistry is a powerful approach for the manipulation of polymer properties. This is particularly true in the case of polyolefins where tacticity and regiochemistry can have dramatic influence on the thermal, rheological, and crystallization properties. The authors Hillmyer, Kobayashi and Pitet demonstrate the regio- and stereoselective ring-opening metathesis polymerization (ROMP) of 3-substituted cis-cyclooctenes (3RCOEs, R = methyl, ethyl, hexyl, and phenyl). Later studies by Hillmyer and Cramer demonstrated that the regioselectivity was due to the steric interactions between the substituent and the NHC-ligand, but with a significant contribution from the solvent polarity. In this study the authors also found different rate limiting step for the different ring sizes.
Read more about this topic: Ring Opening Metathesis Polymerisation
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