Ring-opening metathesis polymerisation

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Ring-opening metathesis polymerization (ROMP) is a type of olefin metathesis chain-growth polymerization that produces industrially important products. The driving force of the reaction is relief of ring strain in cyclic olefins (e.g. norbornene or cyclopentene). A variety of heterogeneous and homogeneous catalysts have been developed. Most large-scale commercial processes rely on the former while many fine chemical syntheses rely on the homogeneous catalysts.[1]

Mechanism[edit]

The ROMP reaction is catalyzed the formation of metal-carbene complexes as first reported by Nobel Prize winner Yves Chauvin and his colleague Jean-Louis Hérisson[2][3] although a hydride mechanism has also been reported.[4] The initiation of the carbene species occurs through numerous pathways; solvent interactions, substituent interactions, and co-catalysts all can contribute to the production of the reactive catalytic species[4][5][6]

The driving force of the reaction is relief of ring strain, after formation of the metal-carbene species, the carbene attacks the double bond in the ring structure forming a highly strained metallacyclobutane intermediate. The ring then opens giving the beginning of the polymer: a linear chain double bonded to the metal with a terminal double bond as well, the new carbene reacts with the double bond on the next monomer, thus propagating the reaction[2]

ROMP reaction giving polynorbornene. Like most commercial alkene metathesis processes, this reaction does not employ a well-defined molecular catalyst.

Solvent effects[edit]

The choice of solvent can be a vital role in the formation of the carbene species, the identity of alcohols affects the activation of RuCl3-based catalysts. Depending upon the alcohol used, activation produced either a ruthenium-hydride or a ruthenium-carbene, the nature of these intermediates affects the properties of the resulting polymer.[4] Solvent can also affect the tacticity of the polymer product.[7]

Substituent effects[edit]

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.[8][9] 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.[5] 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.[4]

The position of the substituent in the ring complex has a correlation to the poisoning effect on the catalyst.[10][11] 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.[8] 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.[5]

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.[4] 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).[12] 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;[13] in this study the authors also found different rate limiting step for the different ring sizes.

Industrial applications[edit]

Ring-opening metathesis polymerization of cycloalkenes has been commercialized since the 1970s,[1] some examples of polymers produced on an industrial level through ROMP catalysis are Vestenamer or trans-polyoctenamer which is the metathetical polymer of cyclooctene; Norsorex or polynorbornene is another important ROMP product on the market; Telene and Metton are polydicyclopentadiene products produced in a side reaction of the polymerization of norbornene.[9]

The ROMP process is quite useful because a regular polymer with a regular amount of double bonds is formed, the resulting product can be subjected to partial or total hydrogenation or can be functionalized into more complex compounds.[9]

See also[edit]

References[edit]

  1. ^ a b Lionel Delaude, Alfred F. Noels (2005). "Metathesis". Kirk-Othmer Encyclopedia of Chemical Technology. Weinheim: Wiley-VCH. doi:10.1002/0471238961.metanoel.a01. 
  2. ^ a b Grubbs, R.H.; Tumas, W. (1989). "Polymer Synthesis and Organotransition Metal Chemistry". Science. 243 (4893): 907–915. Bibcode:1989Sci...243..907G. doi:10.1126/science.2645643. PMID 2645643. 
  3. ^ Hérisson, J.L.; Chauvin, Y. (1971). "Catalyse de transformation des oléfines par les complexes du tungstène. II. Télomérisation des oléfines cycliques en présence d'oléfines acycliques". Die Makromolekulare Chemie. 141 (1): 161–176. doi:10.1002/macp.1971.021410112. 
  4. ^ a b c d e Mutch, A.; Leconte, M.,; Lefebvre, F.; Basset, J.M. (1998). "Effect of alcohols and epoxides on the rate of ROMP of norbornene by a ruthenium trichloride catalyst". Journal of Molecular Catalysis A: Chemical. 133 (1–2): 191–199. doi:10.1016/S1381-1169(98)00103-4. 
  5. ^ a b c Slugovc, C.; Demel, S.; Riegler, S.; Hobisch, J.; Stelzer, F. (2004). "Influence of functional groups on ring opening metathesis polymerization and polymer properties". Journal of Molecular Catalysis A: Chemical. 213 (1): 107–113. doi:10.1016/j.molcata.2003.10.054. 
  6. ^ Zhang, D.; Huang, J.; Qian, Y.; Chan, A.S.C. (1998). "Ring-opening metathesis polymerization of norbornene and dicylopentadiene catalyzed by Cp2TiCl2/RMgX". Journal of Molecular Catalysis A: Chemical. 133 (1–2): 131–133. doi:10.1016/S1381-1169(98)00087-9. 
  7. ^ Samak, B.A.; Amir-Ebrahimi, V; Corry, D.; Hamilton, J.G.; Rigby, S; Rooney, J.J.; Thompson, J.M. (2000). "Dramatic solvent effects on ring-opening metathesis polymerization of cycloalkenes". Journal of Molecular Catalysis A: Chemical. 160 (1): 13–21. doi:10.1016/S1381-1169(00)00228-4. 
  8. ^ a b Hammond, P. 10.569 Synthesis of Polymers Fall 2006 materials; Massachusetts Institute of Technology OpenCourseWare, 2006.
  9. ^ a b c Mol, J.C. (2004). "Industrial applications of olefin metathesis". Journal of Molecular Catalysis A: Chemical. 213 (1): 39–45. doi:10.1016/j.molcata.2003.10.049. 
  10. ^ Wright, D.L.; Schulte II, J.P.; Page, M.A (2000). "An Imine Addition/Ring-Closing Metathesis Approach to the Spirocyclic Core of Halichlorine and Pinnaic Acid". Organic Letters. 2 (13): 1847–1850. doi:10.1021/ol005903b. PMID 10891173. 
  11. ^ Alcaide, B.; Almendros, P.; Alonso, J.; Aly, M.F (2001). "A Novel Use of Grubbs' Carbene. Application to the Catalytic Deprotection of Tertiary Allylamines". Organic Letters. 3 (23): 3781–3784. doi:10.1021/ol0167412. PMID 11700137. 
  12. ^ Shingo Kobayashi, Louis M. Pitet, and Marc A. Hillmyer http://pubs.acs.org/doi/abs/10.1021/ja201644v
  13. ^ Martinez, Henry (2012). "Selectivity in Ring-Opening Metathesis Polymerization of Z -Cyclooctenes Catalyzed by a Second-generation Grubbs Catalyst". ACS Catalysis. 2 (12): 2547–2556. doi:10.1021/cs300549u.