Park, Hyeon; Choi, Tae-Lim. “Fast Tandem Ring-Opening/Ring-Closing Metathesis Polymerization from a Monomer Containing Cyclohexene and Terminal Alkyne” J. Am. Chem. Soc. 2012, 134 (17), 7270-7273. DOI: 10.1021/ja3017335
Sometimes two wrongs can make a right. Tae-Lim Choi and his group at Seoul National University have designed a monomer, which embodies this phrase by using two unsuitable functional groups synergistically for a tandem relay Ring-Opening / Ring-Closing Metathesis (RO/RCM) polymerization. Monomer 1 contains two functional groups that are known to be problematic substrates for metathesis: the cyclohexene moiety polymerizes slowly, if at all, due to low ring strain1 and the terminal alkyne is known to be resistant to polymerization (Scheme 1). However, the combination of these functional groups creates a monomer that readily undergoes polymerization.
Scheme 1. Tandem relay metathesis polymerization of monomer 1.
Initially the authors optimized for polymerization of monomer 1 with bis-pyridine catalyst 2, and discovered the reactivity to be similar to norbornene. With these optimized conditions, experiments were performed to understand the mechanism of the polymerization. Some initial control experiments showed the polymer produced was regio-regular with head-to-tail junctions. Having this information in hand, two distinct mechanistic pathways can be envisioned where either the terminal alkyne (Pathway A) or the cyclohexene (Pathway B) are involved in the initial step with the ruthenium catalyst. Due to the fast polymerization, pathway A, “alkyne first”, seems the most plausible to the authors owing to these reasons: the alkyne is less hindered than the cyclohexene, recent reports have shown the ruthenium carbene to react more readily with alkynes over alkenes2, and finally reaction with the alkyne would generate a ruthenium diene, which is irreversible, whereas the carbene generated through pathway B, “cyclohexene first”, would be in equilibrium thereby slowing down the reaction. Furthermore, because the initial reaction with the alkyne proceeds quickly, the close proximity of the ruthenium carbene to the alkene can increase the rate of the cyclohexene opening, and secondly the irreversible formation of diene 1-A thermodynamically drives the polymerization forward.
Scheme 2. Potential mechanisms for tandem relay metathesis polymerization
The authors also investigated both block copolymerization and post functionalization of the new polymer. Since the polymerization of monomer 1 is a living polymerization, the addition of another monomer to create block polymers was explored and shown to be successful while retaining a narrow PDI (Scheme 3a). Furthermore, considering a diene is introduced into the polymer backbone, the polymer was shown to undergo post modification via a [4+2] cycloaddition (Scheme 3b). Interestingly, the cycloaddition with tetracyanoethylene only proceeded with trans-dienes.
- Block copolymerization
- Cycloaddition with the polymer
In conclusion, using unproductive functional groups for polymerization may seem counterintuitive, but a consideration for the spatial relationship of functional groups within a molecular framework can be a powerful approach. I am sure as chemists continue to take advantage of synergistic elements within the architecture of a monomer, the discovery of polymers with unique properties will expand in unforeseen directions.
1Hejl, A.; Scherman, O.A.; Grubbs, R.H. Macromolecules, 2005, 38, 7214.
2Kim, K.H.; Ok, T.; Lee, K.; Lee, H.-S.; Chang, K.T.; Ihee, H.; Sohn, J.-H. J. Am. Chem. Soc. 2010, 132, 12027.