Alternating Ring-Opening Metathesis Copolymerization by Grubbs-Type Initiators with Unsymmetrical N-Heterocyclic Carbenes

• Published in: Chemistry : a European journal 2009-09, Vol.15 (37), p.9451-9457
• Main Authors: Lichtenheldt, Martin, Wang, Dongren, Vehlow, Kati, Reinhardt, Ingrid, Kühnel, Christa, Decker, Ulrich, Blechert, Siegfried, Buchmeiser, Michael R.
• Format: Article
• Language: English
• Subjects: metathesis; polymerization; ring-opening polymerization; ruthenium; synthesis design
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200900384

A series of RuIV–alkylidenes based on unsymmetrical imidazolin‐2‐ylidenes, that is, [RuCl2{1‐(2,4,6‐trimethylphenyl)‐3‐R‐4,5‐dihydro‐(3H)‐imidazol‐1‐ylidene}(CHPh)(pyridin)] (R=CH2Ph (5), Ph (6), ethyl (7), methyl (8)), have been synthesized. These and the parent initiators [RuCl2(PCy3){1‐(2,4,6‐trimethylphenyl)‐3‐R‐4,5‐dihydro‐(3H)‐imidazol‐1‐ylidene}(CHC6H5)] (R=CH2C6H5 (1), C6H5 (2), ethyl (3)) were used for the alternating copolymerization of norborn‐2‐ene (NBE) with cis‐cyclooctene (COE) and cyclopentene (CPE), respectively. Alternating copolymers, that is, poly(NBE‐alt‐COE)n and poly(NBE‐alt‐CPE)n containing up to 97 and 91 % alternating diads, respectively, were obtained. The copolymerization parameters of the alternating copolymerization of NBE with CPE under the action of initiators 1–3 and 5–8 were determined by using both a zero‐ and first‐order Markov model. Finally, kinetic investigations using initiators 1–3, 6, and 7 were carried out. These revealed that in contrast to the 2nd‐generation Grubbs‐type initiators 1–3 the corresponding pyridine derivatives 6 and 7 represent fast and quantitative initiating systems. Hydrogenation of poly(NBE‐alt‐COE)n yielded a fully saturated, hydrocarbon‐based polymer. Its backbone can formally be derived by 1‐olefin polymerization of CPE (1,3‐insertion) followed by five ethylene units and thus serves as an excellent model compound for 1‐olefin polymerization‐derived copolymers. Alternate your chemistry! RuIV–alkylidenes based on unsymmetrical imidazolin‐2‐ylidenes (see figure) were used for the alternating copolymerization of norborn‐2‐ene with cis‐cyclooctene and cyclopentene. Alternating copolymers, that is, poly(norborn‐2‐ene‐alt‐cyclooctene)n and poly(norborn‐2‐ene‐alt‐cyclopentene)n containing up to 97 and 91 % alternating diads, were obtained.