A phase-separable second-generation hoveyda-grubbs catalyst for ring-opening metathesis polymerization

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ABSTRACT: Polyisobutylene-supported second-generation Hoveyda-Grubbs catalyst is shown to be an effective nonpolar phase tag for ring-opening metathesis polymerization (ROMP). The catalytic activities of the supported Ru–carbene complex in ROMP are comparable to those of their homogeneous counter- parts. The separability of these catalysts leads to lower Ru contamination in the polymer products in comparison to the nonsupported Hoveyda-Grubbs catalyst. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

KEYWORDS: metathesis; phase separation; polyisobutylene-supported N-heterocyclic carbene; ROMP; second-generation Hoveyda-Grubbs catalyst.

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Inductively coupled plasma mass spectrometry (ICP-MS) analysis was used to determine the content of Ru leaching in the polymer products. The results are listed in Table 1. In these experiments, a portion of the solid polymer product was digested to form a solution of Ru in nitric acid. In that process, the Ru content of that solid sample was diluted to form nitric acid solutions that had Ru contents of 0.4 – 0.6 ppm when products formed by polymerizations using the PIB-bound Ru catalyst were analyzed. These ppm values varied depending on the amount of sample digested and the volume of the nitric acid. A similar analysis of a polymerization product isolated from hexane/dichloromethane polymerization using a low molecular weight Grubbs-Hoveyda complex yielded a nitric acid solution with 10.1 ppm Ru. Using these concentrations of Ru in the analysis solutions, the concentration of the Ru in the solid polymers could be calculated: polymer 15 (entry 1), 127 ppm of Ru; polymer 16 (entry 2), 111 ppm of Ru; polymer 17 (entry 3), 167 ppm of Ru; and polymer 18 (entry 4), 228 ppm of Ru. All of the polymerization products formed using the PIB-supported complex had much lower Ru leaching and lower Ru concentrations in the polymer products than a polymer formed using a low molecular weight Grubbs-Hoveyda complex.

Table 1. Results for the ROMP of various monomers using Ru complex 9 and 3.

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To further establish that the low Ru contamination of the product was due to the use of a soluble polymer-bound Ru catalyst and to show that the use of PIB ligands did not significantly affect the polymerization reaction, we carried out ROMP polymerizations analogous to those in Table 1 using the analogous low-molecular-weight Ru complex 3. A ROMP reaction was carried out with monomer 13 using complex 3 under the conditions listed in Table 1. The reaction mixture was worked up using a procedure identical to that used in a ROMP of monomer 13 with complex 9. After solvent precipitation, the product homopolymer 17 obtained was highly colored as shown in Figure 2. ICP-MS analysis of this solid polymer showed that it contained 73% of the starting Ru (4764 ppm Ru) (Table 1, entry 5). This is in contrast to the white polymer product formed using complex 9, which contained 3.3% of the starting Ru (167 ppm Ru) (Fig. 2).