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Ring-Opening Metathesis Polymerization of Butyl Substituted Trans- Cyclooctenes Wesley S

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Ring-Opening Metathesis Polymerization of Butyl Substituted Trans- Cyclooctenes Wesley S Ring-Opening Metathesis Polymerization of Butyl Substituted trans- Cyclooctenes Wesley S. Farrell and Kathryn L. Beers* Materials Science and Engineering Division, National Institute of Standards and Technologyǂ, Gaithersburg, Maryland 20899, United States ǂOfficial contribution of the National Institute of Standards and Technology; not subject to copyright in the United States. ABSTRACT: The synthesis and ring-opening metathesis polymerization of highly-strained 3- and 1-butyl-trans-cyclooctenes was investigated with the goal of quickly preparing regioregular polymers with narrow molar mass distributions, which could serve as precursors for high precision short-chain branched polyethylene. 3-butyl-trans-cyclooctene was unable to be polymerized in a regioregular manner, however 1-butyl-trans-cyclooctene did yield regioregular product, although the dispersities were higher than expected. Investigation by NMR provided evidence that the nature of the propagating alkylidene is likely non-uniform throughout the course of the reaction, leading to the broadened molar mass distribution. Ring-opening metathesis polymerization (ROMP) of cyclic performed, as it provides sufficient steric bulk to serve as a olefins followed by hydrogenation is a powerful method for the model for alkyl chains of varying lengths. production of high precision model polyolefins; it is a chain- growth polymerization, and the stereo- and regiochemistry of We first explored the ROMP of 3-butyl-trans-cyclooctene (1) the resulting polymer may be controlled through judicious using G1 in the presence of excess PPh3 in order to provide catalyst choice and monomer design.1-3 Recently, ROMP of 3- polymers with narrow molar mass distributions. Monomer 1 alkyl substituted cis-cyclooctenes (cCOEs) using highly active was prepared through a three step isomerization15 beginning and functional group tolerant ruthenium-based Grubbs second with 3-butyl-cis-cyclooctene.16,17 1H and 13C nuclear magnetic and third generation catalysts (G2 and G3, respectively) was resonance (NMR) spectroscopy revealed the presence of two reported to yield perfectly stereo- and regioregular species in equal amounts in the product mixture, which polyalkenamers which, upon hydrogenation, provided highly correspond to the two possible isomers for monomer 1.16 4,5 regioregular short-chain branched polyethylene (SCB PE). Polymerization of 1 at room temperature with the G1/PPh3 This lead to a series of reports regarding the use of various system ([M]o = 0.5 mol/L) at 50 °C lead to the isolation of cCOE monomers with other functional groups in the allylic polymer, with longer reaction times leading to higher yields and 6- position for the generation of functionalized regioregular PE, number average molar mass (Mn) values closer to the theoretical 10 and has been extended to include group 6 based values based on the initial monomer:catalyst ratio, although the monoaryloxide pyrrolide (MAP) catalysts as well.11 dispersity (Ð) increased with time as well (Table S1).16 Unfortunately, 1H and 13C NMR spectroscopy revealed that the The driving force behind ROMP is the release of ring strain. resulting polymers were highly regioirregular, with no Thus, polymerizations of 3-substituted cCOE monomers are preference for cis or trans geometry about the double bonds in typically performed at high concentrations (~2 mol/L) or in bulk the polymer backbone being observed. Employing N- monomer with reaction times from 1 to 20 h in order to ensure heterocyclic carbene (NHC) ligated G3 as the catalyst did not high conversion.12 On the other hand, trans-cyclooctene lead to any improvement in the regiochemistry of the resulting (tCOE) exhibits significantly higher ring strain than its cis polymer as judged by 1H NMR.16 Accordingly, further isomer (16.7 kcal/mol vs. 7.4 kcal/mol for the unsubstituted investigation into the polymerization of 1 was not performed. monomers),13 and may be polymerized in a living fashion to Seeking to prevent the possibility of different orientations of the high molar masses in minutes using the less active first monomer during the [2+2] cycloaddition step, we considered a generation Grubbs catalyst (G1) in the presence of excess monomer with the butyl group in the 1 position, which would triphenyl phosphine (PPh ), which serves to reduce the rate of 3 also prevent the possibility for the presence of multiple isomers. secondary metathesis (k ) and increase the rate of initiation s Historically, ROMP of trisubstituted olefins has been (k ).14 Motivated to prepare high molar mass, regioregular i challenging, given the large degree of steric bulk at the reactive polyalkenamers with narrower mass distributions, we explored olefin, often requiring high temperatures, high concentrations, the ROMP of various butyl-substituted tCOE monomers, and and long reaction times. For example, polymerization of 1- report the results here. In particular, the goal was to provide a methyl-trans-cyclooctene (neat) at 50 °C for 23 h was reported monomer with high ring strain coupled with enough steric bulk by Katz and coworkers using the well-defined tungsten initiator to inhibit contributions to the mass distribution from cross (Ph) C=W(CO) , which produced a regioregular polyalkenamer metathesis, and thus access narrower mass distributions. We 2 5 (within the detection limits of 13C NMR) in 49 % yield. The elected to employ butyl as the alkyl group for the studies authors in this work proposed that the regioregularity of the Scheme 1. Synthesis and ROMP on Monomer 2. 13 Figure 1. Partial C (150 MHz, 25 °C, CDCl3) NMR of resulting polymer originates from the higher stability of a poly(1-butyl-trans-cyclooctene). tertiary propagating tungsten alkylidene, relative to a secondary.18 It was shown that the same catalyst could NMR) was observed, and high yields were obtained, in very effectively polymerize the even more highly strained 1- short reaction times (1 m). When larger [M]o:initiator ratios methylcyclobutene as well, also with a high degree of were employed, polymers with Mn values as high as 356 kDa ‘translational invariance’.19 More recently, it has been reported were obtained,27 and initial monomer concentrations as low as that 1,5-dimethyl-1,5-cyclooctadiene may be polymerized 0.1 mol/L provided high polymer yields.16 It is likely that the using various ruthenium based catalysts in a regioregular reason the observed Mn values do not track with those expected fashion. However, given the low ring strain and steric bulk of based on the ratio of monomer to catalyst is due to the fact that this monomer, long reaction times (24 h) and elevated the measurements are relative to polystyrene standards; the temperatures (50 °C) and concentrations (2.0 mol/L) were mean square radius of polystyrene is significantly different than necessary, and polymers with high molar masses (> 30 kDa) the poly(1-butyl-trans-cyclooctene), so a deviation such as this have not been disclosed.20,21 Cyclobutenes bearing a variety of is not unexpected.27 Polymerizations at low temperatures were polar functional groups in the 1 position have also been also successful, however lower yields were observed, despite polymerized with a high degree of regioregularity by Sampson the fact that longer reaction times and higher concentrations and coworkers with functional group tolerant ruthenium were employed. Changing solvent to dichloromethane (DCM) catalysts,22,23 including examples of alternating copolymers.24 also resulted in low yields relative to THF, both at room temperature and below.16 Unfortunately, Ð values were rather With knowledge of the behavior of monomer 1 and the reported high for all polymerizations, regardless of the conditions work regarding ROMP of strained 1-substituted cycloolefins, employed, the reasons for which are discussed below. we sought to expand upon the pioneering work of Katz18 by exploring the ROMP of 1-butyl-trans-cyclooctene (2), which At first glance, NMR spectroscopy suggested that the resulting was prepared as summarized in Scheme 1.15,16 Reaction of 2 polymers are perfectly regioregular; the 1H NMR spectrum 13 with G1/PPh3 yielded no polymer, even at elevated displays one triplet in the olefin region, while the C NMR temperature, likely due to sufficient steric repulsion. The more displays only two olefinic signals (Figure 1). Both cis and trans active catalyst G3, on the other hand, which was chosen for its isomers likely exist, but the extent of each is difficult to ability to yield polymers with narrow molar mass distributions determine. The olefinic resonances observed by both 1H and 25 13 due to its extremely high ki value, successfully and quickly C NMR for these isomers would not be expected to be polymerized monomer 2 at room temperature in distinguishable, given the similar magnetic environment caused tetrahydrofuran (THF) (Scheme 1), which was chosen as the by the presence of the butyl group on the alkene. However, solvent due to its ability to suppress secondary metathesis.26 At close inspection of the 1H NMR does show overlapping triplets room temperature, nearly complete conversion (>90% by 1H for the methyl protons on the end of the butyl group, and the 13C NMR spectrum displays more signals in the aliphatic region a c 16 Entry [M]o:[Ru]o Theor. Mn Mn Ð Yield than would be expected for perfect stereoselectivity, (kDa)b (kDa)c (%) indicating the presence of both cis and trans stereoisomers. Further investigation into this issue was not deemed necessary, 1 25 4.2 4.8 1.3 95 as the presence of multiple stereoisomers would not impact the structure of the product after hydrogenation. 2 93 15.5 29.6 1.9 90 Closer inspection of 1H NMR data for lower molar mass 3 304 50.6 51.3 1.6 79 polymers prepared from monomer 2 revealed that the actual 4 1015 168.8 116.0 1.6 79 5d 4021 668.7 356.1 2.0 70 Table 1.
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