Cis-Selective Ring-Opening Metathesis Polymerization with Ruthenium Catalysts

Abstract

Using a C-H activated, ruthenium-based metathesis catalyst, the cis selective ROMP of several monocyclic alkenes, as well as norbornene and oxanorbornene-type monomers is reported. The cis content of the isolated polymers depended heavily on monomer structure and temperature. By lowering the temperature, cis content as high as 96% could be obtained.

Ring-opening metathesis polymerization (ROMP) is a powerful methodology for the preparation of a wide range of synthetic polymers including block,¹ brush,² and cyclic architectures.³ Furthermore, ROMP can also be used to prepare polymers with specific microstructures comprising various tacticities (e.g. atactic, isotactic, syndiotactic), double bond arrangements (cis/ trans), or different relative monomer configurations (e.g. head to tail, head to head, etc.).⁴ Control of these microstructures is essential for preparing polymers with well-defined properties. For instance, higher content of cis double bonds (%cis) is typically associated with lower melting and glass transition temperatures while inducing slower rates of crystallization.^5–7 Likewise, the properties of conjugated polymers can be adjusted by varying the percentage of cis double bonds they contain.⁸

Several metathesis catalysts, based on Re, Os, Mo, and W, have been shown to give high cis content in the ROMP of norbornene and norbornadiene derivatives.^9,10 Many of these catalysts have also demonstrated an ability to generate polymers with well-defined tacticities. Although %cis varies significantly with catalyst, monomer, solvent,¹¹ and temperature,¹² Ru-based initiators, such as (PCy₃)₂Cl₂Ru=CHPh, give almost exclusively trans polymers.^13,14 Indeed, this has been a serious limitation for previous generations of Ru-based metathesis catalysts, as highlighted by Schrock and coworkers.^10c The best literature examples of stereoselective ROMP with Ru catalysts include alternating copolymerization of norbornene with cycloalkenes to give polymers with up to 75% cis double bonds.^15,16 Our group described similar %cis values for sulfonate and phosphate substituted NHC-based catalysts as well.¹⁷

We recently reported on a new class of Ru-based metathesis catalysts where the N-heterocyclic carbene (NHC) ligand is chelated to the metal center through a Ru-C bond formed via C-H activation.¹⁸ These catalysts showed remarkable selectivity for the formation of cis olefins during a wide variety of cross-metathesis reactions. Our initial ROMP experiments with the C-H activated catalysts revealed no significant increase in cis content compared to standard catalysts such as 1 (Figure 1). However, after having discovered the improved activity, stability, and selectivity of nitrato-complex 2,¹⁹ we decided to investigate the ROMP behavior of this catalyst more closely. Herein, we show that the cis-selectivity of 2 extends to the ROMP of various monomers and consequently establish that Rubased metathesis catalysts are capable of forming polymers with high cis content. We also demonstrate that classic NHC-based Ru catalysts (e.g. 1) can give polymers with unexpectedly high cis selectivity in certain situations.

Figure 1.

Catalysts 1 and 2. Mes = 2,4,6-trimethylphenyl.

When 2 was added to a solution of norbornene (3) in THF at room temperature (RT), an immediate increase in the viscosity of the solution occurred. Isolation of the resulting polymer (poly-3) and subsequent characterization by ¹H and ¹³C NMR spectroscopy revealed that it contained ca. 88% cis double bonds (Figure 2).²⁰ In contrast, poly-3 prepared using 1 showed %cis values of 58% (Table 1).²¹ These later values are typical of NHC-supported Ru-based metathesis catalysts. Importantly, an even higher selectivity of ca. 96% cis could be obtained with 2 by lowering the temperature of the monomer solution prior to the addition of the catalyst. This trend was also observed when norbornadiene (4) was reacted with 2 at different temperatures (Figure 3). The almost exclusive formation of cis poly-4 with 2 is particularly noteworthy since 1 gave no detectable amount of the cis isomer.²² However, poly-4 prepared with 2 was atactic, as evidenced by the lack of long-range order in the ¹³C NMR spectrum (see the Supporting Information).

Figure 2.

(A) ¹³C NMR spectrum (CDCl₃) of poly-3 prepared from 3 (0.5 mmol) and 2 (0.005 mmol) in THF (2 mL) at RT. “ccc” and “cct” represent cis-cis-cis and cis-cis-trans triads consistent with literature reports.⁴ (B) ¹³C NMR spectrum of poly-3 prepared from 2.

Table 1.

Polymerization of 3–10 with catalysts 1 and 2.^a

Monomer	Catalyst	Cis,b%	Yield,c %	Mn,d kDa	PDId
3	1	58	88	112	1.65
	2	88	94	347	1.87
4	1	<5	93	—e	—e
	2	75	88	—	—
5	1	93	78	95.5	1.21
	2	86	91	—	—
6	1	78	95	179	1.24
	2	61	40	137	1.21
7	1	58	78	—	—
	2	84	73	—	—
8	1	50	64	144	1.08
	2	69	81	328	1.09
	2	80f	79	—	—
9	1	81	95	484	1.49
	2	91	78	629	1.33
10	1	66	>95	463	1.5
	2	74	93	183	1.2
	2	80f	79	—	—
11	1	67	>95	—	—
	2	76	47	—	—
	2	91f,g	80

^aConditions were monomer (1 mmol) and catalyst (0.01 mmol) in THF (4 mL,. 0.25 M) at RT.

^bDetermined by ¹H NMR and ¹³C NMR spectroscopy.

^cIsolated yield.

^dDetermined by multiangle light scattering (MALS) gel permeation chromatography (GPC).

^eHere and below: not determined due to insolubility of the isolated polymer in THF or DMF.

^fReaction performed at −20 °C.

^g0.3 mol% catalyst was used.

Figure 3.

Change in %cis with temperature for poly-3 and poly-4 polymerized with 2. Conditions were monomer (0.5 mmol) and 2 (0.005 mmol) in THF (2 mL). Cis content was determined by ¹H NMR spectroscopy.

Having established that 2 could furnish polymers with high cis content for both 3 and 4, we turned our attention to more complex monomers. Many of these monomers have been polymerized with very high cis selectivity and tacticity control using Mo- and W-derived catalysts, but formed predominantly trans polymers when (PCy₃)₂Cl₂Ru=CHPh was used.¹³ Gratifyingly, we found that in almost every case, 2 yielded a polymer with high cis content approaching 90%. In the cases where cis-selectivity with 2 at RT was below that value, conducting ROMP at −20 °C increased %cis by 6–15% (Table 1). In general a lower fraction of cis double bonds was observed for polymers prepared using 1. However, in the case of monomers 5, 6, and 9, high cis content was achieved without the use of a specially designed catalyst! This is particularly surprising since the closely analogues (PCy₃)₂Cl₂Ru=CHPh is known to give poly-5 with only 11% cis double bonds.¹³ In contrast to poly-5 and poly-9 prepared by Mo-based catalysts,¹⁰ no long-range order was observed using either of the Ru-based initiators. With 2, the formation of atactic polymers can be explained by fast carbene epimerization relative to the rate of propagation. This result is typical of Ru-based catalysts, and only under special circumstances is tacticity control achieved.^14,23

Experimental molecular weights (M_n) for polymers prepared with 2 were generally higher than the predicted values, which is indicative of incomplete catalyst initiation or a high rate of propagation (k_p) relative to the rate of initiation (k_i). This could be qualitatively observed as a solution of 2 and 3 remained purple (the color of 2), even after complete conversion of the monomer. Based on the relatively low initiation rate constant of 2, this result was expected.²⁴

In contrast to norbornene and norbornadiene-type monomers, cyclooctadiene (COD, 12), cyclopentene (13), and cis-cyclooctene (14) are significantly more difficult to polymerize via ROMP due to their lower ring-strain.²⁵ Furthermore, the Z-selective ROMP of these monomers is particularly challenging due to the prevalence of intra- and intermolecular chain-transfer reactions and secondary metathesis events.^4,26 In fact, the Z-selective ROMP of 12 has only recently been reported using a Mo metathesis catalyst.^10a,27 Given the strong preference of 2 for cis -selective polymerization of bicyclic monomers, the next logical step was to attempt the ROMP of more difficult substrates, such as 12–14.

When 12 was exposed to 2 (1 mol%) in C₆D₆ (0.6 mL), only minimal conversion (<20%) was observed after 24 h at RT. Surprisingly, increasing the temperature did not result in higher conversions, despite the fact that no catalyst decomposition was observed by ¹H NMR spectroscopy. Increasing the substrate concentration and switching the solvent to THF also did not increase the conversion of 12, nor did repeating the reaction in neat 12. However, polymerizing 12 with 2 in THF at RT over a period of 3 days provided a modest amount of poly-12 (19% yield). Isolation and subsequent analysis of poly-12 via ¹³C NMR spectroscopy revealed that it contained 96% cis double bonds, a value comparable to that obtained with the Mo-based system (Table 2). Similar to the ROMP of 3 and 4, increasing the temperature of the polymerization of 12 resulted in polymers with lower cis content, although it never went below 80%. The extraordinariness of the above result is highlighted by the fact that 1 yielded poly-12 with 90% trans selectivity.²⁶

Table 2.

Polymerization of 12,13, and 15 with catalysts 1 and 2.^a

Mono- mer	Cat- alyst	Time (h)	Cis,b %	Yield,c %	Mn,d kDa	PDId
cycloocta- diene (12)	1	1	10	88	22.9	1.64
	2	36	96	19	99.1	1.60
cyclopen- tene (13)	1	5	15	68	11.1	1.47
	2	3	48	24	102	1.40
trans- cyclooctene (15)	1	1	18	49	—e	—
	2	1	70	44	—	—

^aSee supporting information for reaction conditions.

^bcis content of polymer determined by ¹H NMR and ¹³C NMR spectroscopy.

^cIsolated yield.

^dDetermined by MALS GPC.

^eNot determined due to insolubility of the isolated polymer in THF or DMF.

Subsequent to our experiments with 12, we found that 2 was also effective at polymerizing 13, although the isolated yield of poly-13 was still low (Table 2). Characterization of poly-13 by ¹³C NMR spectroscopy revealed 48% cis content, which is significantly lower than the cis content of poly-12 prepared by 2. Similar levels of cis selectivity have been reported in copolymerizations with 3, although these generally resulted from incomplete incorporation of 13.^15d Switching to 1 produced poly-13 with only 15% cis double bonds. Thus, the use of 2 resulted in a significant improvement in the %cis of poly-13, albeit to a lesser extent than was anticipated.

Unfortunately, no conversion of 14 was observed when it was exposed to 2 under a variety of conditions.²⁸ This was surprising since the strain energy of 14 (7.4 kcal/mol) is greater than that of 13 (6.8 kcal/mol).²⁵ Nevertheless, we reasoned that a more significant increase in strain energy, resulting from the use of trans-cyclooctene (15), would provide access to the desired polymer.²⁹ Indeed, reaction of 2 with 15 at RT in THF resulted in the immediate and high yielding production of poly-15. Characterization of this polymer revealed a cis content of 70%, a value that is among the highest reported for ruthenium-based catalysts.³⁰ Notably, poly-15 prepared from 1 contained ~82% trans double bonds.

As mentioned above, secondary metathesis events are common in non-rigid polymers, because the active chain end is capable of intra – (“back-biting”) and intermolecular chain transfer reactions. Taking this into account, the cis selective polymerizations of 12,13, and 15 with 2 are remarkable. Indeed, given the very high %cis of poly-12 and no erosion of cis content over the course of polymerization, one should conclude that 2 is less prone to isomerizing or reacting with internal double bonds in polymers while displaying high kinetic selectivity for the formation of cis double bonds. Our molecular weight data also supports this argument, as poly-12/13 prepared from 2 had much higher molecular weights compared to poly-12/13 prepared from 1. Such a result is consistent with a reduction in the number of chain transfer events, which tend to lower molecular weight.³¹ The importance of controlling secondary metathesis is reinforced by examination of the polymers prepared from 1. In the case of poly-5/6/9, where secondary metathesis is suppressed due to steric effects, catalyst 1 yielded polymers with relatively high cis content. In contrast, poly-12/13 have no protection against secondary metathesis and thus the thermodynamically favored trans olefin is eventually formed when these polymers are prepared from 1. Although we have not specifically investigated the mechanistic origin of Z-selectivity in ROMP, calculations performed on an analogue of 2 indicate that steric pressure exerted by the NHC on side-bound ruthenacycles is responsible for the observed Z-selectivity during cross-metathesis.^15c,32 It is likely that a similar mechanism is also responsible for the selectivities observed above.

In conclusion, we have demonstrated the cis selective ROMP of several monomers using Ru-based catalysts. The resulting polymers were recovered in moderate to high yield and cis content ranged from 48–96%. While the cis content varied significantly based on monomer structure, our C-H activated catalyst (2) gave polymers with significantly higher %cis values compared to those prepared by a more traditional Ru metathesis catalyst (1), while also showing qualitatively reverse stereoselectivity compared to (PCy₃)₂Cl₂Ru=CHPh. These results culminated in the highly cis selective polymerization of 12, thereby proving that cis selective ROMP is possible with Ru catalysts, even with monomers that are prone to secondary metathesis. Future work in our laboratory will focus on improvements to both the activity and cis selectivity of 2, with an emphasis on the application of this exciting new class of catalysts towards the development of novel polymer architectures.