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Reports: DNI752722-DNI7: Ruthenium and Osmium Alkylidyne Complexes as Catalysts/Initiators for the Ring-Opening Alkyne Metathesis Polymerization

Felix R. Fischer, PhD, University of California Berkeley

Since its discovery in the mid-sixties, the development of stable, well-defined, and functional group tolerant olefin metathesis catalysts has greatly influenced the fields of organic synthesis and polymer science. Although alkene metathesis has found a wide range of applications, alkyne metathesis has only recently become the focus of attention. Moreover, living ring-opening olefin metathesis polymerization (ROMP) has had a great impact among others in the areas of biomimetic synthetic polymers, self-assembled nanomaterials, and monolithic supports. In contrast, the application of ring-opening alkyne metathesis polymerization (ROAMP) to the field of polymer synthesis has remained limited due to the lack of tunable substrates, and commercially available well-behaved catalytic systems.

Description: Macintosh HD:Users:felixf:Documents:Dropbox:Funding:2012:ACS_PRF_2012:grafics:figure2.png

Figure 1. Alkyne metathesis catalysts based on tungsten and molybdenum alkylidyne complexes.

Initial attempts using tungsten and molybdenum alkylidyne complexes 1 to 5 (Figure 1) led to the formation of polymers with broad weight distributions. In an effort to increase the selectivity of ROAMP for the activation of strained over unstrained alkynes, we incorporated a permanent electron donating ONO pincer ligand 6 that stabilizes the high oxidation state of the molybdenum carbyne complex and irreversibly blocks one of the catalyst's active sites. Deprotonation of the ONO pincer ligand 6 with potassium benzyl followed by addition to [TolC¼Mo(OR)3(dme)] in toluene quantitatively converted 7 to the desired product 8, by 1H and 19F NMR. The product was isolated in 36% yield after recrystallization from diisopropyl ether at –35 ¼C. Crystals of 8 are stable in air for several hours and can be stored for indefinite time under an atmosphere of nitrogen.

Figure 2. ORTEP representation of the X-ray crystal structure of 8. Thermal ellipsoids are at the 50% probability level. Color coding: C (gray), O (red), F (green), Mo (turquoise). Hydrogen atoms are omitted for clarity.

We studied the ROAMP of 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e][8]annulene (9) (Figure 3), a highly solubilized ring-strained alkyne, with 8. Addition of 8 to a solution of 9 in toluene ([9]/[8] = 10) at 24 °C does not lead to the formation of polymeric species within 24 h. 1H and 19F NMR indicate that the ROAMP catalyst 8 quantitatively reacts with a half life of t1/2 < 5 min with one equivalent of 9 to form the initiated complex 10 (n = 1)(Figure 3). Instead, at 90 °C, the initiation reaction is instantaneous and the living ROAMP of monomer 9 in toluene is completed in less than 2 h, as determined by 1H NMR. Precipitation of the resulting polymers in MeOH affords poly-9 in greater than 90% isolated yield. GPC analysis for various monomer/catalyst loadings at 90 ¼C in toluene shows a polydispersity index (PDI) of ~1.02, the lowest value reported for ROAMP (Table 1). The molecular weights of poly-9 determined by GPC, calibrated to polystyrene standards, are proportional to the initial [9]/[8] loading and show a unimodal distribution (Figure 4). No evidence for branching or the formation of cyclic polymers could be observed by 1H NMR analysis. End-group analysis reveals that GPC overestimates the Mn of poly-9. An estimated correction factor ~0.5–0.7 correlates well with the degree of polymerization determined by NMR analysis and the expected molecular weight based on the [9]/[8] loading.

Figure 3. ROAMP of 9 with catalyst 8.

Table 1. Molecular weight analysis of poly-9.

[9]/[8]

T (°C)

Mn

theory

Mn

GPC

Mw

GPC

Xn

PDI

GPC

10

90

4,000

8,600

9,200

11

1.06

20

90

8,100

15,700

16,200

23

1.04

50

90

20,200

29,200

30,000

47

1.02

100

90

40,400

54,200

55,300

99

1.02

Figure 4. GPC traces of poly-9.

To expand the substrate scope of ROAMP with catalyst 8 we synthesized ring-strained monomer 10 (Figure 5) featuring solubilizing triethylene glylcol chains. The Mn and the PDIs for polymers obtained from the ring opening of 10 are comparable to 9 and are summarized in Table 2. The observed rate constant for the ROAMP of 10 at 90 °C is slower (kp,obs = 0.144 M–1 s–1) than for 9 resulting in a t1/2 ~ 38 min. With two chemically distinct monomers at hand we studied the performance of ROAMP catalyst 8 in the synthesis of amphiphilic block-copolymers. 10 (20) equiv. of 9 were heated with 8 at 90 °C for 30 min. Prior to the addition of 10 (20) equiv. of 10, an aliquot was removed from the reaction mixture and analyzed by GPC. GPC analysis reveals an increase in Mn upon addition of 10 to the living chains of poly-9. The PDI of poly-9-block-poly-10 is exceptionally low (1.08) and matches the catalyst performance achieved for the respective homopolymers. End-group analysis reveals that the ratio of monomers in poly-9-block-poly-10 scales linearly with the monomer loading.

Figure 5. Synthesis of block-copolymers poly-9-block-poly-10 using ROAMP catalyst 8.

Table 2. Molecular weight analysis of poly-10 and block-copolymers poly-9-block-poly-10.

[9]/[10]/[8]

T (°C)

Mn

theory

Mn

GPC

Mw

GPC

Xn[9]/Xn[10]

PDI

GPC

0/10/1

90

5,400

5,700

6,100

0/9

1.08

10/0/1

90

4,000

3,300

3,800

10/0

1.15

10/10/1

90

8,300

11,000

11,800

11/12

1.07

20/0/1

90

8,100

14,400

15,000

20/0

1.04

20/20/1

90

13,400

25,400

27,200

20/20

1.07

Progress over the last funding period involved the synthesis of a molecularly well-defined ROAMP catalyst based on a molybdenum benzylidyne ONO pincer complex [TolC¼Mo(ONO)(OR)]ŸKOR (R = CCH3(CF3)2) 8. The catalyst is capable of selectively ring-opening strained alkynes in a controlled polymerization to yield high molecular weight polymers with exceptionally low PDIs (1.02). Mechanistic studies reveal that the ROAMP catalyst 8 meets all the criteria for a controlled living polymerization: the initiation reaction is quantitative and ~103 timers faster than the propagation (ki > kp), the concentration of catalytically active complex is constant throughout the reaction, and all propagating chains grow at the same rate. Furthermore, we demonstrate for the first time the synthesis of structurally well-defined block-copolymers through a controlled living ROAMP. The catalyst developed herein provides an unprecedented control and access to highly functionalized derivatives of poly-(arylene ethynylene) for applications in advanced thin-film electronics/photonics, molecular sensing, and nano-patterning.