Synthesis of Cis,syndiotactic A-alt-B Copolymers from Two Enantiomerically Pure Trans-2,3-Disubstituted-5,6-Norbornenes.

Cis,syndiotacticA-alt-B copolymers, where A and B are two enantiomerically pure trans-2,3-disubstituted-5,6-norbornenes with "opposite" chiralities, can be prepared with stereogenic-at-metal initiators of the type M(NR)(CHR')(OR")(pyrrolide). Formation of a high percentage of alternating AB copolymer linkages relies on an inversion of chirality at the metal with each propagating step and a relatively fast formation of an AB sequence as a consequence of a preferred diastereomeric relationship between the chirality at the metal and the chirality of the monomer. This approach to formation of an alternating AB copolymer contrasts dramatically with the principle of forming AB copolymers from achiral monomers and catalysts.

Introduction

Copolymers in which monomers A and B are incorporated in an alternating manner, poly(A-alt-B), are rare.[1−5] Examples are alternating AB copolymers formed from CO and olefins or CO2 and epoxides. In these cases alternation is greatly assisted by the fact that one partner (CO or CO2) does not itself polymerize. A few alternating AB copolymers have been formed through ring-opening metathesis polymerization (ROMP) of cyclic olefin monomers,[6−28] but in these circumstances both A and B usually can be homopolymerized and the stereochemistry of the C=C bond in the polymer is not fixed. One exception is the alternating AB copolymer which has all trans C=C bonds and <5% AA errors formed from a norbornene-like monomer (B) that is slow to homopolymerize and cyclooctene or cycloheptene (A). The most successful initiators are of the type Mo(NR)(CHCMe2Ph)[OCMe(CF3)2]2 (R = 2,6-Me2C6H3 or 2,6-i-Pr2C6H3), two well-defined alkylidene initiators that contain Mo or W out of many that have proven useful for preparing stereoregular polymers from norbornenes and norbornadienes.[29−35]

Among the well-defined Mo or W initiators are those that contain a stereogenic metal, e.g., 1a–1c. These initiators can produce a special category of stereoregular A-alt-B copolymers made from a racemic chiral monomer where A and B are enantiomers. These “A-alt-A*” copolymers have a basic cis, syndiotactic structure (eq ), which is readily proven through 1H and 13C NMR studies.[36,37] Only one example of a stereoregular A-alt-A* copolymer has been reported in the older literature.[38,39] The cis structure is formed when 1a or 1b reacts with monomer to yield all cis metallacycles in trigonal bipyramidal (TBP) intermediates in which the terphenoxide and the imido ligands are in apical positions, while syndiotacticity results from an inversion of chirality at the metal center with each step in the polymerization. Inversion of chirality at the metal forces the olefin to approach first one side of the M=C bond and then the other. Incorporation of enantiomers in an alternating fashion is a consequence of one enantiomer of the racemic monomer reacting more rapidly with one enantiomer (at the metal center) of each propagating species. We have called this mode of control of polymer structure “stereogenic metal control”; although the chirality of the chain end nearest the metal that results from last inserted monomer is not necessarily irrelevant, the determining feature is the lowest energy diastereomeric combination of chirality at the metal and chirality of the monomer. The “errors” in the cis,syndiotactic-poly(A-alt-A*) structure arise through formation of AA and A*A*cis,syndiotactic and trans,isotactic dyads. Trans,isotactic dyads arise through formation of a trans metallacyclobutane intermediate (instead of a cis metallacyclobutane), which “flips over” before opening, a rearrangement that preserves the configuration at the metal and leads to a trans C=C linkage.[37] This mechanistic proposal is based on the fact that polymerization of (+)-DCMNBE (DCMNBE = 2,3-dicarbomethoxynorbornene) by 1a yields a polymer that contains ∼75% trans,isotactic dyads and 25% cis,syndiotactic dyads, while 1c yields a polymer that contains ∼92% trans,isotactic dyads and ∼8% cis,syndiotactic dyads (Figure ).[37] The olefinic protons in trans,isotactic polymer are inequivalent and on the same C=C bond (and therefore coupled to each other with JHH ∼ 16 Hz[37]) while the olefinic protons in cis,syndiotactic polymer are inequivalent and on different C=C bonds (and therefore not coupled to each other). It was also shown that W(O)(CH-t-Bu)(OHMT)(Pyr)(PMe2Ph) polymerizes (+)-DCMNBE to give only cis,syndiotactic-poly[(+)-DCMNBE].

Figure 1

Olefinic region of the 1H NMR spectrum of the poly[(R,R)-DCMNBE] prepared from initiator 1c. Reprinted with permission from ref (37). Copyright 2012 American Chemical Society.

An interesting question is whether stereogenic metal control will direct formation of a copolymer where A and B are not strictly enantiomers, but have similar structures and reactivities toward homopolymerization, are enantiomerically pure, and have “opposite” chirality. If A and B are significantly different chemically, the resulting polymer could be further manipulated through selective reactions that involve one of the two components within the polymer. We show here that several such cis,syndiotacticA-alt-B copolymers can be prepared with Mo (primarily) and W alkylidene initiators.

Results and Discussion

Initial screening experiments employed the four monomers shown in Figure , where the A monomers have the (2R,3R) configuration and the B monomers have the (2S,3S) configuration. The 13C NMR spectra of cis,syndiotactic-poly(A-alt-B) should reveal four different olefinic carbon resonances, and 1H NMR spectra could reveal up to four first order resonances for four different olefinic protons that are coupled pairwise (eq ). (Overlap of proton resonances could result in non first order 1H NMR spectra.) Racemic A is known to be polymerized by 1a to give cis,syndiotactic-poly[A(,)-A(,)];[37] we find that cis,syndiotactic,alt polymers are also formed from racemic A, B, and B (see Supporting Information).

Figure 2

First four monomers employed in this study.

Copolymerization of a mixture of 25 equiv of A and 25 equiv of B with 1a (0.1 M in toluene-d8) as the initiator was complete within seconds to give cis,syndiotactic-poly(A-alt-B). Its partial 13C NMR spectrum in CDCl3 showed primarily four different olefinic resonances (Figure , right), while its 1H NMR spectrum showed four overlapping first order (pseudo triplet) olefinic proton resonances (Figure , left). (See Supporting Information for details.) The broad resonance shown between 5.45 and 5.50 ppm in Figure we propose is half of the pattern that arises from trans,isotacticAA and BB “errors” (see Figure ). The other half of the pattern, along with any (minor) pattern that is characteristic of cis,syndiotacticAA and BB errors (see Figure ), is buried under the main pattern of four triplets for cis,syndiotactic-poly(A-alt-B) around 5.30 ppm. If we assume that only trans,isotacticAA and BB dyad resonances are present under the main four triplet resonance, we can estimate that ∼94% of the polymer contains cis,syndiotactic-poly(A-alt-B) dyads. The olefinic carbon resonances for any errors cannot be identified reliably in the partial carbon NMR spectrum shown in Figure .

Figure 3

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

Copolymerization of 25 equiv of (R,R)-2,3-(CO2Me)2-norbornene (A) and 25 equiv (S,S)-2,3-(CO2CH2CF3)2-norbornene (B) with 1a (0.1 M in toluene-d8) as the initiator was also complete within seconds. The 13C NMR spectrum of the resulting polymer again showed primarily four different olefinic carbon resonances (Figure , right), while its 1H NMR spectrum showed four overlapping first order (pseudo triplet) olefinic proton resonances (Figure , left), consistent with the formation of cis,syndiotactic-poly(A-alt-B). A virtually identical cis,syndiotactic-poly(A-alt-B) polymer was prepared employing 1b as the initiator (see Supporting Information). It is clear from the spectra in Figure that this cis,syndiotacticA-alt-B copolymer contains more trans,isotactic errors than the cis,syndiotacticA-alt-B copolymer described above, most likely as a consequence of the more significant differences in reactivity between A and B than between A and B. The 5.50 ppm resonance was integrated, and the % cis,syndiotactic-poly(A-alt-B) dyads were calculated to be ∼90%. Two olefinic carbon resonances for the AA and BB “errors” in this case can be seen at ∼133.0 and 128.3 ppm (Figure ). The 19F NMR spectrum of cis,syndiotactic-poly(A-alt-B) also reveals two types of overlapping fluorine resonances for AB and BB errors (see Supporting Information), integration of which suggests that the % cis,syndiotactic-poly(A-alt-B) dyads is ∼80%.

Figure 4

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

Figure 5

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

A third cis,syndiotactic polymer was prepared through copolymerization of a mixture of 25 equiv of (R,R)-2,3-(CH2OAc)2-norbornene (A) and 25 equiv of (S,S)-2,3-(CO2Et)2-norbornene (B) with 1b as the initiator (Figure ). The 13C NMR spectrum showed primarily four different olefinic carbon resonances, while the 1H NMR spectrum showed two essentially first order triplet resonances for protons coupled to one another, along with a second order resonance at ∼5.23 ppm for two coupled olefinic protons. On the basis of the carbon NMR spectrum we can estimate the number of AA and BB errors to be on the order of 5%.

A fourth example is cis,syndiotactic-poly(A-alt-B). The 1H NMR spectrum of cis,syndiotactic-poly(A-alt-B) (Figure ) provides little evidence that the polymer is relatively regular. However, inspection of the 13C NMR spectrum shows primarily four olefinic resonances, consistent with a relatively high percentage (estimated ∼90%) of the proposed structure. The complexity seen in the 1H NMR spectrum can be traced to the overlap and second order nature of the proton resonances. At least four carbon resonances for AA and BB errors can be seen in the 13C NMR spectrum.

Figure 6

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

Twelve other Mo and W initiators were explored for making cis,syndiotactic-poly(A-alt-B), but none was as efficient as 1a or 1b, at least according to proton NMR spectroscopy (see Supporting Information for details). The failure of more than two initiators (so far) to produce cis,syndiotacticA-alt-B copolymers of the type described here is not surprising if one considers the complexity of the stereoregular ROMP reaction[29] and the need to control formation of AA and BB errors. The requirements that the metal has a stereogenic center, that its configuration must switch with each insertion of A or B, and that the polymerization be controlled primarily by the chirality of the stereogenic metal are demanding.

Four additional enantiomerically pure monomers (Figure ) were prepared, and five AB combinations were found to give copolymers with >90% alternating AB dyads using 1b as the initiator, according to their 1H and 13C NMR spectra (see Supporting Information for a complete list of reactions employing A and B). The percentage of trans,isotactic and/or cis,syndiotactic errors was estimated to be in the range of 5–10%.

Figure 7

Four additional monomers.

Copolymerization of A and B ((S,S)-(CO2-t-Bu)2-norbornene) using 1b yielded a CDCl3-soluble polymer whose 1H NMR spectrum showed primarily two resonances, a triplet at 5.34 ppm and a second order resonance at 5.23 ppm that integrated to three times its relative intensity (Figure ). Weak and broad resonances near 5.50 and 5.45 can be attributed to trans,isotactic errors. The presence of primarily four olefinic resonances in the 13C NMR spectrum at 134.0, 133.3, 130.8, and 130.6 ppm suggests that the cis,syndiotactic-poly(A-alt-B) structure is of the order of 90%.

Figure 8

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

Copolymerization of A and (S,S)-(CH2OAc)2-norbornene (B) proceeded smoothly to give another CDCl3 soluble polymer. The 1H NMR spectrum of the isolated polymer showed two pairs of overlapping olefinic proton resonances and weak resonances at 5.45–5.50 for trans,isotactic errors (Figure ). However, the 13C NMR spectrum showed primarily four olefinic resonances, which confirm that the polymer has largely the cis,syndiotactic,alt structure. Given the successful copolymerization of (R,R)-(CH2OAc)2-norbornene (A) with (S,S)-(CO2Et)2-norbornene (B; Figure ), the formation of cis,syndiotactic-poly(A-alt-B) is not surprising. The resonance attributed to trans,isotactic errors is much more pronounced when 1a is used as the initiator (see Supporting Information).

Figure 9

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

The copolymer derived from A ((R,R)-(CO2-t-Bu)2-norbornene) and B ((S,S)-(CH2OMe)2-norbornene) showed two higher ordered olefinic proton resonances of equal intensity in CDCl3, (Figure ) along with resonances for AA and BB errors. The two overlapping proton resonances were not well-resolved, making assessment of the errors in the polymer structure difficult. However, the 13C NMR spectrum showed primarily four olefinic resonances, consistent with formation of largely cis,syndiotactic-poly(A-alt-B). Copolymerization of A and B gives cis,syndiotactic-poly(A-alt-B) (Figure ), the proton NMR spectrum of which resembles that of cis,syndiotactic-poly(A-alt-B) (Figure ). The second order olefinic proton resonance at 5.33 ppm was shifted to higher frequency with respect to two coupled triplet proton resonances, revealing the resonances for trans,isotactic errors. Copolymerization of A with B gave cis,syndiotactic-poly(A-alt-B) (Figure ), which contains <10% errors.

Figure 10

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

Figure 11

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

Figure 12

1H NMR (500 MHz, CDCl3, left) and 13C NMR (125 MHz, CDCl3, right) spectra of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only).

It is important to establish whether the tungsten analogue of 1b (1b) is an equally efficient catalyst. Addition of 50 equiv of rac-DCBNBE (DCBNBE = 2,3-dicarbo-t-butoxynorbornene) to a toluene solution of 1b led to full consumption of the monomer within 10 min. Only two pseudo triplet olefinic proton resonances (3JHH = 10 Hz) are present in the 1H NMR spectrum of the resulting polymer (Figure a). The 13C NMR spectrum is also sharp and free of any significant fine structure associated with structural irregularities (see Supporting Information). These results are consistent with a cis,syndiotactic,alt structure for the polymer. The two small broad resonances assigned to trans,isotactic dyads, in the olefin region of poly(rac-DCBNBE) prepared from 1a, are absent from the spectrum of poly(rac-DCBNBE) prepared from 1b. The reason is that polymerization of (S,S)-DCBNBE with 1b gives cis,syndiotactic-poly[(S,S)-DCBNBE] with the 1H NMR spectrum shown in Figure b; there is no evidence for a trans,isotactic structure. The olefinic proton resonances of cis,syndiotactic-poly[(S,S)-DCBNBE] are located in the middle of the olefinic proton resonances for poly(rac-DCBNBE) (Figure a), which makes it difficult to assess the percentage of microstructural errors formed in this copolymer using 1H NMR spectroscopy. However, when racemic DCENBE (DCENBE = 2,3-dicarboethoxynorbornene) and rac-DCMNBE are polymerized by 1b under similar conditions, the olefinic proton triplet resonances are broadened, Figures c and 13e. The positions of the minor component within the olefinic proton resonances of poly(rac-DCENBE) and poly(rac-DCMNBE) are visible and can be unambiguously ascribed to cis,syndiotactic dyads in the largely cis,syndiotactic,alt structure (compare Figures c–13e).

Figure 13

1H NMR (500 MHz, CDCl3) spectra for cis,syndiotactic polymers synthesized using 1b (olefinic resonances only).

As reported previously,[37] (+)-DCMNBE and rac-DCMNBE are polymerized at approximately the same rate using W(O)(CHCMe3)(OHMT)(Pyr)(PMe2Ph). In contrast, (−)-DCBNBE and rac-DCBNBE are polymerized at different rates. The sharp olefinic resonances in the 1H and 13C spectra of poly(rac-DCBNBE) prepared from 1a as the initiator are consistent with a lower percentage of cis,syndiotactic errors. A similar trend is seen when one inspects the 1H and 13C NMR spectra of poly(rac-DCBNBE) using 1b (Figure a).

The proton and carbon NMR spectra of the A-alt-B copolymers derived from 1a, 1b, and 1b are compared in Figures and 15. Initiators 1b and 1b appear to yield the highest percentages of cis,syndiotacticA-alt-B structures with trans,isotactic errors being formed when 1b is employed and cis,syndiotactic errors being formed when 1b is employed.

Figure 14

13C NMR spectra (125 MHz, CDCl3) of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only) formed with initiators 1a, 1b, and 1b.

Figure 15

1H NMR spectra (500 MHz, CDCl3) of cis,syndiotactic-poly(A-alt-B) (olefinic resonances only) formed with initiators 1a, 1b, and 1b.

Conclusions

Cis,syndiotacticA-alt-B copolymers, where A and B are two enantiomerically pure trans-2,3-disubstituted-5,6-norbornenes with “opposite” chiralities, can be prepared with stereogenic-at-metal initiators of the type M(NR)(CHR′)(OHMT)(pyrrolide) (R = 1-adamantyl or 2,6-Me2C6H3; R′ = CMe2Ph; M = Mo or W). The errors when Mo initiators are employed are primarily trans,isotacticAA and BB dyads, while the errors when a W initiator is employed are cis,syndiotacticAA and BB dyads. Formation of a high percentage of alternating AB copolymer linkages relies on an inversion of chirality at the metal center with each propagating step and faster formation of an AB sequence than an AA or BB sequence as a consequence of a preferred diastereomeric relationship between the chirality at the metal and the chirality of the monomer.

Experimental Section

Representative Polymerization

A mixture of 21.0 mg (0.1 mmol, 25 equiv) of A and 34.6 mg (0.1 mmol, 25 equiv) of B in 0.5 mL of toluene-d8 was added to a solution of 3.0 mg (0.004 mmol) of 1b in 0.5 mL of toluene-d8. The reaction mixture thickened within seconds. 1H NMR spectroscopy was used to monitor the course of the reaction. Once complete, the reaction mixture was exposed to air and poured into 35 mL of MeOH. The precipitated cis,syndiotactic-poly(AB) was allowed to settle. The solvent was decanted, and the polymer was dried in vacuo.