Stiffening the Helix

In principle, stable, regular polyisocyanides can be formed by polymerization reactions that are under (i) thermodynamic control (driving force = AGab) or (ii) kinetic control (driving force = AGBz — AGAz) (Fig. 12.8). The different possibilities are depicted in Fig. 12.8, for a polymerization of monomer M to a polymer with monomer units in conformations P-MA and P-MB, which might correspond to structures that differ in helicity, backbone (s-cis)-(s-trans) isomerism or imine syn-anti isomerism (only two distinct conformations are considered for simplicity). If the barrier between two conformations AGABz is sufficiently small (at a certain temperature), the different conformations are kinetically accessible and the polymer structure is under thermodynamic control. The thermodynamically formed conformations are situated on the left side of Fig. 12.8c. Polymerization reactions under thermodynamic control can yield well-defined polymers when the free energy of the most stable conformation is sufficiently lower than that of competing conformations (high AGAB); in the case of cooperativity, only small energy differences are sufficient [7].

Only recently has the literature provided some clear examples of regular well-defined polyisocyanides formed under thermodynamic control at elevated temperatures.

Polymerization of aryl isocyanides using the Pd-Pt catalyst as reported by Taka-hashi et al. is typically done in refluxing THF [29, 30, 65, 66]. Several observations indicate that this type of polymerization is under thermodynamic control. Firstly, as previously mentioned, polymers formed by NiCl2 at room temperature can be converted into a better defined regular conformation at higher temperature, while polymerization of the monomer in refluxing THF with the Pd-Pt catalyst at @66 °C already directly leads to this thermodynamic, regular structure [60]. Secondly, from a detailed study on the helix-sense-selective polymerization using chiral oligomer complexes derived from isocyanide 11 [65], it appeared that the rate constants for propagation are virtually identical, independent of whether the monomer that is incorporated has the same or the opposite chirality as the one constituting the initiating oligomer. This observation rules out the possibility of kinetic control. Finally, a nonlinear relation was observed between the amount of chiral monomer excess and the induced helical sense in the polymerization [67]. This is indicative of a thermodynamically driven ''majority rules'' mechanism, of the type observed in the polymerization of polyisocyanates [68, 69]. However, it appeared that only achiral isocyanides with substantial steric bulk could be polymerized with an ongoing helix sense from a chiral oligomer complex, whereas less bulky achiral isocyanides only showed little preference for

Fig. 12.8 Influence of kinetic and thermodynamic parameters on the polymer structure: (a) Energy diagram for the incorporation of monomer M in conformation A and B of the polymer, with energy levels PMa and PMB; (b) Energy diagram for the conversion of a monomer

unit in conformation A to conformation B in the polymer; (c) Influence of the degree of kinetic control (AGBz — AGaz) and the kinetic (AGabz) and thermodynamic stability (AGab) of the monomer units in the polymer on the obtained polymer structures.

a single screw sense [65]. In this case it remains the question whether for the bulky isocyanides, the kinetics rather than the thermodynamics play a role in the helix-sense-selective polymerization.

Yashima showed that the polymerization of phenyl isocyanide 17 (Scheme 12.4) bearing an l-alanine residue can be performed under both kinetic and thermodynamic control [70]. Whereas the polymerization with NiCl2 in toluene and CCl4 yielded the kinetic product with a positive Cotton effect, the polymerization in THF and even more so in toluene at 100 ° C yielded the thermodynamic product, which gave a negative Cotton effect. Under kinetic control, in apolar solvents, hydrogen bonding is thought to play a role in the transition state. In contrast, in polar solvents or at high temperature, hydrogen bonding is suppressed and the thermodynamic product is formed. The role of hydrogen bonding was confirmed by the fact that for the polymer of phenyl isocyanide 18, which is incapable of hydrogen bonding, a negative cotton effect was observed independent of the polymerization conditions. Remarkably, apart from being manifested by CD spectros-copy, the helix sense of the polymers in self-assembled layers on highly ordered pyrolytic graphite, could be visualized by AFM (See Chapter 11, Fig. 11.4).

17 18

Scheme 12.4

A special case of a thermodynamically formed helix is the previously discussed poly(4-carboxy phenylisocyanide) 14. The polymer does not form a regular structure by itself, but only upon complexation with an optically active amine, allowing one helix sense to become thermodynamically more favorable than the other. Interestingly, in water the helix structure was retained even without chiral amines present, meaning that the thermodynamically formed structure was kinetically trapped [63].

Polymerization is under kinetic control when the transition state energy AGaz for the incorporation of a monomer into the desired configuration P-MA is sufficiently smaller than that for other configurations P-MB, that is (AGaz — AGBz) is large. When the formed structure is thermodynamically stable, that is AGab is large, with P-MA being the lower energy conformation (in this case the polymer could also be formed under thermodynamic control) or when the structure is kinetically trapped, that is AGABz is large, stable well-defined polymers are formed (Fig. 12.8).

Polymerizations catalyzed by nickel at room temperature with bulky monomers are believed to be under kinetic control although in many cases it is not very clear to what extent regular polymers are formed as has been discussed in Section 12.3.

One of the most striking examples of kinetic control is the earlier mentioned polymerization of t-butyl isocyanide, which yielded a mixture of M- and P-helices but no mixed M-P-polymers, as was shown by isolating the two polymers by column chromatography using a chiral support [10, 36]. The bulkiness of the t-butyl group steers the kinetic control and provides kinetic stability to the formed polymers. Phenyl isocyanide was reported to kinetically give a 41-helix during polymerization, however, because of the lack of steric bulk in the side-chain this helix structure is not stable and subsequently unfolds into a random coil polymer [43]. The initial formation of a helix illustrates the important role of the nickel catalyst in providing kinetic control over the reaction, presumably via the merry-go-round mechanism. Deming and Novak also showed that in the polymerization of less bulky isocyanides no complete stereo control is obtained; for a racemic mixture of methylbenzylisocyanide, R and S isomers were mixed in the same helix [71].

For bulky monomers, the occurrence of kinetic control in the nickel(II) mediated polymerization of isocyanides was nicely illustrated by an inhibition experiment [52, 72]. Achiral monomers, which rapidly polymerize, were copolymerized with a slowly polymerizing bulky chiral isocyanide. Instead of imposing its own helix sense (say P) on the achiral monomer, it was found that the chiral isocya-nide promoted the formation of a polymer with the opposite screw sense (M). This intriguing result was explained by kinetic inhibition of the formation of one helix type by the bulky monomer. Whereas the achiral isocyanides will normally form both M and P helices, one of the two helices is inhibited in the copoly-merization (say P) because of the incorporation of the slowly polymerizing chiral monomer. A variation on this experiment was performed by Amabilino et al. [73], who showed that diastereomers of polymers 19 and 20 could be formed by kinetic inhibition of the growth of the normally occurring helix using the slow polymerizing isocyanide 22 of the same chirality as co-monomer.

Work of the same authors also revealed that apart from steric bulk, other interactions between the monomers can influence the polymerization in a well defined way [74, 75]. Polyisocyanides 19-21 are derived from promesogenic monomers (Scheme 12.5), which are able to induce cholesteric and chiral smectic C phases in nematic and smectic C liquid crystals, respectively [74, 75]. Upon polymerization, in most cases the handedness of the polymers turned out to be the same as that of the monomer induced LC phases. The long range chiral induction by the stereogenic center in the tail was explained by stereoselective interaction of the incoming monomer with the growing polymer in a similar fashion as observed in the LC-phase. It is the rigid nature of the phenyl benzoate group that allows the transfer of chirality from the side chain to the isocyanide func-

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