Helical Polyguanidines Prepared by Helix-Sense-Selective

Apr 11, 2007 - The polymerization of N-(n-hexyl)-N'-phenylcarbodiimide was studied using a number of homochiral catalysts based on binaphthol derivati...
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Macromolecules 2007, 40, 3575-3580

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Helical Polyguanidines Prepared by Helix-Sense-Selective Polymerizations of Achiral Carbodiimides Using Enantiopure Binaphthol-Based Titanium Catalysts Hong-Zhi Tang, Eva R. Garland, and Bruce M. Novak* Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695

Jiangtao He and Prasad L. Polavarapu Department of Chemistry, Vanderbilt UniVersity, NashVille, Tennessee 37235

Frank Chen Sun and Sergei S. Sheiko Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290 ReceiVed August 15, 2006; ReVised Manuscript ReceiVed March 13, 2007

ABSTRACT: The polymerization of N-(n-hexyl)-N′-phenylcarbodiimide was studied using a number of homochiral catalysts based on binaphthol derivatives. All polymers showed optical activity due to the formation of a predominant screw sense controlled by the chirality of the catalyst. The helicity and chirality of these materials were studied by VCD, ECD, and polarimetry. Structures calculated using density functional theory were used to compare with their simulated VCD and experimental VCD, thus allowing the absolute screw sense to be determined. AFM imaging showed epitaxial ordering of these rigid chains on graphite surfaces and single chains measuring nearly 3 mm were observed in spin-cast samples on silicon.

Introduction Polyguanidines, an important class of synthetic helical polymers, continue to attract interest due to its promising properties. Polyguanidines adopt 61 helical structures and hence possess rigid polymer backbones, which behave as mesogens.3 For example, polyguanidines display various lyotropic and thermotropic liquid crystalline phases1,2 and optical switching phenomena. Single screw sense helical polyguanidines can be prepared from homochiral carbodiimides using achiral titanium and copper catalysts.4-7 We have pursued this approach and reported two typical helical polyguanidines. One is poly{N-[(R)2,6-dimethylheptyl]-N′-hexylguanidine}, whose kinetically controlled conformation (KCC) of a random coil ([R]20 365 ) +7.5°, hexane) evolved to a thermodynamically controlled conformation (TCC) of a helical structure ([R]20 365 ) -157.5°) when annealed.6 The other is poly{N-(1-anthryl)-N′-[(R)- and/or (S)3,7-dimethyloctyl]guanidines} possessing much bulkier side groups, whose KCCs are exactly the same as their TCCs. Their helical structures maintain the same conformations in different solvents at various temperatures even upon annealing at high temperatures.7 We recently turned our attention to the helix-sense-selective polymerization of achiral carbodiimide monomers to synthesize single screw sense helical polyguanidines.7-11 During our efforts to pursue this approach, we have synthesized a series of enantiopure binaphthol-based monoalkoxy and dialkoxy titanium catalysts, as shown in Chart 1.7-11 Using catalyst S5, we prepared a helical polyguanidine, poly[N-n-dodecyl-N′-(1naphthyl)guanidine], which displays birefringent, cholesteric mesophases, and its films are highly opalescent due to the Bragg reflection of visible light from the pitch of the frozen cholesteric domains.9 We also obtained a regio- and stereoregular polyguani* Corresponding author. E-mail: [email protected].

dine, poly[N-(1-anthryl)-N′-n-octadecylguanidine], prepared by the helix-sense-selective polymerization catalyzed by R7 complex, which shows solvent-driven and thermodriven switching phenomena observed in electronic circular dichroism (ECD) and optical rotations without changing its P-handed screw sense.10,11 The systematic study of the helix-sense-selective polymerizations of achiral carbodiimides using chiral binaphthol-based Ti(IV) catalysts, however, is still lacking. Determination of the absolute screw senses of helical polymers is extremely challenging. The success, to date, is limited to helical polyisocyanides.12-15 The general approach is an introduction of large chromophore side chains to give exciton coupling bisignate Cotton effects in ECD spectra. Theoretical CD calculations are also performed to confirm the assignments. The success in helical polyisocyanides is probably due to the extremely short distances between two coupling chromophore side chains, compared to other helical polymer systems. By analyzing the crystal structures of oligomers, the assignments of screw senses of helical poly(quinoxaline-2,2′diyl)s16 and helical N-alkylated poly(benzamide)s17 were also reported. We recently developed a new method to determine the screw sense of helical polyguanidine by means of the vibrational circular dichroism (VCD) technique and theoretical modeling.11 In this contribution, we report the helical polyguanidines prepared by the helix-sense-selective polymerizations of achiral carbodiimides catalyzed by chiral dialkoxy- and monoalkoxytitanium complexes (Chart 1). Systematic studies of the polymerizations were carried out; i.e., the catalyst, solvent, and concentration effects were investigated. The decomposition properties were studied by thermogravimetric analyses (TGA). The helical structures of the resulting polyguanidines were studied by means of ECD and VCD. The screw senses of the resulting helical polyguanidines were assigned using the VCD

10.1021/ma0618777 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007

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Macromolecules, Vol. 40, No. 10, 2007 Table 1. Conditions and Results of Helix-Sense-Selective Polymerizations of Monomer 1a

polymer poly-1-1 poly-1-2-1 poly-1-2-2 poly-1-3 poly-1-4 poly-1-5R poly-1-5S poly-1-6 poly-1-7

catalyst R1 R2 R2 R3 R4 R5 S5 R6 R7

[mon.]/[cat.]

time (h)

yield (%)

imineb (cm-1)

solubility in chloroform

273 317 150 57 317 208 205 208 201

75 19 135 75 146 2 2 2 2

20 30 31 36 9 18e 70e 67 85

1622 1624 1629 1624 1620 1624 1624 1624 1626

poor good good poor poor good good good good

c [R]25 D (deg)

-109 -54 +189 -307 -100 +755

decomp tempd (°C) 174 166 216 178 173 168 195 196 177

a The neat polymerizations were carried out in a drybox under a nitrogen atmosphere at room temperature. b FT-IR spectra were recorded using thin polymer films drop-casted from polymer solutions in chloroform on sodium bromide crystals. c Measured in chloroform, c ) 0.1 g/100 mL, path length ) 100 mm. The polymer solutions were filtered using 1.0 or 0.45 µm syringe filters. The optical rotations of poly-1-1, poly-1-3, and poly-1-4 have been omitted due to their poor solubilities. d 5% weight loss temperatures. The TGA measurements were carried out in a nitrogen flow at an increasing temperature rate of 10 °C/min. e Runs using R5 and S5 were carried out at different temperatures (fluxuations in the interior drybox temperature) leading to different yields and optical rotations.

Chart 1

technique and theoretical modeling. The visualizations of single helical polyguanidines by atomic force microscopy (AFM) were also performed on graphite, mica, and alkylated silicon wafer substrates. Results and Discussion Helical Poly[N-(n-hexyl)-N′-phenylguanidine]s Prepared by Helix-Sense-Selective Polymerizations. We have carried out the neat helix-sense-selective polymerizations of achiral carbodiimide monomer, N-(n-hexyl)-N′-phenylguanidine (1), using the chiral titanium catalysts in Chart 1. The carbodiimide polymerizations have relatively low ceiling temperatures (low ∆H and very high ∆S of polymerization) depending on the steric demands of the side chains; hence, the polymerization/depolymerization equilibria are often a concern. To push the equilibrium to the right, high (neat) monomer concentrations are often used. The conditions and results are summarized in Table 1. Dialkoxy-Ti(IV) catalysts show higher polymerization activities compared to the monoalkoxy-Ti(IV) ones, evidenced by the shorter polymerization times. The polymerizations were quenched using wet solvents when the magnetic stir bars became immovable. The polymers from monoalkoxy-Ti(IV) catalysts are less soluble in chloroform than those from dialkoxy-Ti(IV) catalysts. This may indicate that the monoalkoxy-Ti(IV) catalysts yielded higher molecular weight helical polyguanidines than the dialkoxy-Ti(IV) catalysts. The exceptions are the helical polyguanidines, poly-1-2-1 and poly-1-2-2, from catalyst R2, which display good solubility in chloroform. Reproducing the polymerization provided similar results. The estimations of molecular weights and polymer dispersion indexes (PDI) of

these helical polyguanidines by means of GPC eluting with chloroform failed, probably due to the sticking of the polar polymer backbones to the GPC columns. The frequencies of the imine stretching in FT-IR spectra of these helical polymers range from 1620 to 1629 cm-1, suggesting that the regioselectivities and/or the stereoselectivities of these catalysts are slightly different, and the resulting helical polymers have slightly different polymer backbone structures. TGA data show that these helical polyguanidines are stable below +170 °C but decompose completely upon heating, probably into monomers. The starting decomposition temperatures are in the range 168-216 °C. Generally, the helical polyguanidines from (R)-catalysts show positively signed optical rotations in chloroform except for those from catalysts R2 and R6. Reproducing the experiments gave similar results. VCD Measurements and Determinations of the Screw Senses of the Resulting Helical Polyguanidines. The vibrational absorbance and vibrational circular dichroism (VCD) spectra of five polymers, poly-1-2-1, poly-1-5R, poly-1-5S, poly-1-6, and poly-1-7, were measured in CDCl3 solution. The absorbance (A) and VCD (B) spectra are shown in Figure 1. The absorbance spectra of all five polymers are essentially the same. There are large differences among the VCD spectra of these five polymers. First, we can classify these five polymers into two categories according to the signs of peaks at 1640, 1623, and 1590 cm-1. Polymers poly-1-2-1, poly-1-5S, and poly1-6 have a (+)-1640, (-)-1623, and (-)-1590 cm-1 pattern. Polymers poly-1-5R and poly-1-7 have the opposite pattern. These two categories are consistent with their optical rotation signs at 589 nm. Polymers poly-1-2-1, poly-1-5S, and poly-1-6

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Helical Polyguanidines 3577 Table 2. Optimized Dihedral Angles (deg) in the Two Model Molecules

Figure 1. Experimental vibrational absorbance (A) and VCD (B) spectra of helical polyguanidines in CDCl3. The typical concentration is around 20 mg/mL. The path length is 200 µm. In these spectra, solvent spectra were subtracted out.

Figure 2. Structures of two model molecules used in calculations.

have negatively signed optical rotations while polymers poly1-5R and poly-1-7 have positively signed optical rotations at the sodium D line. The peak intensities of these three peaks at 1640, 1623, and 1590 cm-1 are also different. Poly-1-7 has the strongest VCD peak intensity (also the largest optical rotation magnitude), suggesting that the catalyst R7 is more efficient in preparing a helical polymer. Two model molecules A and B with two repeat units (Figure 2) were built to predict the absorbance and VCD spectra of helical polymers. The major difference in these two model molecules is the position of benzene ring. In molecule A, the benzene ring is attached to the imine nitrogen atom, while the benzene ring is attached to the amine nitrogen in molecule B. The initial geometry of these two molecules is optimized at molecular mechanics level, which has a P-handedness. The

dihedral angles

model A

model B

C1N2C3N4 C1′N2′C3′N4′ N2C3N4C5 N2′C3′N4′C5′ C3N4C5C6 C3′N4′C5′C6′ N2C3N2′C3′ C3N4C3′N4′

-147.4 -153.7 -169.2 -169.7 +67.3 +69.5 -104.0

-164.0 -172.5 -142.3 -149.6 +43.7 +34.9 -122.7

structures were further optimized by density functional theory using B3LYP functional and 6-31G(d) basis set. The dihedral angles, which define the structure of repeat units, are C1N2C3N4, N2C3N4C5, and C3N4C5C6. The dihedral angle which defines the relative position of two consecutive repeat units is N2C3N2′C3′ for molecule A and C3N4C3′N4′ for molecule B. The optimized dihedral angles are listed in Table 2. The structures of these two model molecules are very close to each other. The initial geometry of models A and B is optimized at molecular mechanics level and further optimized by density functional theory using B3 LYP functional and 6-31G(d) basis sets. Their FT-IR and VCD spectra were calculated by density functional theory using B3 LYP functional and 6-31G(d) basis sets. To determine the handedness of these helical polyguanidines, their experimental VCD spectra need to be compared with predicted VCD spectra. The comparison is shown for the spectra of poly-1-7 and the model molecules A and B in Figure 3. The spectra of model molecule A are very close to the experimental spectra. Model molecule A also has a lower energy than model molecule B. The signs of three major VCD peaks around 1600 cm-1 agree well for polymer poly-1-7 and model molecule A. We therefore believe that the handedness of poly-1-7 in chloroform should be of a P screw sense the same as that of the model molecule A. On the basis of the VCD spectra, we then assigned a P-handedness for poly-1-5R and an Mhandedness for poly-1-2-1, poly-1-5S, and poly-1-6, which have opposite VCD patterns compared to polymers poly-1-5R and poly-1-7. Although the binapthyl ligands for catalysts R2, R5, R6, and R7 possess the same configuration of R (M-handed), the resulting helical polyguanidines take either P- or M-screw senses in choloroform solutions. The cause remains unclear. Helical Poly[N-(n-octadecyl)-N′-phenylguanidine]s Prepared by Helix-Sense-Selective Polymerizations. The helixsense-selective polymerizations of achiral monomer, N-(noctadecyl)-N′-phenylcarbodiimide (2), were performed in solvents of toluene and chloroform at room temperature in a drybox in a nitrogen atmosphere. No significant solvent effect was observed. Compared to the neat polymerizations of monomer 1, the solution polymerizations of 2 are sluggish. This is probably due to the equilibrium occurring between the polymerization and depolymerization in this polymerization process.8 By comparing poly-2-2 to poly-2-4 and poly-2-6, it is evident that higher concentrations of monomer facilitate the polymerization rate and afford higher molecular weight helical polyguanidines. The frequencies of imine stretching in these polymers vary slightly from 1627 to 1633 cm-1, indicating that these helical polyguanidines possess slightly different polymer backbones, e.g., the regioregularity and the stereoregularity. These polymers show positively signed optical rotations in the range of +38° to +130°. The molecular weights and PDIs of the resulting helical polymers also depend on the catalyst type; i.e., monoalkoxy-Ti(IV) catalysts yield higher molecular weight helical polyguanidines.8 The relatively narrow PDI of poly-2-2

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Macromolecules, Vol. 40, No. 10, 2007 Table 3. Conditions and Results of Helix-Sense-Selective Polymerizations of Monomer 2a

polymer poly-2-1 poly-2-2 poly-2-3 poly-2-4 poly-2-5 poly-2-6

catalyst R1 R1 R2 R2 R7 R7

concb (M) 10-2

2.1 × 3.4 × 10-2 2.1 × 10-2 6.4 × 10-3 2.1 × 10-2 6.4 × 10-3

solvent

time (days)

yield (%)

Mnc

PDI

imined (cm-1)

e [R]25 D (deg)

toluene chloroform toluene chloroform toluene chloroform

3 2.5 11 5 12 14

35 59 21 6 15 62

23500 1116200 27200 39100 5100 1600

30.8 1.8 26.4 27.4 3.3 6.5

1633 1633 1627 1628 1627 1627

+130 +69 +38 +59 +127 +89

a The polymerizations were carried in a drybox under a nitrogen atmosphere at room temperature. The ratio of [monomer]/[catalyst] is 50. b Catalyst concentration. c Molecular weights and PDIs were estimated using GPC eluting with chloroform at 30 °C relative to standard polystyrenes. d FT-IR spectra were recorded using the thin polymer films drop-casted from polymer solutions in chloroform on sodium bromide crystals. e Measured in chloroform, c ) 0.1 g/100 mL, path length ) 100 mm.

Figure 4. ECD spectra of poly-2-1 (A) and poly-2-5 (B) in isooctane at various temperatures (c ) 4.2 × 10-5 M, path length ) 10 mm).

Figure 3. Comparison between the experimental absorbance (A) and VCD (B) of poly-1-7 and the predicted spectra of model molecules A and B.

is likely due to the purification process. To dissolve the higher molecular weight polymers, a great amount of chloroform was used, and thus lower molecular weight polymers were lost in the reprecipitation process. ECD Measurements. The long flexible n-octadecyl side groups give the polymer solubility in nonpolar isooctane. This makes it possible to study the conformations of the resulting polyguanidines by means of the ECD technique. We have measured the UV-vis and ECD spectra in isooctane at various temperatures. As an example, the ECD spectra of poly-2-1 in isooctane at various temperatures are shown in Figure 4A. Two positively signed ECD peaks with the intensity of 2.8 and 5.2

M-1 cm-1, at room temperature match the absorption maxima of imine (210 nm) and phenyl (250 nm) chromophores, respectively. This suggests that poly-2-1 adopts a helical structure in isooctane at room temperature. No variation in the shape and nearly no decrease in the intensity of the ECD and UV-vis spectra were observed upon heating the isooctane solution up to +80 °C, indicating that the helical structure of poly-2-1 is very stable during the heating process. Polymers poly-2-2, poly-2-3, and poly-2-4 gave almost the same ECD and UV-vis spectra in isooctane at various temperatures (see Supporting Information). This suggests that all these polymers take almost the same helical structures in isooctane at various temperatures. Significant decreases in the ECD intensity of poly2-5 and poly-2-6, however, were observed upon heating, and an example is shown in Figure 4B. This indicates that the helical structures of poly-2-5 and poly-2-6 are rather fluxional upon heating, probably due to the lower molecular weights. Direct Observation of Single Helical Polyguanidine Chains. Visual conformations of the large helical polyguanidine, poly2-2, were undertaken using tapping mode atomic force micros-

Macromolecules, Vol. 40, No. 10, 2007

Figure 5. AFM images (A: a general view; B: a closer view) of poly2-2 on graphite spin-coated from a dilute chloroform solution at room temperature. Large white blobs in the images are most likely clusters of molecules that did not adsorb to the graphite surface. The single helical polymers are highlighted with white threads to help differentiate these molecules. The area circled shows where the molecules begin to crystallize with one another to form a network.

Figure 6. AFM images (A: a general view; B: a closer view) of poly2-2 on mica spin-coated from a chloroform dilute solution at 50 °C.

copy (TMAFM) on various substrates including graphite, mica, and alkylated silicon wafer. These three substrates show how the interaction between the substrate and the poly-2-2 directly affects the surface conformation of the polymers. Depending on the substrate used, epitaxial alignment of the polymer chains along with intra- and intermolecular crystallization is seen. Samples were prepared by spin-coating dilute chloroform solutions at room temperature or 50 °C. Figure 5A shows the AFM image of poly-2-2 on graphite. Poly-2-2 appears to form a network, making it very hard to discern the end of one polymer from another. Helical polyguanidines lay in such sharp angles due to the alkyl groups interacting with the graphite substrate leading to the epitaxial alignment of the polymers. Single helical polymer chains were found to range in size from 400 nm to greater than 1 µm in size. Figure 5B shows a closer view of one of the junctions between two polymer chains. The polymer chains do not only join together at their ends; sometimes they also appear to crystallize together in the middle of the polymer chains. When two or more polymer chains join together, a thickening of the contact region can be seen (circled). Figure 6A shows the AFM height images of poly-2-2 on mica. The polymers were spin-coated at a higher temperature of +50 °C to separate the polymer chains and prevent them from crystallizing together in solution. However, the helical polymers still form a network on mica. The structure of the network is different from that on graphite. First, there is no epitaxial alignment of the polymer chains. Second, the ends of the helical polymer chains and the junction points between some polymer chains appear to be thicker due to the intra- and intermolecular

Helical Polyguanidines 3579

Figure 7. AFM images (A: a general view; B: stretched out molecules) of poly-2-2 on an alkylated silicon wafer spin-coated from a chloroform dilute solution at 50 °C.

crystallization. Figure 6B is a closer view of single polymer chains on mica. The end of the helical polymer chain is thicker than the rest of the molecule (circled). This is probably due to the polymer chain crystallizing with itself instead of crystallizing with other molecules into a network structure. The length of this molecule is ∼1.3 µm. Also circled is a polymer chain that seems to have completely crystallized upon itself, adopting a conformation of a shorter thicker rod instead of a very long chain. Figure 7A shows the AFM images of poly-2-2 that was spincoated on an alkylated silicon wafer. Again, it can be seen that the polymer is still crystallizing with itself and other molecules. However, the polymer chains are much more curved and bent than those imaged on the mica surface. The crystallization of the ends into rodlike objects is apparent as well as a greater amount of intermolecular crystallization not at the ends. Polymers in this image are clearly very long, but it is impossible to discern what part of the network is from which polymer chain. The polymer chains also have been stretched out, which can occur during the spin-coating process. Figure 7B is an image of the stretched out molecules. From this image, the much thicker ends of the molecules are still observed; however, the areas between the ends of the polymer are just single polymer chains. Circled is a section that is a single polymer chain and not a collection of two or more polymer chains crystallized together. This section of the helical polymer chain is ∼3 µm in length. Conclusions Helical polyguanidines were prepared by helix-sense-selective polymerizations of achiral carbodiimide monomers catalyzed by enantiopure binaphthol-based mono- and dialkoxytitanium complexes. Dialkoxy-Ti(IV) catalysts show higher polymerization activities in the neat polymerization of monomer 1 than monoalkoxy-Ti(IV) catalysts. Solution polymerizations of monomer 2 are very sluggish due to the equilibrium occurring between the polymerization and depolymerization. Higher concentrations of monomer afforded higher polymerization rates and higher molecular weights of the resulting helical polymers. No significant solvent effect was observed in the solution polymerizations of monomer 2. The helical structures of the polyguanidines from monomer 1 were studied by means of VCD measurements. On the basis of the VCD and optical rotation data and the modeling calculations, the screw senses of these helical polyguanidines were assigned. Polymers poly-1-5R and poly-1-7 take P-handed helical structures in chloroform, while polymers poly-1-2-1, poly-1-5S, and poly-1-6 adopt opposite handedness. Although the binapthyl ligands for catalysts R2, R5, R6, and R7 possess

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the same configuration of R (M-handed), the resulting helical polyguanidines take either P- or M-screw senses in choloroform solutions. The cause remains unclear. The helical structures of the polyguanidines from monomer 2 were studied by means of UV-vis and ECD measurements in isooctane at various temperatures. The helical structures of polymers with higher molecular weights, including poly-2-1, poly-2-2, poly-2-3, and poly-2-4, are more stable at higher solution temperatures compared to those with lower molecular weights, including poly-2-5 and poly-2-6. This is the first study of molecular weight effects on the helical structures of polyguanidines. The single polymer chain conformations of the huge helical polyguanidine, poly-2-2, with molecular weight greater than 1 million, were directly observed using tapping mode AFM on various substrates including graphite, mica, and alkylated silicon wafer. The length of one single polymer chain is greater than 3 µm. Acknowledgment. We thank Professor A. Clay Clark at NCSU for use of his CD instrument. Supporting Information Available: All experimental details and related data. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) Kim, J.; Novak, B. M.; Waddon, A. J. Macromolecules 2004, 37, 1660-1662.

Macromolecules, Vol. 40, No. 10, 2007 (2) Kim, J.; Novak, B. M.; Waddon, A. J. Macromolecules 2004, 37, 8286-8292. (3) Nieh, M.-P.; Goodwin, A. A.; Stewart, J. R.; Novak, B. M.; Hoagland, D. A. Macromolecules 1998, 31, 3151-3154. (4) Goodwin, A.; Novak, B. M. Macromolecules 1994, 27, 5520-5522. (5) Shibayama, K.; Seidel, S. W.; Novak, B. M. Macromolecules 1997, 30, 3159. (6) Schlitzer, D. S.; Novak, B. M. J. Am. Chem. Soc. 1998, 120, 21962197. (7) Tang, H.-Z.; Lu, Y.; Tian, G.; Capracotta, M. D.; Novak, B. M. J. Am. Chem. Soc. 2004, 126, 3722-3723. (8) Tang, H.-Z.; Novak, B. M.; Boyle, P. D.; Garland, E. R. Manuscript in preparation. (9) Gong, G.; Lu, Y.; Novak, B. M. J. Am. Chem. Soc. 2004, 126, 40824083. (10) Tang, H.-Z.; Boyle, P. D.; Novak, B. M. J. Am. Chem. Soc. 2005, 127, 2136-2142. (11) Tang, H.-Z.; Novak, B. M.; He, J.; Polavarapu, P. L. Angew. Chem., Int. Ed. Engl., in press. (12) de Witte, P. A. J.; Castriciano, M.; Cornelissen, J. J. L. M.; Scolaro, L. M.; Nolte, R. J. M.; Rowan, A. E. Chem.sEur. J. 2003, 9, 17751781. (13) Cornelissen, J. J. L. M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Macromol. Chem. Phys. 2002, 203, 1625-1630. (14) van Beijnen, A. J. M.; Nolte, R. J. M.; Naaktgeboren, A. J.; Zwikker, J. W.; Drenth, W.; Hezemans, A. M. F. Macromolecules 1983, 16, 1679-1689. (15) Takei, F.; Hayashi, H.; Onitsuka, K.; Kobayashi, N.; Takahashi, S. Angew. Chem., Int. Ed. Engl. 2001, 40, 4092-4094. (16) Ito, Y.; Miyake, T.; Hatano, S.; Shima, R.; Ohara, T.; Suginome, M. J. Am. Chem. Soc. 1998, 120, 11880-11893. (17) Tanatani, A.; Yokoyama, A.; Azumaya, I.; Takakura, Y.; Mitsui, C.; Shiro, M.; Uchiyama, M.; Muranaka, A.; Kobayashi, N.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 8553-8561.

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