Detecting the Shape Change of Complex Macromolecules during

Biomacromolecules 2000, 1, 6-16

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Articles Detecting the Shape Change of Complex Macromolecules during Their Synthesis with the Aid of Kinetics. A New Lesson from Biology Virgil Percec* and Marian N. Holerca Roy&Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 Received January 18, 2000

The synthesis and living ring opening metathesis polymerization initiated with RuCl2(dCHPh)(PCy3)2 of three 7-oxanorbornene monomers containing two tapered 3a and respectively two conical 3b and 3c dendritic side groups is described. 3a and the corresponding polymer 4a self-assemble in a cylindrical shape, while 3b and 3c self-assemble in spherical shapes. The polymerization of 3a proceeds via a cylindrical growing chain and occurs with the same rate constant regardless of the initial monomer concentration and the initial ratio between 3a and the initiator. The polymers resulting from 3b and 3c exhibit, depending on the degree of polymerization, spherical and cylindrical shapes. The shape change of the propagating macromolecules that resulted from 3b and 3c were detected by the change in the rate constant of propagation. The implication of this kinetic method for the detection of shape change in the design of novel complex synthetic nanoscale functional macromolecules inspired from biology is discussed. Introduction In the past 10 years, one of the topics of research in our laboratory involves the use of Nature as a model for the rational design of novel synthetic methods and architectural motifs that are employed in the construction of novel molecular, supramolecular and macromolecular complex functional nanosystems. Scheme 1 outlines some representative examples of novel concepts that were accomplished by using Nature as a model. One of the first elucidated examples of biological selforganized assemblies from the living matter is the cell membrane.1 The fluid mosaic model of the cell membrane has a bilayer structure self-organized in water from linear phospholipids and globular proteins. The biological membrane plays a crucial role in almost all cellular phenomena at the physiological temperature. Above the physiological temperature the membrane undergoes an order-disorder transition and the bilayer structure is disorganized. Below the physiological temperature when the membrane crystallizes the motion of the bilayer is frozen. Consequently, both above and below the physiological temperature the biological membrane created from linear lipids cannot perform its functions and as a consequence the normal performance of the living organism is disturbed. Archaebacteria including halophiles, extreme thermophiles, thermoacidophiles, and methanogens grow under rather * To whom correspondence may be addressed: e-mail, percec@ sas.upenn.edu.

extraordinary conditions of high salt, low pH, high temperature, or complete lack of oxygen and differ significantly from other forms of life.2,3 These unusual microorganisms have the capability to survive, for example, at a very high temperature specially due to the macrocyclic rather than linear topology of the lipids forming their cell membrane. Therefore, the cell membrane of archaebacteria has supported the hypothesis that the macrocyclic topology, shown in the top left part of Scheme 1, stabilizes a liquid crystalline state more than a linear topology. This hypothesis suggested that the cyclic and not the linear topology, which was unanimously accepted by the scientific community for over 100 years,4-6 is the most suitable for the stabilization of a liquid crystalline phase. Research carried out in our laboratory has demonstrated this concept for the field of thermotropic main chain liquid crystalline polymers.7-10 The left-top part of Scheme 1 outlines both the fluid mosaic model of the cell membrane and the rodlike collapsed shape of the cyclic topology responsible for the formation of the thermotropic liquid crystals. Scheme 1 outlines several other examples that were developed by using Nature as a model. For example the use of a willow as a model for a tree, i.e., a tree with flexible rather than rigid branches, allowed the demonstration that hyperbranched polymers and dendrimers can also adopt extended and/or elongated rather than only spherical shapes.11-14 These series of experiments impacted the field of dendrimers since it advanced the concept of a dendritic

10.1021/bm005507g CCC: $19.00 © 2000 American Chemical Society Published on Web 02/23/2000

Shape Change of Complex Macromolecules

Biomacromolecules, Vol. 1, No. 1, 2000 7

Scheme 1. Selected Examples in Which Nature Was Used as a Model for the Design of Complex Molecular, Macromolecular, and Supramolecular Nanosystemsa

a

For more information on these systems, see the references cited.

building block as an efficient tool for the construction of macromolecular and supramolecular systems with complex shapes. 3- and 4-helix bundles of proteins15 and superlattices such as the hexagonal supperlattice encountered in the supramolecular structure of the muscle16 were also used as models in our laboratory for the rational design of novel synthetic architectural motifs. Recently we have reported that the

design of the first example of a three-cylindrical bundle supramolecular dendrimer and its co-assembly provided a mechanism for the rational construction of hexagonal columnar superlattices.17 Complexes of nucleic acids with proteins including rodlike and icosahedral viruses,18-20 represent some of the most investigated and best understood biological supramolecular assemblies.

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Biomacromolecules, Vol. 1, No. 1, 2000

Figure 1. TEM micrographs of two topological isomers of DNA complexed with recA proteins: (a) catenane;22 (b) knot.23

The elucidation of the topological isomers of the cyclic DNA was made possible by experiments that are based on the ability of DNA to complex proteins. For example, the complexation of DNA with recA protein increases the diameter of the nucleic acid to provide sufficient visualization in order to reliably score the signs of nodes, etc., and to allow the stereostructural analysis by transmission electron microscopy, after electrophoresis provided an overview of the population. This elegant series of experiments led to the development of the field of biochemical topology.21 Figure 1 shows the stereostructural analysis of two topological stereoisomers of the cyclic DNA coated with recA protein. This analysis demonstrated that the complex of the DNA stereoisomer from Figure 1a is a catenane,22 while the one from Figure 1b is a knot.23 We believe that the elaboration of the synthetic macromolecules which can be visualized and analyzed with the same details as the topological stereoisomers of DNA can have the same impact on the field of polymer science as the development of biochemical topology had on the field of molecular biology. We have used rodlike (Scheme 2a) and icosahedral (Scheme 2b) complexes of nucleic acids with proteins, which are the simplest examples of viruses, as a model for the rational design of nanoscale cylindrical and respectively spherical macromolecules with controlled size, shape, and internal and external structure.24 Scheme 2 illustrates the rational design of tapered (c) and conical (d) quasi-equivalent building blocks and their use in the construction of cylindrical and spherical macromolecules. Both the tapered and the conical building blocks are synthesized from AB2 or AB3 first generation minidendritic or higher generation monodendrons suitably functionalized on their periphery with alkyl groups and an endo-receptor X on their inner core. Tapered building blocks self-assemble in supramolecular cylinders which self-organize in a 2-D columnar hexagonal p6mm lattice.25 Conical building blocks self-assemble in spherical objects which self-organize in 3-D Pm3n26 and Im3n27 cubic lattices. The analysis of these lattices by X-ray diffraction experiments provides information on the size of

Percec and Holerca

Figure 2. TEM micrographs of a columnar hexagonal 2-D lattice selforganized from supramolecular cylinders. Reprinted with permission from ref 28. Copyright 1997 American Association for the Advancement of Science.

the supramolecular cylinder or sphere and on the number and size of the tapered and conical building blocks that form the supramolecular objects. Subsequently, the endo-receptors X of the tapered and conical building blocks are replaced with a polymerizable group, and a cylindrical or spherical polymer is synthesized. The lattices of the resulting polymers are analyzed by X-ray diffraction to provide information on the shape and size of the polymers. These polymers can be visualized by transition electron microscopy (TEM) in a lattice28 (Figure 2) or as single cylindrical29 and spherical30 macromolecules on a surface or in a disordered assembly (Figure 3). Also, they can be analyzed in solution.29 The ability of these giant polymers with complex architecture to self-organize in lattices resembles the behavior of proteins. The capability to analyze macromolecules in ordered and disordered states both in bulk and in solution has no precedent in the field of polymer science. As illustrated in Scheme 2, conical building blocks are able to generate both spherical and cylindrical polymers.24 The ability of tapered and conical building blocks to construct either cylindrical or spherical objects can be explained by their quasi-equivalence.24 Quasi-equivalence is a concept that was conceived by Klug and Caspar31,32 to explain the mechanism via which a large number of identical protein building blocks could self-assemble into closed containers of predetermined size such as an icosahedral virus capsid.33 Purposeful switching among different conformational states exerts self-control in the construction and action of protein assemblies. Therefore, quasi-equivalent building blocks are chemically identical subunits which self-control their shape by switching between more stable unsociable and less stable associable conformational states during the process of selfassembly. The energy to drive the change from the more stable unsociable to the less stable associable conformation is provided by the intersubunit interactions which conserve essential bonding specificity. Therefore, quasi-equivalence

Shape Change of Complex Macromolecules Scheme 2. Schematic Representation of the Self-Assembly of Flat-Tapered and Conical Monodendrons into Supramolecular Cylindrical and Spherical Dendrimers and Their Self-Organization into Hexagonal Columnar (Φh) (2-D) and Cubic (Cub) (3-D) Lattices

discards the constraints of strict equivalence while retaining the physical essentials of specificity to allow the assembly of chemical units into a system of minimum free energy. The icosahedral symmetry was already known to Leonardo da Vinci (Figure 4a) in about 150034 and was recognized as the most efficient of all closed shells that can be constructed by the architect R. Buckminster Fuller35 who used it to design (Figure 4c) and build his famous geodesic domes (Figure 4d). The self-assembly of icosahedral viruses (Figure 4b) from quasi-equivalent protein subunits is illustrated in Figure 4e. In this figure a shell of icosahedral symmetry is constructed from 60 identical left-handed enantiomorphic structural units.32 Three classes of connections are possible in this surface lattice by the following specific bonding interactions: thumb to pinkie ) pentamer bond; ring finger to middle finger ) trimer bond; index finger to index finger ) dimer bond. Any two of these three classes of bonds would hold this structure together. The underlined triangles mark equivalent subdivisions defined by the 5- and 3-fold axes at


their vertexes. These quasi-equivalent triangles are able to self-organize into the pentagon and hexagon elements required for the construction of the icosahedral shapes (Figure 4-e). The most recent addition to the icosahedral symmetry was provided by fullerenes.36,37 The transplant of the quasiequivalence concept from biochemistry to synthetic chemistry was made in our laboratory in order to explain the selfcontrol both of the shape of tapered and conical building blocks and of the resulting supramolecular objects24 as well as to explain the self-control that occurs during the conventional radical polymerization of quasi-equivalent building blocks in their self-assembled and self-organized state of their supramolecular assemblies.37 Briefly, in ideal dilute solution the free radical polymerization of these quasi-equivalent building blocks proceeds with an extremely slow rate. However, due to the suppression of the termination reaction by the bulky and sterically hindered growing polymer radicals, polymers with Mw/Mn e 1.10 are obtained. In a self-assembled and self-organized state, a dramatic increase of the rate of polymerization occurs due to the self-encapsulation of the polymerizable groups in a reactor created by the dendritic coat of the monomers. Although a linear increase of Mn is obtained as a function of conversion, polymers with broad molecular weight distribution are obtained.24,29,38 The kinetics of the conventional free radical polymerization process was investigated at 60 and 90 °C. At this temperature even the most compact conical building block distorts its shape. Presently, we are involved in the elucidation of the mechanism of polymer shape change as a function of the degree of polymerization (Scheme 2)24 and of the kinetics of polymerization in dilute ideal solution and in selfassembled and self-organized states by using combinatorial libraries of polymerizable building blocks of different shape and size.29,38,39 These experiments employ living polymerization mechanisms and/or catalytic systems which tolerate a large variety of functional groups and provide living active or dormant species that are able to propagate the polymerization process over a long reaction time and broad range of temperatures. One of the most successful catalysts employed in these kinetic investigations40,41 is the RuCl2(dCHPh)(PCy3)2 elaborated by Grubbs et al.42 This paper describes the synthesis of three 7-oxanorbornene monomers substituted with two second generation monodendron-based building blocks (3a) and respectively with two conical second generation monodendron-based building blocks (3b and 3c). The kinetics of their polymer-

Figure 3. Visualization of cylindrical29 and spherical30 single macromolecules by scanning force microscopy (SFM).

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Figure 4. The use of quasi-equivalent units in the construction of (a) the first icosahedron by Leonardo da Vinci,34 (b) the human wart icosahedral virus with the underlined icosahedral lattice showing the 5-fold and 6-fold symmetry axes marked, (c) a Fuller geodesic dome built on a plane, (d) the geodesic dome located in Expo Montreal, Canada, (e) a drawing demonstrating the strict equivalence in a shell with icosahedral symmetry constructed from 60 identical left-handed enantiotropic units, and (f) the C60 molecule.

ization initiated with RuCl2(dCHPh)(PCy3)2 is discussed as a function of the shape of their quasi-equivalent side groups and of the shape of the resulting polymers. Results and Discussion Scheme 3 describes the synthesis of the 7-oxanorbornene monomers 3a, 3b, and 3c. Their synthesis follows a reaction pathway that was previously elaborated in our laboratory.40,41 The synthesis and characterization of the second generation monodendritic benzyl alcohols 1a, 1b, and 1c were reported previously.29 The benzyl alcohols 1a, 1b, and 1c were esterified with the furan-maleic anhydride Diels-Alder adduct 2. The synthesis of 2 was described in a previous publication from our laboratory.41 This esterification proceeds in a two-step one-pot reaction via an ester-acid intermediate. As explained previously,41 this reaction scheme would allow the placement of two dissimilar monodendritic substituents

on the same 7-oxanorbornene monomer unit. This monomer would yield a copolymer by the homopolymerization of a single 7-oxanorbornene monomer. Experiments on this line are in progress in our laboratory and will be reported in due time. In the present case, the anhydride 2 was reacted with 2 equiv of 1a, 1b, or 1c and a catalytic amount of the supernucleophilic catalyst DMAP in CH2Cl2 for 48 h at 23 °C. After 48 h, 1H NMR analysis indicated the complete conversion of 1a-c. DCC and an additional amount of DPTS were added to perform the esterification of the resulting mono-acid with the remaining equivalent of 1. Purification by column chromatography (SiO2, 3:1 hexanes/ethyl acetate) afforded 76.3, 74.4, and 71.1% of 3a, b, and c. Scheme 4 outlines the polymerization of 3a-c initiated with RuCl2(dCHPh)(PCy3)2 at 23 °C in CH2Cl2. The kinetics of polymerization was monitored as reported in a previous publication from our laboratory.41 When the polymers were



Scheme 3. Synthesis of Monomers 3a, 3b, and 3c

used for differential scanning calorimetry (DSC) and X-ray diffraction measurements, the growing polymer chains were end-capped by their reaction with ethyl vinyl ether (Scheme 4). Figure 5 presents kinetic experiments for the ring-opening polymerization (ROMP) of 3a initiated with RuCl2(dCHPh)(PCy3)2 at different initial concentrations of 3a (i.e., [3a]0) and different initial molar ratios between [3a]0 and RuCl2(dCHPh)(PCy3)2 (i.e., [I]0). Conversion and ln([M]0/[I]0) in time plots demonstrate a living polymerization process (Figure 5-c) for monomer 3a. The rate constant of propagation, kp, obtained at different initial concentrations [3a]0 (presented on the figures) is constant within experimental error. Figure 5d shows an experiment in which additional 3a was added under Ar at 70% conversion. These experiments demonstrate that in ideal solution the ROMP polymerization of 3a follows a living polymerization mechanism. Table 1 and Figure 6 summarize the thermal characterization of 3a and 4a with different molecular weights. The characterization of 3a and 4a with different molecular weights by size exclusion chromatography (SEC) and X-ray diffraction experiments are presented in Table 2. Briefly, 3a

and 4a self-assemble in supramolecular columns that selforganize in a columnar hexagonal lattice. Therefore, although the propagating chain of 4a is sterically hindered, the polymerization process follows a living mechanism. The mechanism of self-assembly and self-organization of 3a and 4a is outlined in Scheme 5. The other two monomers investigated, i.e., 3b and 3c, selfassemble in spherical shapes (Scheme 6). According to X-ray experiments, 42 units of 1b and 23 units of 1c self-assemble in a spherical shape.29 Therefore, up to a degree of polymerization that is close to the number of monomer units that self-assemble in a sphere, the corresponding polymers will also exhibit a spherical shape. Above this degree of polymerization (i.e., about 21 for 3b and 12 for 3c, which correspond to 42 units of 1b and 24 units of 1c) both polymers 4b and 4c should exhibit a cylindrical shape.24 The conversion (%) and ln([M]0/[I]0 in time plots for the polymerization of 3b and 3c are shown in Figure 7. As we can see from Figure 7, the ROMP of both 3b and 3c proceeds via two sets of rate constants of propagation that are conversion dependent, i.e., for 3b kp,cub ) 22.42 × 10-3 L mol-1 s-1 and kp,col ) 46.31 × 10-3 L mol-1 s-1

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Scheme 4. Ring Opening Metathesis Polymerization (ROMP) of Monomers 3 Initiated by RuCl2(dCHPh)(PCy3)2

Figure 5. (a) Conversion (%) and ln([M]0/[I]0) as a function of time for the polymerization of 3a with a initial monomer concentration [3a]0 ) 0.364 mol/L and [3a]0/[I]0 ) 50. (b) Same for [3a]0 ) 0.182 mol/L. (c) Same for [3a]0 ) 0.091 mol/L. (d) The dependence of Mn,GPC on conversion for ROMP of 3a with readdition of monomer at 70% initial conversion.

while for 3c kp,cub ) 5.5 × 10-3 L mol-1 s-1 and kp,col ) 23.3 × 10-3 L mol-1 s-1. In both cases, kp,cub is lower than kp,col. The change in kp for 3b corresponds to a conversion of 22%. This conversion corresponds to a theoretical DP of 22 which represents 44 single 1b monodendrons or 22 repeat units of 3b in a sphere. This result is in excellent agreement with the X-ray data that showed that 42 molecules of the 1b monodendrons self-assemble in a sphere.29 For the case of 3c, the change in kp occurs at 17% conversion. For the experimental conditions used in this polymerization, this conversion corresponds to a theoretical degree of polymerization of 9, which means that 18 units of 1c self-assemble in one sphere. In agreement with the kinetic results, X-ray

data have shown that 23 molecules of 1c form a sphere. These kinetic experiments demonstrate that the rate constant of propagation is dependent on the shape of the propagating macromolecule. The cylindrical macromolecules generated from 3a add the monomer 3a to their growing chain with the same rate constant regardless of the initial monomer concentration and of the initial monomer-to-initiator ratio (Figure 5). However, both monomers 3b and 3c propagate via two rate constants. The first one corresponds to the range of monomer-to-initiator ratios that correspond to the polymer shapes that are lower than a sphere and the second one to the monomer-to-initiator ratios that correspond to polymer shapes that are cylindrical. The steric constraint around the



Table 1. Characterization of 3,4-3,5-G2-ON (3a) and Poly(3,4-3,5-G2-ON) (4a) by DSC and X-ray Diffraction Experiments phasea transitions (°C) and enthalpy change (kcal/mru) [M]0/[I]0 1b 30 50 75 100 150 200 400

heating

cooling

k 44.50 (30.37) LCc 65.66 (10.07) φH 164.96 (6.89) i k 14.26 (1.27) LCc 52.53 (3.07) φH 168.16 (1.44) i k -4.26 (3.20) k 48.40 (1.06) φH 106.40 (0.09) i k -3.73 (3.84) k 95.60 (-0.18) φH 106.24 (0.12) i k -2.33 (4.13) k 50.11 (2.08) φH 112.81 (0.14) i k 2.50 (6.34) g 40.62 φH 111.93 (0.14) i k -2.26 (7.97) g 42.20 φH 117.66 (0.14) i k 2.26 (5.95) g 40.55 φH 116.00 (0.24) i k -4.33 (4.02) g 39.89 φH 128.08 (0.13) i k 2.11 (4.83) g 41.42 φH 127.93 (0.23) i k -4.32 (6.01) g 42.15 φH 128.56 (0.22) i k 2.86 (6.36) g 41.55 φH 129.54 (0.23) i k -3.93 (5.52) g 40.56 φH 131.86 (0.22) i k 2.73 (6.78) g 41.92 φH 132.06 (0.22) i k -0.73 (6.39) g 43.62 φH 132.26 (0.18) i k 2.64 (5.65) g 41.02 φH 132.90 (0.21) i

i 157.12 (1.20) φH 45.87 (3.17) LCc 2.24 (3.33) k i 92.20 (0.05) φH -8.13 (3.41) k i 94.01 (0.21) φH 36.06 g -4.20 (6.14) k i 98.20 (0.28) φH 38.06 g -4.06 (6.09) k i 102.53 (0.31) φH 37.06 g -3.80 (5.33) k i 102.86 (0.30) φH 38.24 g -4.25 (6.66) k i 103.13 (0.29) φH 36.58 g -4.53 (6.70) k i 102.46 (0.19) φH 33.47 g -4.13 (5.07) k

a k ) crystalline, g ) glassy, φ ) columnar hexagonal, i ) isotropic. b DP ) 1 corresponds to the monomer 3a. c LC is a second columnar hexagonal H phase which was not yet characterized by XRD.

Scheme 5. Schematic Representation of the Self-Assembly Process of Monomer 3a and of Its Corresponding Polymer 4a into Cylinders and the Subsequent Self-Organization into a Columnar Hexagonal p6mm Lattice

Figure 6. Phase behavior for polymers 4a as a function of theoretical degree of polymerization. Table 2. Characterization of 3,4-3,5-G2-ON (3a) and Poly(3,4-3,5-G2-ON) (4a) by SEC and X-ray Diffraction Experiments [M]0/[I]0 mol/mol 1c 30 50 75 100 150 200 400

% conversion

Mn,th

Mn,GPC

Mw/Mn

a,a Å

µb

90 90 90 90 90 90 90

2261 61000 101700 152600 203400 305000 406800 813600

2300 45200 69200 95500 158900 189800 246500 511300

1.02 1.04 1.05 1.15 1.06 1.17 1.18 1.31

36

2.0

41

2.5

41 41

2.5 2.5

a a ) lattice parameter, calculated as a ) 2(d 100 + d110 x3 + 2d200)/3 x3. b µ ) number of monomer units in a cylindrical layer of 4.4 Å thickness. c DP ) 1 corresponds to the monomer 3a.

growing chain is higher for the propagating chain corresponding to fragments of sphere, and therefore kp,cub is lower than kp,col. Therefore, the change in shape from fragment of sphere to column is accompanied by a self-acceleration of

the polymerization reaction that is generated by the change in propagating chain shape which releases the steric hindrance. In conclusion, these experiments demonstrate that kinetic experiments can be used to determine the shape change of complex macromolecules during their synthesis. This is possible only due to the quasi-equivalence of the monomeric building blocks. We expect that more compact and rigid building blocks that are not able to distort their shape from spherical to cylindrical under the polymerization conditions should not undergo this change in shape and therefore the living polymerization process will be interrupted

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Scheme 6. Schematic Representation of the Self-Assembly Process of Monomer 3b and of Its Corresponding Polymer 4b into Cylinders and the Subsequent Self-Organization into a Cubic Pm3n Lattice

Figure 7. (a) The dependence of conversion (%) and ln([M]0/[I]0) on time for the ROMP of monomer 3b. (b) The dependence of conversion (%) and ln([M]0/[I]0) on time for the ROMP of monomer 3c. The experimental conditions are on the figure.

at an exact number of repeat units required to enclose a single spherical shape. The nonstatistical termination of a selfassembly process is known to occur during the self-assembly of cylindrical and spherical viruses.20 Although the conical building blocks employed in this experiment do not interrupt the living polymerization process, they have demonstrated that the shape change of complex macromolecules can be detected with the aid of kinetics. This new concept will contribute to the elucidation of the polymerization mechanism in a self-assembled state24,38 and in the design of novel complex synthetic nanoscale functional macromolecules inspired from biology. Experimental Section Materials. 1-Bromododecane (98+%, Lancaster) and chromatographic SiO2 (Fisher) were used as received. THF and Et2O (Fisher, A. C. S. reagents) were refluxed over sodium ketyl and freshly distilled before use. CH2Cl2 (Fisher, ACS reagent) was refluxed over CaH2 and freshly distilled before use. Benzene and o-dichlorobenzene (o-DCB) (both Fisher, ACS reagents) were shaken with concentrated H2SO4, washed twice with water, dried over MgSO4 and finally distilled over CaH2 or sodium ketyl, respectively. Methanol (MeOH), ethanol (EtOH), CHCl3, H2SO4, dimethylformamide (DMF), KOH, MgSO4, and NaHCO3 (all Fisher, ACS reagents) were used as received. 1,3-Dicyclohexylcarbodiimide (DCC, 99%), LiAlH4 (95%), neutral chromatographic Al2O3, SOCl2 (97%), 3,4-dihydroxybenzoic acid (98%), and

LiAlH4 (95+%) (all from Aldrich) were used as received. Maleic anhydride (99%, Aldrich) was recrystallized from CHCl3. Furan (Aldrich, 99%) was washed with 5% KOH, dried over MgSO4, and pump-thaw distilled. 4-(Dimethylamino)pyridine (DMAP) (99%) and p-toluene sulfonic acid used in the synthesis of 4-dimethylaminopyridinium ptoluenesulfonate (DPTS) (98.5%) (both from Aldrich) were dried by azeotropic distillation with benzene. Bis(tricyclohexylphosphine)benzylidineruthenium(IV) dichloride (Grubbs catalyst) (Strem) was used as received. General Methods. 1H-NMR (200 MHz) and 13C-NMR (50 MHz) spectra were recorded on a Varian Gemini 200 spectrometer. CDCl3 was used as solvent and TMS as internal standard unless otherwise noted. Chemical shifts are reported as δ, ppm. The purity of products was determined by a combination of techniques including thin-layer chromatography (TLC) on silica gel plates (Kodak) with fluorescent indicator and high-pressure liquid chromatography (HPLC) using a Perkin-Elmer Series 10 high-pressure liquid chromatograph equipped with a LC-100 column oven, Nelson Analytical 900 series integrator data station and two PerkinElmer PL gel columns of 5 × 102 and 1 × 104 Å. THF was used as solvent at the oven temperature of 40 °C. Detection was by UV absorbance at 254 nm. Weight average (Mw) and number average (Mn) molecular weights were determined with the same instrument from a calibration plot constructed from polystyrene standards. Thermal transitions of samples that were freeze-dried from


benzene were measured on a Perkin-Elmer DSC-7 differential scanning calorimeter (DSC). In all cases, the heating and cooling rates were 10 °C min-1 unless otherwise noted. First-order transition temperatures were reported as the maxima and minima of their endothermic and exothermic peaks. Glass transition temperatures (Tg) were read at the middle of the change in heat capacity. Indium and zinc were used as calibration standards. An Olympus BX-40 optical polarized microscope (100× magnification) equipped with a Mettler FP 82 hot stage and a Mettler FP 80 central processor was used to verify thermal transitions and characterize anisotropic textures. X-ray diffraction experiments were performed using an Rigaku X-ray generator with 12 kW rotating anode. The point focus beam was monochromatized to Cu KR with a graphite crystal. Densities, F, were determined by flotation in glycerol/H2O or glycerol/MeOH. Synthesis. 4-Dimethylaminopyridinium p-toluenesulfonate (DPTS),43 exo,exo-5,6-(dicarboxylic anhydride)-7-oxabicyclo[2.2.1]hept-2-ene41 (2), and 3,5-bis[3,4-bis(n-dodecyl-1-oxy)benzyloxy]benzyl alcohol (1)29were synthesized as previously described. General Procedure for the Synthesis of MonodendronSubstituted Oxanorbornenes. exo,exo-5,6-Bis[[[3,5-bis(3,4(dodecyl-1-oxy)benzyloxy)]benzyloxy]carbonyl]-7-oxabicyclo[2.2.1]hept-2-ene (3a). Compound 3a was synthesized according to a modified literature procedure.40 A 50-mL pearshaped flask equipped with a Teflon-coated magnetic stirring bar was flame-dried and flushed with Ar. Compound 1a (3.171 g, 3.0 mmol), compound 2 (0.249 g, 1.5 mmol), and DMAP (0.0275 g, 0.225 mmol) were dissolved under Ar in 8 mL of CH2Cl2. The reaction mixture was stirred for 48 h at 23 °C at which time 1H-NMR indicated complete alcoholysis of the anhydride. DCC (0.371 g, 1.8 mmol) and DPTS (0.096 g, 0.3 mmol) were added to the reaction mixture. After 24 h, the reaction mixture was filtered and the solvent was removed by rotary evaporator. The crude product was chromatographed on SiO2 using 4:1 hexanes/ ethyl acetate and then precipitated into cold MeOH to yield 2.58 g (76.3%) of a white powder. Mp 52-53 °C. Purity (HPLC), 99+%; TLC (3:1 hexanes/EtOAc), Rf ) 0.4. 1HNMR, δ (CDCl3, TMS, ppm): 0.88 (t, 24H, CH3, J ) 7.0 Hz), 1.2-1.6 (m, 144H, (CH2)9), 1.78 (m, 16H, CH2CH2OPh), 2.84 (s, 2H, OCHCHCO2), 3.95 (t, 16H, J ) 7.3 Hz, CH2OPh), 4.85 (s, 8H, PhCH2OPh), 4.92, 5.05 (dd, 4H, J ) 16.2 Hz, PhCH2OCO), 5.25 (s, 2H, OCHCHCO2), 6.46 (s, 2H, dCH), 6.60 (bs, 4H, ArH ortho to CH2OCO), 6.78 (m, 14H, ArH). 13C-NMR, δ (C6D6, TMS, ppm): 14.4 (CH3), 22.5 (CH2CH3), 26.0-30.2 [(CH2)8], 32.6 (CH2CH2CH3), 47.3 (C(O)CH), 66.8, 67.3 (ArCH2OC(O)), 68.0-71.1 (CH2OAr), 71.3-71.5 (ArCH2OAr), 79.8 (C(O)CHCH(O)CHd ), 105.4 (ArC para to CH2OCO), 106.7 (ArC ortho to -CH2OCO), 114.4-115.4 (ArC ortho to -OCH2CH2-), 122.4-122.5 (ArC meta/para to -OCH2CH2-), 134.2, 134.3 (ArC ipso to -CH2-OAr), 136.8 (CHdCH), 139.3 (ArC ipso to CH2OCO), 159.5-151.7 (ArC ipso to -OCH2CH2-), 163.2 (ArC meta to CH2OCO), 171.4 (CdO). exo,exo-5,6-Bis[[[3,4-bis(3,4-(dodecyl-1-oxy)benzyloxy)]benzyloxy]carbonyl]-7-oxabicyclo[2.2.1]hept-2-ene (3b).


From 1b (3.171 g, 3.0 mmol), 2 (0.249 g, 1.5 mmol), DMAP (0.0275 g, 0.225 mmol), DCC (0.371 g, 1.8 mmol), and DPTS (0.096 g, 0.3 mmol) in 8 mL of CH2Cl2, 2.51 g (74.4%) of a white powder was obtained. Mp 55-57 °C. Purity (HPLC), 99+%. TLC (5:1 hexanes/EtOAc), Rf ) 0.3. 1H-NMR, δ (CDCl , TMS, ppm): 0.88 (t, 24H, CH , J ) 3 3 6.9 Hz), 1.2-1.7 (m, 144H, (CH2)9), 1.82 (m, 16H, CH2CH2OPh), 2.80 (s, 2H, OCHCHCO2), 3.95 (m, 16H, CH2OPh), 4.83, 4.95 (dd, 4H, J ) 16.4 Hz, PhCH2OCO), 5.00 (m, 8H, PhCH2OPh), 5.23 (s, 2H, OCHCHCO2), 6.41 (s, 2H, dCH), 6.80-7.0 (m, 18H, ArH). 13C-NMR, δ (C6D6, TMS, ppm): 14.2 (CH3), 22.4 (CH2CH3), 26.3-30.2 [(CH2)8], 32.5 (CH2CH2CH3), 47.8 (C(O)CH), 65.2, 67.9 (ArCH2OCO), 68.3-71.4 (CH2OAr), 71.8-72.6 (ArCH2OAr), 79.1 (C(O)CHCH(O)CHd), 110.8 (ArC ortho to -CH2OCO), 113.6 (ArC meta to CH2OCO), 114.4-115.4 (ArC ortho to -OCH2CH2-), 118.3 (ArC ortho to -CH2OCO), 122.4122.5 (ArC meta/para to -OCH2CH2-), 134.2, 134.3 (ArC ipso to -CH2-OAr), 136.8 (CHdCH), 129.4 (ArC ipso to CH2OCO), 148.4 (ArC para to CH2OCO), 148.5 (ArC meta to CH2OCO), 150.5-151.7 (ArC ipso to -OCH2CH2-), 171.4 (CdO). exo,exo-5,6-Bis[[[3,4-bis(3,4,5-(dodecyl-1-oxy)benzyloxy)]benzyloxy]carbonyl]-7-oxabicyclo[2.2.1]hept-2-ene (3c). From 1c (6.120 g, 4.0 mmol), 2 (0.332 g, 2 mmol), DMAP (0.0366 g, 0.3 mmol), DCC (0.494 g, 2.4 mmol), and DPTS (0.127 g, 0.4 mmol) in 10 mL CH2Cl2, 4.56 g (71.1%) of a white powder was obtained. Mp 46-48 °C. Purity (HPLC), 99+%. TLC (5:1 hexanes:EtOAc), Rf ) 0.45. 1H-NMR, δ (CDCl3, TMS, ppm): 0.88 (t, 36H, CH3, J ) 6.7 Hz), 1.11.7 (m, 216H, (CH2)9), 1.83 (m, 24H, CH2CH2OPh), 2.81 (s, 2H, OCHCHCO2), 3.7-4.1 (m, 24H, CH2OPh), 4.815.10 (m, 16H, PhCH2OCO, PhCH2OPh), 5.23 (s, 2H, OCHCHCO2), 6.45 (s, 2H, dCH), 6.61-7.0 (m, 22H, ArH). 13C-NMR, δ (C D , TMS, ppm): 14.0 (CH ), 22.5 (CH 6 6 3 2 CH3), 26.6-30.8 [(CH2)8], 32.5 (CH2CH2CH3), 47.2 (C(O)CH), 66.5, 66.9, 67.2 (ArCH2OCO), 68.2-71.0 (CH2OAr), 71.2-71.5 (ArCH2OAr), 79.6 (C(O)CHCH(O)CHd), 102.8, 103.7 (ArC ortho to -CH2OCO), 114.4-115.4 (ArC ortho to -OCH2CH2-), 122.4-122.5 (ArC meta/para to -OCH2CH2-), 134.2, 134.3 (ArC ipso to -CH2-OAr), 136.8 (CHd CH), 132.6, 135.3 (ArC ipso to CH2OCO), 139.3 (ArC para to CH2OCO), 150.5, 151.2 (ArC ipso to -OCH2CH2-), 153.2 (ArC meta to CH2OCO), 170.1 (CdO). General Procedure for the Ring Opening Metathesis Polymerization of Monomers 3 Using RuCl2(dCHPh)(PCy3)2 Initiator (5). Polymerization of exo,exo-5,6-Bis[[[3,5-bis(3,4-(dodecyl-1-oxy)benzyloxy)]benzyloxy]carbonyl]-7-oxabicyclo[2.2.1]hept-2-ene (3a). Monomer 3a (typically, 0.15 g) was placed in a flame-dried Schlenk tube containing a Teflon-coated magnetic stirring bar. The tube was sealed with a rubber septum and placed under vacuum, followed by an Ar backflush. Freshly distilled CH2Cl2 was transferred to the tube to form a solution with [M]0 ) 0.182 M (unless specified otherwise). The solution was degassed by successive freeze-pump-thaw cycles. The RuCl2(d CHPh)(PCy3)2 initiator was added under Ar to the stirred monomer solution as indicated by the desired degree of polymerization. The tube was sealed and the mixture was

16


stirred from 3 to 72 h at 23 °C depending on the desired DP. The conversion and molecular weight was followed by sampling the reaction mixture followed by gel permeation chromatography (GPC) analysis. The propagating carbene was observed by 1H NMR (19.5 ppm, multiplet, 1H). The reaction was terminated by addition of about 2 mL of ethyl vinyl ether and stirring for 1 h. The polymer was purified by passing through a basic Al2O3 column using hexanes eluent. Precipitation into cold MeOH resulted in a white polymer. Thermal transitions and corresponding enthalpy changes are listed in Table 1. 1H-NMR, δ (CDCl3, TMS, ppm): 0.88 (t, 24H, CH3, J ) 6.9 Hz), 1.27 (overlapped peaks, 144H, (CH2)9), 1.78 (m, 16H, CH2CH2OPh), 3.033.18 (bs, 2H, OCHCHCO2), 3.56-3.80 (bs, 16H, CH2OPh), 4.48-5.02 (b, 22H, PhCH2OPh, CO2CH2Ph; OCHCHCO2), 5.80-5.95 (b, 2H, dCH, cis and trans), 6.30-7.21 (bm, 18H, ArH). 13C-NMR, δ (C6D6, TMS, ppm): 14.5 (CH3), 22.4 (CH2CH3), 27.3-30.6 ((CH2)8), 32.6 (CH2CH2CH3), 54.0 (C(O)CH), 68.1-68.6 (ArCH2OC(O), CH2OAr), 72.1 (ArCH2OAr, meta to -CH2OCO), 78.6-79.4 (C(O)CHCH(O)CHd , β to cis), 81.7-83.4 (C(O)CHCH(O)CHd, β to trans), 108.4 (ArC ortho to -CH2OCO), 114.8-115.2 (ArC ortho to -OCH2CH2-), 128.1-129.8 (ArC para to -OCH2CH2-, ArC meta to -OCH2CH2-), 136.6-137.2 (CHdCH), 139.7 (ArC ipso to CH2OCO), 154.5 (ArC meta to CH2OCO), 160.4 (ArC ipso to -OCH2CH2-), 171.6 (CdO). Acknowledgment. We thank the National Science Foundation (DMR-9708581) for financial support and Professor S. Z. D. Cheng of University of Akron for the density measurements and access to his X-ray facilities. References and Notes (1) Singer, S. J.; Nicolson, G. L. Science 1972, 175, 720. (2) Yamauchi, K.; Kinoshita, M. Prog. Polym. Sci. 1993, 18, 763. (3) De Rosa, M.; Trincone, A.; Nicolaus, B.; Gambacorta, A. Life Under Extreme Conditions; di Prisco, G., Ed.; Springer-Verlag: Berlin, 1991; p 61. (4) Reinitzer, F. Monatsh. Chem. 1888, 9, 421. (5) Percec, V.; Tomazos, D. In ComprehensiVe Polymer Science, First Suppl.; Allen, G., Ed.; Pergamon Press: Oxford, 1992; p 299. (6) Percec, V. Macromol. Symp. 1977, 117, 267. (7) Percec, V.; Kawasumi, M.; Rinaldi, P. L.; Litman, V. E. Macromolecules 1992, 25, 3851. (8) Percec, V.; Kawasumi, M. J. Chem. Soc., Perkin Trans. 1 1993, 1319. (9) Percec, V.; Turkaly, P. J.; Asandei, A. D. Macromolecules 1997, 30, 943. (10) Percec, V. Pure. Appl. Chem. 1995, 67, 2031. (11) Percec, V.; Kawasumi, M. Macromolecules 1992, 25, 3843. (12) Percec, V.; Chu, P.; Kawasumi, M. Macromolecules 1994, 27, 4441.

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