A Crystalline 2,3-exo-Disyndiotactic Dicyclopentadiene Tetramer

Article pubs.acs.org/crystal

A Crystalline 2,3-exo-Disyndiotactic Dicyclopentadiene Tetramer Arnaldo Rapallo,*,† Giovanni Ricci,† William Porzio,† Gianmichele Arrighetti,‡ and Giuseppe Leone*,† †

CNR, Istituto per lo Studio delle Macromolecole, via E. Bassini 15, I-20133 Milano, Italy CNR, Istituto di Cristallografia Sincrotrone ELETTRA, SS14 Km 163.5, I-34012 Basovizza, Trieste, Italy

‡

S Supporting Information *

ABSTRACT: Cycloolefin oligomers and polymers have attracted attention for functional materials, but they are rarely seen in the crystalline state. A new crystalline cycloolefin dicyclopentadiene tetramer, having a 2,3-exo-disyndiotactic structure, is presented. By combining global optimization techniques based on configurational sampling in generalized statistical ensembles, with very high quality synchrotron X-ray diffraction data, the molecular mass of the crystallizing oligomer was first singled out, and its structure was then determined with a high degree of accuracy. The original procedures and methods developed for this particular case can be considered a suitable reference to tackle the difficult characterization problems posed by the crystalline powders of organic materials in general.

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INTRODUCTION Vinyl-polycycloolefins formed by addition polymerization of cyclic olefins [i.e., cyclopentene, cyclobutene, and norbornene (NB)] can exhibit unique physical features greatly differing from those of linear polyolefins.1 High decomposition temperatures, high plasma etch resistance, good transparency for short wavelength radiation, and small optical birefringence make polycycloolefins of potential interest for the microelectronic industry.2a Of particular interest is vinyl-type poly(norbornene) (PNB), which is a good candidate for applications in flexible flat panel displays and microelectric and microfluidic devices.2b,c PNBs can be obtained with various catalysts based on Co, Ni, Ti, Cr, and Zr;3 up to now, however, crystalline PNBs were obtained only with Ti,4 Zr,5 and Cr complexes.6 Specifically, a 2,3-exo-diheterotactic polymer was obtained with CrCl2(dppa)MAO (MAO = methylalumoxane, dppa = bis-diphenylphosphineamine),6d while a crystalline 2,3-exo-disyndiotactic one was obtained with TiCl4-AlEt2Cl.4b The crystalline structures of the above polymers were determined, resulting in the extremely unusual disyndiotactic polymer structure.4c Indeed, the disyndiotactic PNB chains adopted a remarkable conformation in the crystalline state: wide helices with relatively compact structures were formed, leaving an empty accessible tubular channel at the core, in which guest molecules such as toluene are readily hosted. This feature made the polymer of particular interest, in view of a possible use as a porous material in sensing and recognition/separation technologies. However, these possible applications are offset by the poor solubility and hard processability of the polymeric material. Aiming to overcome this drawback, we conceived the use of functional norbornenes such as 5-vinyl-2-norbornene, 5-ethylidene-2norbornene, norbornadiene, and dicyclopentadiene (DCPD). In addition to the ready availability of these monomers, the resulting polymers would be expected to have double bonds which might be useful for further chemical modification and © XXXX American Chemical Society

functionalization. Following the ideas that the presence of substituents on the NB unit could improve the processability of the obtainable polymers, and that the polymers from substituted norbornenes and their functionalized derivatives (e.g., epoxidized derivatives) could adopt helical structures similar to that found for disyndiotactic PNB, thus yielding interesting polar tubular molecules with apolar cavities, the DCPD polymerization with the catalytic system TiCl4−AlEt2Cl was investigated first.7 In this first stage of our study, low molecular weights (Mw’s) were obtained, the soluble fractions exhibiting a Mw value in the range around 400−760 g/mol, suggesting the presence of oligomers containing from 3 to 6 DCPD units. Very interestingly, however, it was found that the heptane soluble fractions, in particular that obtained at DCPD/Ti = 11, were highly crystalline, contrary to the diethyl ether, and the heptane insoluble fractions were largely amorphous. Unlike the case of the 2,3-exo-disyndiotactic NB heptamer (Chart 1A), previously Chart 1. (A) Crystalline cis-2,3-exo-Disyndiotactic NB Heptamer,8 and (B) Crystalline cis-2,3-Exo-disyndiotactic DCPD Tetramer Obtained with TiCl4−AlEt2Cl

Received: July 10, 2014 Revised: September 4, 2014

A

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Table 1. Polymerization of DCPD with TiCl4/AlEt2Cla diethyl ether soluble fraction

heptane soluble fraction

heptane residue

DCPD/Ti

%

Mwb

Mw/Mnb

%

Mwb

Mw/Mnb

%

Mwb

11 50

24 0

447

1.4

50 29

620 760

1.6 1.4

26 71

6540 insoluble

Mw/Mnb 2.5

Polymerization conditions: heptane; total volume, 16 mL; AlEt2Cl, 4 mmol; TiCl4, 2 mmol; temperature, 0 °C; time, 28 h; monomer conversion, ∼100%. bMolecular weight (Mw, in g/mol) and molecular weight distribution Mw/Mn by SEC.

a

were stopped with methanol containing a small amount of hydrochloric acid; the precipitated polymers were collected by filtration, repeatedly washed with fresh methanol, and finally dried in a vacuum at room temperature to constant weight. Polymer Fractionation. The polymers were fractionated by boiling solvent extraction with a Kumagawa extractor. The polymerization products were first extracted with boiling diethyl ether for about 16 h, to give the ether soluble fraction and then with heptane, to give the heptane soluble fraction and the heptane insoluble fraction. Characterization. 13C and 1H NMR measurements were carried out on a Bruker Avance 400 spectrometer. The spectra were obtained in C2D2Cl4 at 103 °C using hexamethyldisiloxane, HMDS, as internal standard. FTIR measurements were carried out on a PerkinElmer Spectrum two equipped with UATR. The concentration of the polymer solutions was about 10 wt %.Waters GPCV2000 size exclusion chromatography (SEC) system using two online detectors: a differential viscometer and a refractometer. The experimental conditions consisted of three PL Gel Olexis columns, o-dichlorobenzene as the mobile phase, 0.8 mL × min−1 flow rate, and 145 °C temperature. Calibration of the SEC system was constructed using 18 narrow Mw/Mn poly(styrene) standards with molar weights ranging from 162 to 5.6 × 106 g × mol−1. For SEC analysis, about 12 mg of polymer was dissolved in 5 mL of o-DCB with 0.05% of BHT as antioxidant. The wide-angle X-ray diffraction (XRD) data were obtained at 20 °C using a Siemens D-500 diffractometer equipped with a Siemens FK 60−10 2000W tube (Cu Kα radiation, λ = 0.154 nm at a power of 40 kV × 40 mA). The measurement range was from 5 to 35 2θ° at 0.05 2θ° intervals. X-ray patterns of the compound were obtained at the hard beamline of the Italian Synchrotron Elettra (Trieste, Italy). Powder was loaded in a glass capillary of 0.3 mm. Diffraction patterns were recorded in transmission geometry, and the incoming beam was perpendicular to the capillary which rotated around its axis during exposition to improve powder averaging. Bidimensional CCD detector of 165 mm diameter was placed normal to the incident beam direction and about 70 mm from the sample. A 200 × 200 μm wide monochromatic beam at the wavelength of λ = 0.155 nm allowed measurement of patterns up to a 2θ° angle of 44.03 (maximum resolution ≈ 2.07 Å).

isolated as single crystals, and structurally characterized by single crystal X-ray diffraction techniques,8 DCPD crystalline oligomers could be obtained only in the form of powders. This fact, together with the uncertainty on the mass of the crystalline species, made the structural characterization of this intriguing molecular crystal a real challenge, impossible to be tackled by standard direct space techniques for structure determination from powder diffraction data. The ambiguity about the molecular mass presented by the DCPD oligomers hampered the possibility to identify in advance a unique cell just by using the peaks positions in the profile, since a huge number of equally well promising solutions to this indexing problem could be found even confining the search to the most probable triclinic and monoclinic space groups, given the large ranges of volumes, hence cell parameters, to be searched. To overcome these difficulties, the indexing and structure solution steps were to be performed together, using also the crystal energy as a further supporting criterion for both reducing the cell parameters space and selecting feasible structures among which to search for the solution of the crystallographic problem. This was done as described in the text, by the new global optimization direct space method VARICELLA,9 recently introduced to face crystal structure determination problems in cases where the prior information on the system is lacking to a large extent and cannot be entirely retrieved from the available experimental profile. The methodology presented here is original, proven to be successful and of very general applicability. Moreover, the materials are very promising and worth to be an object of further studies aimed to obtain polymers of high molecular mass. For these reasons the presented work can be considered a suitable reference to tackle the difficult characterization problems posed by the crystalline powders involving polymers of bulky species such as NB and DCPD, or their possible chemical derivatives, specially tailored for functional applications.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

According to that already reported for NB, DCPD was initially polymerized at two different monomer/titanium molar ratios, 11 and 50 (see Table 1); the same molar ratios permitted isolation from NB of the 2,3-exo-disyndiotactic heptamer and the 2,3-exo-disyndiotactic polymer, respectively. The products obtained were examined by NMR and FTIR spectroscopy. 13C NMR (see Supporting Information (SI), Figure S1) revealed the extreme complexity of these materials in terms of both molecular microstructure and mass heterogeneity. Vinyl addition polymers or oligomers were obtained (Scheme 1), as indicated by NMR and FTIR spectra.10 The crude products were successively fractionated with boiling diethyl ether and heptane, allowing isolation of different fractions. As mentioned above, low molecular weights were generally obtained, the soluble fractions exhibiting a Mw value in the range around 400−760 g/mol, suggesting the presence of oligomers containing 3−6 units of DCPD. The molecular weights seemed

Materials. AlEt2Cl (Aldrich, 97% pure) and TiCl4 (Aldrich, 99.95% pure) were used as received without further purification. Heptane (Aldrich, 99%) was dried by refluxing for about 10 h over Kdiphenylketyl and then distilled and stored over molecular sieves under nitrogen. Cyclopentadiene dimer (Fluka, ∼90% pure) was heated at 150 °C, and the fraction boiling at 40 °C was collected and kept at −78 °C and then used in the polymerization runs. Deuterated solvent for NMR measurements (C2D2Cl4) (Cambridge Isotope Laboratories, Inc.) was used as received. Polymerization. All manipulations of air- and/or moisturesensitive materials were carried out under an inert atmosphere using a dual vacuum/nitrogen line and the standard Schlenk-line techniques. The experiments were carried out in a 25 mL Schlenk flask containing a stirring bar. The polymerization reactor was first dried by heating at 110 °C, and then a vacuum was applied for 1 h at 40 °C. The reactor vessel was charged with heptane and monomer and brought to the desired polymerization temperature of 0 °C. The polymerization was started by adding AlEt2Cl and TiCl4 in that order. Polymerizations B

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experimentally observed molecular masses. The tentative molecules were chosen to have the double bonds disposed as in Scheme 1. The crystal configurational sampling was performed using the Allinger’s et al. MM3 force field11 in its newest version MM3-2000 as implemented in TINKER 4.2.12 The nonbonded interactions were smoothly switched off between the starting and ending cutoffs of 8 and 10 Å, respectively. Finally, the manually background−subtracted XRD spectrum was used as the reference experimental powder pattern. The cell dimensions were allowed to vary so that the density of the crystals was in the range between 0.8 and 1.2 g/cm3. The P1̅ space group was chosen as the simplest one containing the center of inversion necessary to make the asymmetric unit coexist into the same cell together with its enantiomer, and to minimize the size of the system to be managed, thus reducing the computational burden. The underlying idea was that even if the chosen triclinic space group was incorrect, the crystal built with molecules containing the true number of units, should have anyway shown maximum agreement between the calculated and experimental powder profiles with respect to the other crystals, the features correctly captured in this case being two (number of units and center of inversion), instead of one (center of inversion). This hypothesis was confirmed by the calculations; indeed the best figure of merit (minimum value of the standard disagreement factor Rwp) was obtained from the crystal of tetramers, the other cases showing poorer agreement, as can be seen in Table 2. In Figure 1 the calculated

Scheme 1. Polymerization of DCPD with TiCl4/AlEt2Cl

to increase with increasing the DCPD/Ti molar ratio, as indicated by the increased amount of the heptane insoluble fraction at DCPD/Ti = 50 (71% vs 26% at DCPD/Ti = 11). Concerning the crystallinity of the various isolated fractions, it is worthwhile to note that the diethyl ether soluble and the heptane insoluble ones were amorphous (see SI, Figure S2), while the heptane soluble fractions, in particular that obtained at DCPD/TI = 11, were highly crystalline. Indeed, the powder XRD spectrum contained a high number of peaks up to the highest values of 2θ. The XRD features, the presence of a range of low molecular masses (Mw/Mn = 1.6) as well as the complexity of the 13C NMR spectra, induced us to argue that the sample may contain more than one crystalline species and/ or disordered crystals. Nevertheless, the case of the disyndiotactic NB heptamer8 showed that only molecules of a well-defined size are able to crystallize in a unique form when this particularly bulky monomer is oligomerized. For this reason it was conjectured, by analogy, that also oligomers of the bulkier DCPD give a unique crystalline form, by packing disyndiotactic molecules that, eventually, differ only in the position of the double bond in the cyclopentenes. According to this framework, the high number of peaks in the diffraction pattern was considered to be compatible with a low symmetry crystal, either triclinic or monoclinic. Before any attempt to a structure solution was possible, an original computational strategy was to be implemented in order to assess the mass of the crystallizing species, by taking advantage of the possibilities offered by the VARICELLA method.9 The particular Monte Carlo procedures developed in the mentioned algorithm allow for sampling the cell parameters together with position, conformation, and orientation of the molecules within the crystal, and the use of the crystal energy together with the experimental powder spectrum as a guide to the sampling of the configurational space, permit the selection of physically meaningful structures among which to search for the solution of the crystallographic problem. This made it possible to exploit the powder XRD data for assessing the exact molecular mass of the crystallizing oligomers, given the mass ranges provided by the SEC experiments. In general terms, such a goal is impossible to achieve within the traditional techniques in crystal structure determination from powder XRD data that require prior knowledge of the unit cell. Indeed uncertainties in the correct mass, in the crystal density, and in the space group make it necessary to enlarge the search space of the cell parameters, so that any standard indexing procedure yields a huge number of equally well promising unit cells which cannot be processed on a one-by-one fashion to solve the crystallographic problem. In order to solve the mass dilemma, crystals in the P1̅ space group were concerned, built up with Z = 2 oligomers containing from three to six units, according to the

Table 2. Best Figure of Merit (Minimum Value of Rwp) Found for Oligomers with 3−6 DCPD Units in the Space Group units of DCPD

best figure of merit (minimum Rwp) (%)

3 4 5 6

43 37 54 64

profile for the best P1̅ crystal of tetramers is shown together with the experimental one. From here onward, this crystal structure will be referred to as the reference structure, and the related cell, as the reference cell. The cell parameters corresponding to the best solution were a = 6.8 Å, b = 11.0

Figure 1. Experimental manual background-subtracted XRD spectrum (red line) and calculated profile for the best P1̅ crystal of tetramers (green line). C

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Å, c = 18.7 Å, α = 94.6°, β = 91.1°, γ = 102.2° (V = 1361.83 Å3). Although Figure 1 shows, in absolute terms, quite poor agreement between the calculated and experimental profiles, many features of the experimental spectrum were captured, and the obtained crystal structure showed several interesting features: (i) the independent tetramer inside the cell was found in a reliable conformation, this being a perturbation of the global minimum of the isolated molecule, and (ii) the angles α and β appeared as rough approximations of a right angle. All these facts, together with the impossibility of indexing the peaks at the smallest 2θs, led to the hypothesis that this P1̅ crystal was, actually, an approximation of half of a bigger centersymmetric monoclinic crystal, with Z = 4 and one molecule as asymmetric unit. Monoclinic P21/c was then chosen as the tentative space group, being the most recurring in organic crystals and being the same in which the NB heptamers crystallize into.8 To check these conjectures, very high quality synchrotron data were collected, and a list of peak positions was extracted from them. Then an indexing procedure was performed by a house-made program based on Monte Carlo sampling, by restraining the volumes of the tentative cells to fulfill the already mentioned density limits. Rather than with respect to the disagreement between experimental and calculated peak positions, the cells were ranked according to their similarity with the reference cell. Among many output cell proposals, only one seemed to double the reference cell, so a Pawley refinement was performed on it with the MATSTUDIO package13 using the synchrotron data, and finally obtaining a = 11.381 Å, b = 21.440 Å, c = 13.969 Å, β = 113.56° (V = 3124.43 Å3), with a standard disagreement factor Rwp/bkg = 0.0182. The tetramer obtained in the reference crystal structure was introduced into the Pawley refined cell, replicated according to the symmetry operations of the P21/c space group and a fixed cell sampling with VARICELLA was performed on it. In few steps the correct position and orientation were found and the run was stopped. At this point all the 16 possible tetramers differing in the position of the double bonds in the cyclo(pentene)s were replaced to the asymmetric unit in the crystal, and the systems were energy minimized at a fixed cell, by imposing the symmetry operations of the group and applying the periodic boundary conditions. The abovementioned MM3 force field was employed for these molecular mechanics calculations, and the nonbonded interactions were smoothly switched off by 18 and 20 Å cutoffs. The molecule giving the minimum energy of the crystal (Chart 2) was then chosen as the most representative one for describing the crystal itself. In Table 3 the crystal energies obtained by packing the various tetramers (a−p), differing in the disposition of the double bonds as indicated in terms of the numbering shown in Chart 2, are reported using the energy of the most stable case as the zero energy reference. As can be seen, molecules (a) and (b) yield crystals with essentially the same minimum energy value after optimization, and the same is true if they are both packed together, so, in principle, the crystal could be described in terms of a 50% mixture of these two molecules. The energy differences shown in Table 3 indicate that the real crystal certainly can accommodate into the cell other tetramers as well, so that a single molecule description is chosen, and given in terms of the molecule (a). This is not a problem for this particular crystal, since the shape of all the packed tetramers is actually very similar in all cases, and the coexistence of the

Chart 2. Asymmetric Unit Giving the Minimum Energy Crystal According to the MM3 Force Fielda

a

The carbon numbering according to the systematic molecular naming conventions is also shown.

Table 3. Crystal Energies Obtained by Packing the Various Tetramers (a−p), Differing in the Disposition of the Double Bondsa tetramer (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) a

positions of the double bonds 2-3 1-2 1-2 2-3 2-3 2-3 2-3 1-2 2-3 1-2 2-3 2-3 1-2 1-2 1-2 1-2

2′-3′ 2′-3′ 2′-3′ 2′-3′ 2′-3′ 1′-2′ 1′-2′ 1′-2′ 1′-2′ 2′-3′ 2′-3′ 1′-2′ 1′-2′ 2′-3′ 1′-2′ 1′-2′

1″-2″ 1″-2″ 2″-3″ 2″-3″ 1″-2″ 1″-2″ 1″-2″ 1″-2″ 2″-3″ 1″-2″ 2″-3″ 2″-3″ 1″-2″ 2″-3″ 2″-3″ 2″-3″

1‴-2‴ 1‴-2‴ 1‴-2‴ 1‴-2‴ 2‴-3‴ 1‴-2‴ 2‴-3‴ 1‴-2‴ 1‴-2‴ 2‴-3‴ 2‴-3‴ 2‴-3‴ 2‴-3‴ 2‴-3‴ 1‴-2‴ 2‴-3‴

ΔE (kcal mol−1) 0.00 0.02 0.67 1.16 2.24 2.50 3.19 3.48 3.75 3.98 4.12 4.60 5.21 5.62 6.73 7.39

The most stable case is taken as the zero energy reference.

different kinds of molecules into the real crystal can be conveniently accounted for by just setting an appropriate value of the global temperature atomic factor. All the tetramers (a− d), having the double bonds between the carbon atoms 1‴-2‴ and 2′-3′, gave the lowest energy crystal structures, within about 1.2 kcal/mol with respect to the most stable one. Breaking this peculiar disposition of these double bonds causes the immediate increase of the crystal energy. As an example we can consider tetramer (f). Just changing the position of the double bond in the tetramer (a) from 2′-3′ to 1′-2′ causes an energy increase of 2.50 kcal/mol. These calculations neglect possible further stabilizing effects connected to presence of π electrons in the solid, since they are not modeled within the MM3 force field. Nevertheless, indication that the found packing is efficient in arranging many of the tetramers present in solution can be undoubtedly deduced from the obtained energy data. In order to finalize the structure a 100 ns constant temperature, fixed cell molecular dynamic (MD) simulation at 298 K was performed by TINKER 4.2 on the crystal of tetramers (a), with the same force field settings as those D

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employed in the crystal energy optimizations. An average thermalized conformation to be used as a rigid body in the subsequent Rietveld refinement was thus obtained. The Rietveld refinement was carried out with the MATSTUDIO package, obtaining a structural determination of very high quality, as can be qualitatively appreciated from Figure 2, and

Table 4. Torsional Angles along the Backbone for the DCPD Tetramer (Chart 1B) and the NB Heptamer (Chart 1A) Described in Ref 8 angular value (deg) torsion (I) (II) (III) (IV) (V) (VI) (VII)

7 - 6 - 6′ - 7′ 6 - 6′ - 7′ - 6″ 6′ - 7′ - 6″ - 7″ 7′ - 6″ - 7″ - 6‴ 6″ - 7″ - 6‴- 7‴ 7″ - 6‴- 7‴- CEt1 6‴- 7‴- CEt1 - CEt2

DCPD tetramer

NB heptamer

−78.1 8.7 133.8 9.2 −74.7 −5.3 155.8

−89.6 16.6 149.8 19.9 −88.5 3.5 166.2

the couple of bonds 6′-6, 7′-6″, and 6″-7′, 7″-6‴, respectively, is much greater in the case of the NB heptamer than in the case of the DCPD tetramers. This means that the latter suffer from less intramolecular strain than the heptamer, though some short carbon−carbon distance below 3.5 Å is found in the tetramer as well as in the heptamer.8 These short distances between carbon atoms belonging to different monomers and separated by four or more bonds in the tetramer are reported in Table S1, SI. In a previous molecular modeling study on the isolated NB heptamer,4b it was shown that all its most stable conformations in solution are concave in shape and that the last NBs are embedded into the cavity formed by the initial ones, this making difficult further insertions of NBs during the polymerization process due to steric hindrances, at least at low ratios of NB/Ti in the reaction environment. Similar arguments can explain the low molecular masses found in the powder samples of polymerized DCPD. Among the oligomers formed in solution, only the tetramers can adopt such a molecular conformation that allows for crystal packing. The other terms remain in the semicrystalline powder and contribute to the amorphous part of it, which is the major component, and could be evaluated by the XRD experimental and calculated background profiles to amount to 71%. The ratio of the crystalline to the amorphous part was evaluated by taking as zero reference the intensity at the highest 2θ value.

Figure 2. Synchrotron XRD experimental (red) and calculated (green) spectra, together with background (cyan), and difference spectra (pink). The difference spectrum is shifted upward for figure clarity. In the inset the high-angle portion [30−44° (2θ)] of the spectrum is shown.

quantitatively from the final standard disagreement factor Rwp/bkg = 0.0347. The final cell parameters were a = 11.378 Å, b = 21.427 Å, c = 13.951 Å, β = 113.53° (V = 3118.40 Å3). The rigid-body refinement modified only to a negligible extent the position and orientation of the asymmetric unit in the cell with respect to the MD outcome, this preserving the energetic consistency of the final crystal structure, and strongly increasing the reliability of the crystallographic result. The density of the crystal is 1.199 g/cm3, which is quite high compared to the density of 1.142 g/cm3 of the crystalline NB heptamer described in ref 8. Despite this higher density, connected to the presence of unsaturated carbons in the molecules, no intermolecular carbon−carbon distances are found to be shorter than 3.65 Å (to be compared to the reference C−C distance of 3.4 Å given in ref 14). These facts, together with the high value of 0.7 of the packing factor, defined as the ratio of the van der Waals volume of the molecules to the cell volume, clearly indicate packing closeness, i.e., the strong capacity of this molecule to give very well formed and stable crystals. The overall molecular conformation of the asymmetric unit is mainly characterized by the torsional angles along the backbone. Torsional angles are given in Table 4 according to the carbon numbering shown in Chart 2 and compared with those of the NB heptamer described in ref 8. DCPD tetramer and NB heptamer share some similarity in the sequence of the backbone torsional angles: (I) and (V) are both gauche-type states, (III) is in the anticlinal range, while (II), (IV), and (VI) are cis-type torsions. This indicates that the torsional states along the backbone are mainly dictated by the steric hindrance of the NBs in both cases and that the cyclopentene groups in the tetramer play only a minor role in this respect, due to their particular disposition in the molecule. It is interesting to note that the deviation from zero of the torsions (II) and (IV), which should correspond to perfect eclipsed conformation of

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CONCLUSIONS The polymerization of DCPD with the catalytic system TiCl4/ AlEt2Cl was examined, obtaining vinyl-type oligomers or low molecular weight polymers. Fractionation of the crude products with different boiling solvents permitted isolation of a crystalline powder containing oligomers in the range from 3 to 6 DCPD units. By exploiting the possibility to sample the molecular configurational space together with the cell parameters of model crystal structures, offered by the recently introduced VARICELLA method, the mass of the crystallizing oligomer could be assessed directly from the XRD spectrum, taking into account the range of molecular masses given by the SEC experiments. The crystallizing species was found to be a tetramer having a 2,3-exo-disyndiotactic structure. The crystal structure, constituted by closely packed single molecules, was finally determined with a high degree of accuracy by combining molecular mechanics and molecular dynamics calculations with very high quality synchrotron XRD data. The original procedures and methods developed here can be considered a suitable reference to tackle the difficult characterization problems posed by the crystalline powders of these interesting materials. E

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(9) Rapallo, A. J. Chem. Phys. 2009, 131, 044113. (10) Haselwander, T. F. A.; Heitz, W.; Maskos, M. Macromol. Rapid Commun. 1997, 18, 689−697. (11) Allinger, N. L.; Yuh, Y. H.; Lii, J.-H. J. Am. Chem. Soc. 1989, 111, 8551−8566. (12) Ren, P.; Ponder, J. W. J. Phys. Chem. B 2003, 107, 5933−5947. (13) MATSTUDIO, release 4.0; Accelrys Inc:, San Diego CA,USA, 2003. (14) Bondi, A. J. Phys. Chem. 1964, 68, 441−451.

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: 13C NMR of the heptane soluble fraction. Figure S2: XRD powder patterns of diethyl ether soluble, heptane soluble, and heptane insoluble fractions. Table S1: Shortest distances between carbon atoms belonging to different monomers and separated by four or more bonds in the tetramer. This material is available free of charge via the Internet at http://pubs.acs.org. The crystallographic coordinates are deposited in the Cambridge Crystallographic Data Centre and allocated the deposition number CCDC 1012412.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Italian Ministry of Education, University and Research (PON01_00074 DIATEME), the Italian Ministry of Foreign Affairs, Directorate General for the Country Promotion, through the Executive Programme with Argentina, and Project Regione Lombardia (3667/2013).

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REFERENCES

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dx.doi.org/10.1021/cg501033f | Cryst. Growth Des. XXXX, XXX, XXX−XXX