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Synthesis and Structure of Syndiotactic Poly(3-methyl-1-butene): A Case of 3/1 Helical Conformation for Syndiotactic Polymers Claudio De Rosa,*,† Anna Malafronte,† Miriam Scoti,† Finizia Auriemma,† Ivana Pierro,†,‡ Giuseppe Leone,‡ and Giovanni Ricci‡ †

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Monte S.Angelo, Via Cintia, I-80126 Napoli, Italy CNR-Istituto per lo Studio delle Macromolecole (ISMAC), Via A. Corti 12, I-20133 Milano, Italy

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S Supporting Information *

ABSTRACT: Syndiotactic poly(3-methyl-1-butene) (sP3MB) has been obtained for the first time by hydrogenation of 3,4-poly(isoprene), which in turn has been synthesized with the catalyst FeCl2(2,2′bipyridine)2 activated with methylaluminoxane. The synthesized sample of sP3MB is amorphous but crystallizes by stretching and annealing of the oriented amorphous phase at 60−70 °C. The diffraction image of the annealed fibers shows only three equatorial reflections and a few weak reflections distributed on three nonequatorial layer lines. From these data a model of the conformation of the chains of sP3MB is proposed. The chains of sP3MB assume in the crystalline phase a 3/1 helical conformation, providing the first example of a syndiotactic polymer with 3-fold helical chains. A high degree of disorder is present in the crystals so that the structure can be described by an arrangement of parallel conformationally ordered 3/1 helical chains and disorder in the lateral correlations between the chains.

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INTRODUCTION The discovery that organometallic complexes of transition metals can promote the polymerization of olefins to stereoregular polyolefins1−7 has allowed the production of many novel stereoregular and crystalline polymers that could not be synthesized with Ziegler−Natta catalysts. Because these complexes are soluble in common solvents, the polymerizations are conducted in homogeneous phases, resulting in catalysts characterized by a single catalytic site. This feature affords exceptional control over architecture of macromolecules, including control of concentration of stereo- and regio-defects and efficient uniform placement of constitutional defects, as comonomeric units, along the chains,1−7 with relevant impact on the final material properties. Among the rich library of homogeneous catalysts developed so far, stereorigid zirconocenes and titanocenes with C2 symmetry, such as rac-ethane(indenyl)2MCl2 (with M = Zr or Ti), in combination with methylaluminoxane (MAO),8 produce isotactic polypropylene,1,2 whereas ansa-zirconocenes with Cs symmetry can produce highly stereoregular and almost completely regioregular syndiotactic polypropylene (sPP).9,10 The prototype of this class of Cs symmetric ansa-metallocene is isopropylidene(cyclopentadienyl)(fluorenyl)zirconium dichloride (Me2C(Cp)(9-Flu)ZrCl2, Me = methyl, Cp = cyclopentadienyl, and Flu = fluorenyl).9 With some of these Cs symmetric catalysts completely new sPP samples, having high crystallinity and melting temperature, were obtained.11 The same Cs symmetric zirconocene precursors that once activated with MAO promote prevailingly syndiotactic-specific polymerization of propylene can produce highly syndiotactic © XXXX American Chemical Society

polymers of other 1-olefins, such as syndiotactic poly(1butene) (sPB) 1 2 − 1 5 and poly(4-methyl-1-pentene) (sP4MP).12,16,17 Quite surprisingly, they instead promote isotactic-specific polymerization of branched 1-olefins such as 3-methyl-1-butene and 3-methyl-1-pentene.11,18−20 In particular, in the case of 3-methyl-1-butene (3MB), polymerization with the Cs symmetric catalyst (Me)(Ph)C(Cp)(9-Flu)ZrCl2 (Ph = phenyl) gave a polymer showing X-ray powder diffraction profile of isotactic poly(3-methyl-1-butene) (iP3MB) with low crystallinity.20 The 13C NMR analysis of the obtained polymer indicated the presence of 3MB monomeric units with 4,1 enchainment arising from isomerization of the secondary 2,1 last inserted unit,20 according to a mechanism similar to the isomerization of 2,1 to 3,1 propene units and of 2,1 to 4,1 butene units in isotactic poly(1butene).20−25 Moreover, the 2D NMR spectrum confirmed that the methylene groups of the main chains are largely in meso (m) diads; that is, the regioregular 1,2 sequences of 3MB units are largely isotactic.20 The formation of isotactic poly(3-methyl-1-butene) (P3MB) with Cs symmetric metallocene catalysts, which are syndiospecific in propene, 1-butene, and 4-methyl-1-pentene polymerization, has not been completely explained so far and seems to be related to the accepted chain migratory polymerization mechanism involving skipped insertion of the monomer for these Cs catalysts.9,20,26 For slow monomer Received: July 22, 2018 Revised: October 5, 2018

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DOI: 10.1021/acs.macromol.8b01580 Macromolecules XXXX, XXX, XXX−XXX

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calcium hydride for about 4 h, then distilled trap-to-trap, and stored under nitrogen. Syndiotactic 3,4-poly(isoprene) was synthesized with the catalyst (bipy)2FeCl2/MAO (bipy = 2,2′-bipyridine) as described in ref 28. Hydrogenation Procedure. The hydrogenation was performed in a round-bottom flask equipped with a reflux condenser, a nitrogen inlet port, and a temperature controller.37,39 Typically, 4.8 g of the syndiotactic 3,4-poly(isoprene) was dissolved in 400 mL of o-xylene. The mixture was continuously stirred at room temperature until the polymer was completely dissolved. TsNHNH2 (30 g, 0.16 mol) was then added, and the mixture was refluxed by slowly heating to 120 °C. After 3 days the mixture was allowed to cool spontaneously to room temperature, and TsNHNH2 (30 g, 0.16 mol) was added. This operation is repeated once again. Upon completion of the reaction, the hydrogenated sample was hot-filtered, the volume of the filtered solution was reduced under vacuum, and the dissolved polymer precipitated with methanol and was collected by filtration. The polymer was dried under vacuum at room temperature, and then it was extracted with acetone through a Soxhlet method for 10 h to remove any excess of TsNHNH2 and products originating from TsNHNH2 decomposition. The residual sample of syndiotactic poly(3-methyl-1-butene) was finally dried under vacuum, dissolved in toluene, precipitated into methanol, and dried again under vacuum at room temperature to constant weight (yield = 4.4 g (91.6%), Mw = 392700 g/mol, Mw/Mn = 2.8). Characterization. 13C and 1H NMR measurements were performed on a Bruker Avance 400 spectrometer. The spectra were obtained in C2D2Cl4 at 103 °C (hexamethyldisiloxane, HMDS, as internal standard). The concentration of polymer solutions was about 10 wt %. 13C parameters were: spectral width 17 kHz, 90° pulse, 11.0 μs PL1, and −5.0 dB, with a delay of 16 s. Weight average molecular masses (Mw) and molecular weight distribution (Mw/Mn) were obtained by a high temperature 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, odichlorobenzene as the mobile phase, 0.8 mL min−1 flow rate, and 145 °C temperature. Universal calibration of the SEC system was performed using 18 narrow Mw/Mn poly(styrene) standards with molar weights ranging from 162 to 5.6 × 106 g mol−1. For the analysis, about 12 mg of the polymer was dissolved in 5 mL of odichlorobenzene with 0.05% of 2,6-di-tert-butyl-4-methylphenol (BHT) as the antioxidant. Details of characterization and of conformational energy calculation are reported in the Supporting Information.

insertion, as in the case of 3MB, skipped insertion may prevail, and the change of the steric control from chain migratory syndiotactic to chain-end (prevailingly) isotactic could explain the formation of isotactic instead of syndiotactic P3MB with Cs symmetric metallocene catalysts.20 However, no clear evidence is available to support this hypothesis, as for example (from 13 C NMR) the presence of a single r diad stereodefect in isotactic sequences (...mmrmmm...), typical of chain-end mechanism, or the effect of temperature on the isotacticity of the polymer.20 Because Cs metallocene catalysts fail in the syndiospecific polymerization of branched 1-olefins, such as 3-methyl-1butene, the purpose of this paper is the synthesis for the first time of syndiotactic poly(3-methyl-1-butene) (sP3MB) using a different synthetic strategy based on the synthesis of syndiotactic 3,4-poly(isoprene) and successive hydrogenation of the poly(1,3-diene). The synthesis of stereoregular poly(1,3diene)s with controlled molecular structure has been recently improved thanks to the development of a new generation of catalysts based on complexes of transition metals and lanthanides with various ligands (e.g., phosphines, imines, imino-pyridines, and cheto-imines) in combination with MAO.27 These catalysts have demonstrated to be active and highly stereospecific, giving stereoregular poly(1,3-diene)s (cis1,4-isotactic and syndiotactic, 1,2-isotactic and syndiotactic) from various dienes (such as isoprene, 1,3-pentadiene, 1,3hexadiene, 3-methyl-1,3-pentadiene, 1,3-heptadiene, 1,3-octadiene, and 5-methyl-1,3-hexadiene).27−33 From all these stereoregular poly(1,3-diene)s, a whole series of stereoregular poly(olefin)s and perfectly alternating copolymers of olefins34−36 have been obtained via the hydrogenation reaction.37−39 Some of these new polymers cannot be obtained by simple stereospecific polymerization of the corresponding monomers. In particular, in this paper the preparation of syndiotactic poly(3-methyl-1-butene) (sP3MB) by hydrogenation of syndiotactic 3,4-poly(isoprene) is presented. The syndiotactic 3,4-poly(isoprene) has been, in turn, synthesized with the catalytic system FeCl2(2,2′-bipyridine)2-MAO.28 The syndiotactic stereoregularity of the poly(diene) is preserved after hydrogenation of the double bonds on the side groups (the hydrogenation reaction does not lead to the formation of new stereocenters), and samples of sP3MB able to crystallize have been obtained for the first time. A further aim of this paper is the structural characterization of this new syndiotactic polymer (sP3MB). Although only a very low degree of crystallinity is achieved in stretched fibers of the obtained samples of sP3MB, and the corresponding diffraction patterns show only a few weak and diffuse reflections, a model of the chain conformation of sP3MB in the crystals is proposed. We show that the chains of sP3MB assume a 3/1 helical conformation and represent the first case of 3/1 helix in syndiotactic polymers.

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RESULTS AND DISCUSSION The syndiotactic 3,4-poly(isoprene) was synthesized by polymerizing isoprene with the catalytic system (biScheme 1. Polymerization of Isoprene Catalyzed by (bipy)2FeCl2/MAO and Successive Hydrogenation of the Obtained Syndiotactic 3,4-Poly(isoprene) To Give a Fully Saturated Syndiotactic Poly(3-methyl-1-butene) (sP3MB)

EXPERIMENTAL SECTION

Materials. Manipulations of air- and/or moisture-sensitive materials were performed under an inert atmosphere using a dual vacuum/nitrogen line and standard Schlenk-line techniques. Toluene (≥99.7% pure, Aldrich) was refluxed over Na for about 8 h and then distilled and stored over molecular sieves under nitrogen. o-Xylene (Aldrich, anhydrous grade), p-toluenesulfonyl hydrazide (TsNHNH2, Aldrich), deuterated solvent for NMR measurements (C2D2Cl4) (Cambridge Isotope Laboratories, Inc., Tewksbury, MA), and methylaluminoxane (MAO) (10 wt % solution in toluene, Aldrich) were used as received. Isoprene (98%, Aldrich) was refluxed over

py)2FeCl2/MAO, in toluene at −30 °C (Scheme 1).28 The 13 C NMR spectrum of the syndiotactic 3,4-poly(isoprene) together with the peaks attribution is shown in Figure 1A (C1: 110.0 ppm; C2: 145.3 ppm; C3: 40.9 ppm; C4: 37,1 ppm; C5: 16.4 ppm). The NMR spectrum indicates a content of constitutional 3,4-isoprene units of 85%, with 15% of 1,4-cis B

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Figure 1. 13C NMR spectra (C2D2Cl4, 103 °C, HMDS) of syndiotactic 3,4-poly(isoprene) (A) and syndiotactic poly(3-methyl1-butene) (sP3MB) (B).

Figure 2. X-ray diffraction profiles of the as-polymerized (a) and compression-molded (b) samples of sP3MB.

the first broad peak is evident (indicated by the arrow in Figure 2a), indicating that some weak crystalline reflections hidden by the amorphous haloes are probably present. The DSC thermograms recorded at a scanning rate of 10 °C/min during heating of the as-polymerized sample up to 200 °C, successive cooling down to −40 °C, and second heating are reported in Figure 3. It is apparent that very small and large

isoprene units. The syndiotacticity of the 3,4-isoprene sequences is not very high, with a concentration of the syndiotactic pentad rrrr of nearly 53%. The hydrogenation of the syndiotactic 3,4-poly(isoprene) was then performed by a noncatalytic way, in which the reaction is promoted by diimide (diazene, NHNH), which in turn was generated in situ through the thermolysis of ptoluenesulfonic acid (TsNHNH2)37,39 (Scheme 1). As recently shown, this procedure is an attractive process and extremely efficient in the case of 1,3-diene polymers.34−36 The complete hydrogenation of the pristine 3,4-poly(isoprene) was confirmed by FTIR and NMR (1H and 13C) spectroscopy. The 13C NMR spectrum of the saturated polymer sP3MB, together with the peaks attribution, is shown in Figure 1B: four major resonances are observed at 16.6 ppm (C1/C5), 26.5 ppm (C2), 31.0 ppm (C4), and 37.5 ppm (C3). As is clearly evident, the peaks in the olefinic region (from 100 to 150 ppm), observed in the 13C NMR spectrum of the poly(diene) (Figure 1A) and due to the olefinic carbon atoms, are completely absent in the 13C NMR spectrum of sP3MB, confirming the complete hydrogenation of the diene polymer. Moreover, the prevalent syndiotactic stereoregularity of 3,4poly(isoprene) is preserved after hydrogenation of the double bonds on the side groups (the hydrogenation reaction does not lead to the formation of new stereocenters), and syndiotactic poly(3-methyl-1-butene) (sP3MB) has been obtained for the first time, even though the degree of syndiotacticity of the 3methyl-1-butene sequences is rather low with a concentration of the syndiotactic pentad rrrr of nearly 53%. Furthermore, the macromolecules of sP3MB contain additional disorder due to the presence of constitutional defects represented by alternating propylene−ethylene units −[CH2−CH(CH3)− CH2−CH2]− arising from the hydrogenation of the 1,4-cis isoprene units present in the pristine syndiotactic 3,4poly(isoprene). Therefore, chains of sP3MB are composed of sequences of 3-methyl-1-butene units with prevailing syndiotactic configuration separated by alternating propylene− ethylene units. The X-ray diffraction profile of the as-polymerized sample of sP3MB, shown in Figure 2 (profile a), presents two large peaks at 2θ = 12° and 19°. This suggests that the sample is basically amorphous, as expected from the highly disordered molecular structure with low syndiotacticity and the presence of constitutional defects. However, a shoulder at 2θ ≈ 10° of

Figure 3. DSC thermograms recorded at 10 °C/min during heating of as-polymerized sample (a), cooling from 200 °C down to −40 °C (b), and successive second heating (c).

endothermic signals at 70 and 110 °C are present in the first heating scan (Figure 3a), indicating the presence of small crystallinity in the as-polymerized sample. Crystallization is not observed during cooling from the amorphous state at high temperature (200 °C), as shown by the absence of exothermic peaks in the thermogram of Figure 3b and the absence of endothermic peaks in the successive second heating scan (Figure 3c). Only the glass transition at nearly 20 °C is observed in both heating and cooling scans (Figure 3). Similar results have been obtained in cooling and heating DSC scans at lower scanning rates. Crystallization has not been observed even by cooling from the melt at very low cooling rates of 2 and 1 °C/min. The diffraction profile of a compression-molded sample of sP3MB, obtained by cooling the as-polymerized sample from 150 to 25 °C under a press, shows only the two amorphous halos at 2θ = 12° and 19° without shoulders, confirming that the sample is amorphous (profile b of Figure 2) and does not crystallize by cooling from the melt. Both as-polymerized and compression-molded samples do not crystallize by annealing at C

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Macromolecules various temperatures from 60 °C up to the melting temperature, assumed to be in the range 70−110 °C, as shown by the DSC curve a of Figure 3. Broad diffraction profiles similar to those of Figure 1 have been obtained for samples precipitated or solidified by casting from toluene solution, indicating that also in this condition no crystallization occurs. The difficulty in the crystallization of sP3MB is not surprising considering the low syndiotacticity and the presence of constitutional defects that both produce short regular syndiotactic sequences of 3-methyl-1-butene units. Crystallization of sP3MB has been finally achieved by stretching the amorphous compression-molded sample of Figure 2b and annealing at 60−75 °C of the fibers kept in tension. The highest crystallinity and degree of orientation have been obtained by annealing at 70 °C. The X-ray fiber diffraction pattern, and the corresponding diffraction profile read along the equatorial layer line, of fibers of sP3MB stretched at 250% deformation and annealed at 70 °C for 30 min are shown in Figure 4. The stretching at 250% strain of the

ture,20,40 confirming the syndiotacticity of sP3MB obtained after hydrogentation. The pattern of Figure 4A presents on the equator a strong reflection at 2θ = 11.7°, a less intense reflection at 2θ = 10°, and a very weak reflection at 2θ = 15.9° (Figure 4B). Moreover, two weak and broad reflections are also present on two different layer lines at values of 2θ of nearly 19°. Finally, a strong meridional reflection at 2θ = 20.2° (Bragg distance d = 4.40 Å) is present on the last visible layer line. All observed reflections with the corresponding values of 2θ, spacings, and cylindrical reciprocal coordinates are listed in Table 1. The intensities of scattering along the four layer lines including the equator, at values of the cylindrical coordinates ζ = 0, 0.079, 0.155, and 0.224 Å−1, are reported in Figure 5 as a function of the cylindrical coordinate ξ. On the layer lines at ζ = 0.079 and 0.155 Å−1 the scattering profiles of the amorphous phase are also reported (dashed lines). These amorphous profiles have been obtained from the bidimensional diffraction pattern of the amorphous undeformed compression-molded sample of Figure 2b by reading the intensity along the layer lines at heights ζ = 0.079 and 0.155 Å−1 as a function of ξ. As also evident in the diffraction pattern of Figure 4A, the diffracted intensities read along the layers at heights ζ = 0.079 and 0.155 Å−1 contain a strong contribution of the amorphous scattering (dashed lines in Figure 5B,C). Above the amorphous scattering, clear crystalline reflections at ξ = 0.191 Å−1 (on the layer at ζ = 0.079 Å−1) and at ξ = 0.150 Å−1 (on the layer at ζ = 0.155 Å−1) are present in the profiles of Figures 5B and 5C, respectively (Table 1). For the layer at ζ = 0.224 Å−1 the intensity profile of Figure 5D confirms the presence of a strong meridional reflection (ξ ≈ 0). From the experimental values of ζ = l/c = 0.079, 0.155, and 0.224 Å−1 (Table 1), values of the chain axis c = 12.7−13.4 Å have been evaluated, assuming that the three observed layer lines correspond to first (l = 1), second (l = 2), and third (l = 3) layers (Table 1), and an average value of the chain axis c = 13.0 Å has been assumed. This periodicity is much higher than the value of 5 Å typical of the trans-planar conformation in syndiotactic polymers41 and is also higher than the typical value of 7.4−7.5 Å observed for the common 2-fold helical conformation found in the crystalline forms of other syndiotactic polymers,41 such as sPP,11,41 sPB,13,14,41 and polystyrene (sPS).41,42 In any case, the value of the chain axis c of 13 Å indicates a M/N helical conformation for the macromolecules of sP3MB with the ratio between the numbers of structural units M and turns N of the helix included in the chain axis c different from 2 (M/N ≠ 2) and a line repetition symmetry s(M/N)2. In the symbol of the line repetition symmetry, s indicates the screw symmetry operation and the symbol 2 indicates that besides the screw symmetry s, a 2-fold rotation axis (2) perpendicular to the chain axis is present, as for all syndiotactic polymers.41 The meridional reflection with d = 4.40 Å (Figure 5D) suggests a value of unit height p = c/M ≈ 4.40 Å and a number of structural units in the identity period M = c/p = 13.0/4.40 ≈ 3. Therefore, the conformation of the chains of sP3MB in the crystalline phase is assumed to be a 3/1 helix with line repetition symmetry s(3/1)2. The hypothesis of 3-fold helical conformation is confirmed by the intensities of reflections observed on the three layers l, which, according to the Cochran, Crick and Vand theory,43 for a generic M/N helix, is a function of the order n of the Bessel functions Jn.

Figure 4. Bidimensional X-ray fiber diffraction image (A), and corresponding equatorial profile as a function of the Bragg angle 2θ (B), of fibers of crystalline sP3MB obtained by deformation at 250% strain of the amorphous compression-molded sample of Figure 2b and annealing at 70 °C for 30 min.

amorphous sample produces only orientation of the amorphous phase, whereas annealing at 70 °C allows crystallization, as indicated by the development in the diffraction pattern of Figure 4A of sharp reflections on the equator and a clear sharp reflection on the meridian. Improvement of crystallinity was not observed by annealing at higher temperatures before melting. The diffraction pattern of Figure 4 is different from the diffraction pattern of the isotactic poly(3-methyl-1-butene) reported in the literaD

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Table 1. Diffraction Angles (2θ), Bragg Distances (d), Cylindrical Coordinates of Reciprocal Lattice (ξ and ζ), Reciprocal Values of the Cylindrical Coordinate (1/ζ), Layer Lines (l), and Intensities (I) of Reflections Observed in the X-ray Fiber Diffraction Pattern of sP3MB of Figure 4A, Values of the Chain Axis c Evaluated from 1/ζ = c/l Assuming the Assigned Values of the Layer Line l, and Values of the Lowest Orders of the Bessel Functions n for the 3/1 Helical Conformation 2θ (deg)

d (Å)

ξ (Å−1)

ζ (Å−1)

9.95 11.9 15.9 18.4 19.4 20.2

8.89 7.41 5.57 4.83 4.57 4.40

0.112 0.135 0.180 0.191 0.150 0

0 0 0 0.079 0.155 0.224

1/ζ = c/l (Å)

l

12.66 6.45 4.46

0 0 0 1 2 3

c (Å)

Ia

n (3/1 helix)

12.66 12.90 13.38

s vs mw mw mw ms

0 0 0 1, −2 2, −1 0

a

vs = very strong; s = strong; ms = medium strong; mw = medium weak.

l = mM + nN

and trans conformations, θ1 = 60°, θ2 = 175° and θ1 = −175°, θ2 = −60°; therefore, this minimum corresponds to righthanded and left-handed 2-fold helical conformations (G+G+TT)n and (TTG−G)n, respectively, with s(2/1)2 symmetry,41 which characterize the chains in the crystal structures of the polymorphic forms of sPP,11 sPB,13,14 and sPS.42 The absolute energy minimum is characterized by values of torsion angles θ1 = G′ = 80° = 60° + δ, θ2 = T′ = 200° = 180° + δ that deviate by δ ≈ 20° from the exact staggered values T and G of the s(2/1) helix and corresponds to helical (T′T′G′G′)n conformations with s(M/N)2 symmetry with M/ N ≠ 2.41 Complex helices s(M/N)2 with M/N ≠ 2 characterize the chains in the crystal structure of form II of sPB (s(5/3)2)13,15 and of sP4MP (s(12/7/2).16,17 Figure 8 shows the refinement of the value of the torsion angle θ3 for the fixed values of θ1 and θ2 at the minima of the map of Figure 7. For the absolute minimum at θ1 = 160°, θ2 = −80° (280°), the energy curve of Figure 8A presents a deep minimum at θ3 = 158.8°, whereas for the relative minimum at θ1 = 185°, θ2 = −60° the energy curve of Figure 8B presents a minimum at θ3 = 168.4°. The Newman projections around the bond C5−C3 for these minima are shown in Figure 8A′,B′. In both minima one of the two methyl groups (atom C7) of the lateral group is in a gauche arrangement to both the adjacent backbone methylene carbon atoms C2 and C4 (Figure 8A′,B′). This double gauche arrangement of the lateral groups is analogous to that found in the s(2/1)2 helical conformation of sPB in the stable crystalline form I.13,14 The positions of the two minima of energy are compared in Figure 7 with curves that represent the loci of points (θ1, θ2) that give the 2/1 and 3/1 (or 3/2) helical conformations with unit twist t = 2πN/M = 180° and 120° (or 240° = −120°), respectively (Figure 7), calculated according to general methods.41,47−50 It is apparent that the minimum of lowest energy is near the curve of the s(3/1)2 helical conformation and t = |120°|, suggested by the diffraction data, whereas it is confirmed that the relative minimum of slightly higher energy corresponds to the 2/1 helical conformation, which does not correspond to the experimental chain conformation of the only crystalline phase of Figure 4A found so far for sP3MB. It is worth recalling that a detailed study of the helical conformations with s(M/N)2 symmetry in syndiotactic polymers has revealed important differences with the helical conformation in isotactic polymers based on the values and sign assumed by the unit twist.13,16,41,51,52 In fact, in the geometrical map of the unit twist of syndiotactic polymers, as that of Figure 9, there are two sets of curves of loci of points θ1, θ2 corresponding to any given value of the unit twist t, while in

(1)

According to the selection rule of eq. 1, for the 3/1 helix, Bessel functions with low values of the order n contribute to the first (l = 1) and second (l = 2) layer lines, besides the meridional third layer (l = 3), as observed in the experimental diffraction pattern (Table 1). The same conclusion of 3/1 helix has been obtained using the method of Mitsui,44,45 and the analytical method proposed in refs 41 and 46. It is worth noting that in the described crystal structures of the various polymorphic forms of syndiotactic polyolefins prepared with the Cs symmetric metallocene catalyst,41 the chains are mostly characterized by trans-planar conformation (in sPP11 and sPS42), by 2/1 helical conformation (in sPP,11 sPB,13,14 and sPS42), and by complex helical conformations, as s(5/3)2 in form II of sPB13,15 and s(12/7)2 in sP4MP.16,17 A 3/1 helical conformation has never been found in all the known crystal structures of syndiotactic polyolefins.41 The case of sP3MB is the first example of syndiotactic polymer with chains in 3/1 helical conformation. The hypothesis of 3/1 helical conformation for the chains of sP3MB has been confirmed by calculations of the conformational energy and of Fourier transform. The conformational energy has been calculated on a portion of the chain of sP3MB of Figure 6 having a symmetry s(M/N)2 and applying the equivalence principle41 (see the Supporting Information). The portion of chain is characterized by a sequence of the backbone torsion angles ...θ1θ1θ2θ2... according to the s(M/N)2 symmetry (Figure 6). The map of conformational energy versus the two variables θ1 and θ2 of the backbone is shown in Figure 7. For each point θ1, θ2 of the map, the torsion angle θ3 that defines the conformation of the side groups is scanned from 0 to 360° and the minimum energy is reported. The map of Figure 7 calculated for the helical symmetry s(M/N)2 contains other reasonable solutions for syndiotactic polymers, as the glide plane symmetry, because for values of θ1 and θ2 of 180° the helical symmetry degenerates into the tc glide place symmetry. In Figure 7, the absolute minimum of energy is located at θ1 = 80°, θ2 = 200°, θ3 = 300° and at the symmetrically equivalent point θ1 = −200° (160°), θ2 = −80° (280°), θ3 = 160°, whereas a relative minimum of higher energy (1.0 kJ/mol monomeric units (mu)) is located at θ1 = 60°, θ2 = 175°, θ3 = 290° and at the symmetrically equivalent point θ1 = −175°, θ2 = −60°, θ3 = 170°. Both pairs of minima correspond to the left- and right-handed s(M/N)2 helical conformation, characterized by successions of dihedral angles ...θ1θ1θ2θ2... such as ≈G+G+TT or ≈TTG−G−, with values of θ1 and θ2 nearly trans (≈T) or gauche (≈G+ or ≈G−). For the relative minimum of higher energy, the values of θ1 and θ2 are in the exact gauche E

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Figure 6. Portion of chain of sP3MB with s(M/N)2 symmetry used for the calculation of the conformational energy and definition of the torsion angles θ1, θ2, and θ3 and of the bond angles τ1 and τ2. The torsion angles θ3 and θ3′ within a racemo diad are defined with respect to the carbon atoms C6 and C11, respectively, of the lateral methyl groups as θ3 = C6−C5−C3−C2 and θ3′ = C11−C10−C8− C9. The binary axes perpendicular to the chain axis, crossing the methylene groups in the backbone, impose θ3 = θ3′.

Figure 7. Map of the conformational energy of sP3MB versus the backbone torsion angles θ1 and θ2. The helical s(M/N)2 symmetry is imposed. For each point θ1, θ2 of the map the energy is minimized by scanning θ3 every 5°. Energy curves every 2.5 kJ/(mol of monomeric units) are reported with respect to the absolute minimum of the map assumed as zero. The minima of energy are indicated with stars. The dashed curves join pairs of θ1 and θ2 that give identical values of the unit twist t = 120° (or 240° = −120°), corresponding to the 3/1 right-handed (or 3/2 left-handed) helical conformation, and t = 180°, corresponding to the 2/1 helical conformation.41

these data, it has been concluded that for syndiotactic polymers there exist two different M/N helical conformations characterized by different values of the backbone torsion angles but the same values of unit twist and unit height and, hence, the same number of structural units M and turns N of the helix per chain repeat.41,51,52 The only difference is related to the different orientation of two successive 2-fold axes normal to the chain axis.41,51,52 Figure 9 reports along with the contours lines that represent the loci of points of unit twist t = 120° and 240° (continuous lines) curves representing the loci of points of the unit height of sP3MB p = c/3 = 4.33 Å (dotted lines).41,47−50 The 3/1 helix of the chains of sP3MB must have unit height p = c/M = 13.0/3 = 4.33 Å and, therefore has been selected as that corresponding to the point (θ1, θ2) in Figure 9 that gives contemporarily the experimental values of unit twist t = 120° (or 240°) (the curves of Figures 7 and 9) and unit height p = 4.33 Å (Figure 9). This point is given by the intersection of the curves of the values of t and p in Figure 9. The map of Figure 9 presents 8 intersection points and 2 possible nonequivalent solutions. In fact, the solution a(θ1,θ2) is equivalent to the solution b(θ2,θ1), and the different solution c(θ1′,θ2′) is equivalent to d(θ2′,θ1′) because they correspond to the same

Figure 5. X-ray diffraction profiles versus the cylindrical coordinate ξ of fibers of sP3MB of Figure 4A read along the equatorial layer (l = 0) at the reciprocal coordinate ζ = 0 (A), the first layer line (l = 1) at ζ = 0.079 Å−1 (B), the second layer line (l = 2) at ζ = 0.155 Å−1 (C), and the third layer line (l = 3) at ζ = 0.224 Å−1 (D). On the first and second layer lines at ζ = 0.079 Å−1 (B) and 0.155 Å−1 (C) the intensity profiles of the amorphous phase are also reported (dashed lines).

the case of isotactic polymers only one set of symmetry related curves corresponds to any given value of the unit twist t.13,16,41 For instance, in the map of Figure 9, the continuous contour lines join the same values of the unit twist t = 120° (3/1 righthanded helix) or −120° = 240° (left-handed 3/1 helix with 3/ 2 symmetry). It results that there are two different (nonequivalent) curves relative to the unit twist t = 120°, corresponding to the 3/1 helix, along with the two corresponding different (nonequivalent) curves relative to the unit twist of opposite sign t = 240° = −120°, corresponding to the enantiomorphic 3/2 helix.13,16,41 From F

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Figure 8. Conformational energy versus the torsion angle θ3 for a chain of sP3MB having a fixed conformation of the backbone corresponding to the absolute energy minimum of the map of Figure 7 with θ1 = 160°, θ2 = −80° (A), to the relative energy minimum of the map with θ1 = 185°, θ2 = −60° (B), and to the conformation corresponding to the intersection point (a)̅ of the geometrical map of Figure 9 with θ1 = 165.2°, θ2 = −91.4° (C). Newman projections (A′−C′) around the bond C5−C3 of the conformations of the lateral groups corresponding to the energy minima in A (A′), B (B′), and C (C′).

atoms. 13,41 Among the four remaining solutions, the conformations a(θ 1 ,θ 2 ) and a̅(−θ 2 ,−θ 1 ), b(θ 2 ,θ 1 ) and b̅(−θ1,−θ2), c(θ1′,θ2′), and c(−θ 2′,−θ1′), and d(θ2′,θ1′) and ̅ d̅(−θ1′,−θ2′) correspond to pairs of enantiomorphous helices. This means that two solutions corresponding to two different 3/1 helices (a) and (c) (and the corresponding enantiomorphous helices (a)̅ and (c)) ̅ are possible, but only one solution is energetically feasible. The four solutions correspond to the pairs of torsion angles: (a) θ1 = 91.4°, θ2 = 194.8° (and that equivalent (b) θ1 = 194.8°, θ2 = 91.4°), corresponding to the left-handed 3/1 helical conformation with symmetry s(3/2)2, (a)̅ θ1 = −194.8° (165.2°), θ2 = −91.4° (268.6°) (and that equivalent (b̅) θ1 = −91.4°, θ2 = −194.8°), corresponding to the right-handed 3/1 helix, with symmetry s(3/1)2, (c) θ1 = 81.4°, θ2 = 126.2° (and that equivalent (d) θ1 = 126.2°, θ2 = 81.4°), corresponding to a different right-handed 3/1 helical conformation with s(3/1)2 symmetry and, finally, (c)̅ θ1 = −126.2° (233.8°), θ2 = −81.4° (278.6°) (and that equivalent (d̅ ) θ 1 = −81.4° (278.6°), θ 2 = −126.2° (233.8°), corresponding to a different left-handed 3/1 helical conformation with s(3/2)2 symmetry.

Figure 9. Map of the values of the unit twist t = 120° (M/N = 3/1) and t = −120° = 240° (M/(M − N) = 3/2) (continuous lines), corresponding to a 3/1 helix, and of the value of the unit height p = 4.33 Å (dashed lines) (c = 13.0 Å), as a function of the torsion angles θ1 and θ2.

conformations for chains of the type of Figure 6 with equivalent opposite chirality of the methine carbon G

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Macromolecules It is apparent from the comparison of the energy map of Figure 7 and the geometrical map of Figure 9 that only the solution (a) θ1 = 91.4°, θ2 = 194.8° (and the corresponding enantiomorphous (a)̅ θ1 = −194.8° (165.2°), θ2 = −91.4° (268.6°)) is in the region of the minimum energy, whereas the conformations (c) and (c)̅ are 3/1 helices of very high energy. With the values of the backbone torsion angles θ1 = 165.2° and θ2 = 268.6° (−91.4°) (right-handed 3/1 helix, point (a)̅ of Figure 9), the lowest energy has been obtained for a value of the torsion angle θ3 = 163.6° (Figure 8C), similar to that found in Figure 8A. Also for this conformation, the Newman projection around the bond C5−C3, reported in Figure 8C′, shows that one of the two methyl groups (atom C7) of the side group is in a gauche arrangement to both the adjacent backbone methylene carbon atoms C2 and C4. A model of minimum energy of the chain of sP3MB in 3/1 helical conformation corresponding to values of torsion angles θ1 = 165.2°, θ2 = −91.4°, and θ3 = 163.6° (right-handed) is shown in Figure 10. The model of the 3/1 helix of Figure 10 is proposed on the basis of the diffraction data and conformational energy calculations. The presence of only three equatorial reflections in the pattern of Figure 4 (Table 1) and of very broad and weak reflections on the layer lines indicates that the crystal structure is characterized by a highly disordered packing of conformationally ordered 3/1 helices, as in the well-described cases of solid mesophases.41,51,53,54 This is not surprising considering the low syndiotacticity of the sample of sP3MB and the presence of non-negligible amount of constitutional defects (propene−ethylene alternating units that separate prevailingly syndiotactic sequences of 3-methyl-1-butene units). The few and diffuse reflections and the presence of structural disorder make the determination of the unit cell and of packing mode uncertain. However, in the limit of packing of parallel 3/1 helices characterized by order only along the chain axis, without long-range lateral correlations between the chains (in the relative heights and rotations of chains), the resulting distribution of diffraction intensities in the reciprocal space should be approximated by the Fourier transform of the single helix. The Fourier transform of a single chain of sP3MB in 3/1 helical conformation has been calculated by using the Cochran, Crick, and Vand equation43 and the general equation for the calculation of the structure factors of helical molecules.41 The structure factor of a discontinuous helix M/N can be calculated by eq 2:41,43 ÄÅ ÉÑ ÅÅ i ÑÑ πy F(ξ , Ψ, ζ ) = K ∑ ∑ f j Jn (2π ξrj) expÅÅÅinjjjΨ − ψj + zzz + 2πi ζzj ÑÑÑ ÅÅÇ k ÑÑÖ 2 { n j

Figure 10. Model of 3/1 helical conformation for the chains of sP3MB, with θ1 = 165.2°, θ2 = −91.4°, and θ3 = 163.6° (righthanded).

(2)

Laue function eq 3, and the sum over n should be performed over all Bessel functions:41,43

where ξ, Ψ, and ζ = l/c are the cylindrical coordinates of the points of the reciprocal lattice, rj, ψj, zj are the cylindrical coordinates of the jth atom with scattering factor f j, calculated according to ref 55, and Jn is the Bessel function of order n. It is worth noting that for ζ= l/c, with l an integer number, the sum over j should be extended over the sole atoms in the helical residue, K is equal to 3 (≡ M, the number of residue/period), whereas the sum over the order n of the Bessel functions should be performed for the sole values of n that satisfies the selection rule l = mM + nN (eq 1). On the other hand, for ζ= l/c, with l a continuous variable, eq 2 still holds, but the sum over j should be extended over all atoms in the period, K is the

K=

sin(πNcl) sin(πl)

(3)

In eq 3, Nc is the number of identity period/chain, set equal to 10 in our calculations. In practice, only low orders of Bessel functions need to be considered for the calculation of the structure factors F with eq 2 because for a given argument (2πrjξ), the value of a Bessel function rapidly decreases with increasing the order n.41,43 The distribution of the diffracted intensity on the l-th layer line of a M/N helix corresponds to the square modulus of the structure factor F(ξ,ψ,ζ). H

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Figure 11. Diffracted intensities calculated with eq 4 for the model of single chain of sP3MB in 3/1 helical conformation of Figure 10 for the three layer lines l = 1 (A), l = 2 (B), and l = 3 (C) as a function of the cylindrical coordinate ξ and map of the diffracted intensity versus the reciprocal coordinates ξ and ζ (D). The vertical lines in (A−C) indicate the position ξ of the experimental reflections in the pattern of Figures 4 and 5 (Table 1), whereas the horizontal lines in (D) indicate the experimental values of the coordinates ζ corresponding to the three layer lines.

data support the hypothesis of the 3/1 helical conformation for the chains of the crystalline phase of sP3MB and confirm the hypothesis that the crystal structure of sP3MB is characterized by highly disordered packing of conformationally ordered 3/1 helices without long-range correlations between the chains.

The radial intensity distribution of a single chain corresponds to the average of the square modulus of F(ξ,Ψ,ζ) in eq 2 with respect to ψ, ⟨|F(ξ,Ψ,ζ)|2⟩ψ. Therefore, it is a function of ξ and ζ only and is given by eq 4 (for a righthanded helix):41 ⟨|F(ξ , ψ , ζ )|2 ⟩ψ ÄÅ ÉÑ2 ÅÅ Ñ i y 2 π lz Å j j zzÑÑÑÑ zzÑÑ + = K 2∑ ÅÅÅÅ∑ f j Jn (2πξrj) cosjjj−nψj + j Å c z{ÑÑÑ n Å k ÅÅÇ j ÑÖ ÅÄÅ ÑÉÑ2 ÅÅ i 2πlzj yzÑÑÑ zzÑÑ ∑ ÅÅÅÅÅ∑ f j Jn(2πξrj) sinjjjjj−nψj + zzÑÑ c Å n Å k {ÑÑÖÑ ÇÅ j

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CONCLUSIONS

Syndiotactic poly(3-methyl-1-butene) (sP3MB) cannot be prepared by polymerization of 3-methyl-1-butene with syndiospecific Cs symmetric metallocene catalyst. These catalysts can produce sPP, sPB, and sP4MP by polymerization of the respective monomers but produce the isotactic polymer from the polymerization of 3-methyl-1-butene. In this paper syndiotactic poly(3-methyl-1-butene) has been synthesized for the first time by hydrogenation of 3,4-poly(isoprene), which in turn has been prepared with the catalytic system FeCl2(bipy)2MAO. The synthesized sample of sP3MB is amorphous and does not crystallize by annealing or cooling from high temperature. Crystallization of sP3MB has been achieved by stretching of compression-molded samples at 250% deformation and annealing of the oriented amorphous at 60−70 °C. The X-ray fiber diffraction pattern of the annealed crystalline fibers shows few weak reflections distributed on three nonequatorial layer lines and only three reflections on the equator. These data indicate a highly disordered crystal structure. However, the fiber diffraction data indicate that the chain of sP3MB assumes an ordered 3/1 helical conformation. This hypothesis is supported by calculations of conformational energy. sP3MB represents the first case of syndiotactic polymer with chains in 3/1 helical conformation.

(4)

The calculated diffracted intensities for the model of single chain of sP3MB in 3/1 helical conformation of Figure 10 are reported for the three layer lines in Figure 11 versus the cylindrical coordinate ξ and in a map versus the cylindrical coordinates ξ and ζ. The vertical lines in Figure 11A−C indicate the position ξ of the experimental reflections in the pattern of Figure 4A (Table 1), whereas in Figure 11D the horizontal lines indicate the experimental values of the coordinates ζ corresponding to the three layer lines (Table 1). It is apparent that the calculated diffracted intensities for a single chain in 3/1 helical conformation well reproduce the position ξ of diffraction maxima observed in the diffraction pattern of Figures 4 and 5 (Table 1). Even on the equator the calculated maxima of diffracted intensities fall in the regions at ξ = 0.11−0.18 Å−1 of the three reflections observed in the experimental pattern of Figures 4 and 5A (Table 1). These I

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(11) De Rosa, C.; Auriemma, F. Structure and physical properties of syndiotactic polypropylene: A highly crystalline thermoplastic elastomer. Prog. Polym. Sci. 2006, 31, 145. (12) Albizzati, E.; Resconi, L.; Zambelli, A. (Himont Inc.) Eur Pat Appl 387609, 1991; Chem. Abstr. 1991, 114, 62980a. (13) De Rosa, C.; Venditto, V.; Guerra, G.; Pirozzi, B.; Corradini, P. Polymorphism and Chain Conformations in the Crystalline Forms. of Syndiotactic Poly(1-butene). Macromolecules 1991, 24, 5645. (14) De Rosa, C.; Venditto, V.; Guerra, G.; Corradini, P. Crystal structure of form I of syndiotactic poly(1-butene). Makromol. Chem. 1992, 193, 1351. (15) De Rosa, C.; Scaldarella, D. Crystal Structure of Form II of Syndiotactic Poly(1-butene). Macromolecules 1997, 30, 4153. (16) De Rosa, C.; Venditto, V.; Guerra, G.; Corradini, P. Chain Conformation and Unit Cell in the Crystalline Phase of Syndiotactic Poly(4-methy1−1-pentene). Macromolecules 1992, 25, 6938. (17) De Rosa, C.; Venditto, V.; Guerra, G.; Corradini, P. Crystal structure of syndiotactic poly(4-methyl-1-pentene). Polymer 1995, 36, 3619. (18) Grisi, F.; Longo, P.; Zambelli, A.; Ewen, J. A. Group 4 Cs symmetric catalysts and 1-olefin polymerization. J. Mol. Catal. A: Chem. 1999, 140, 225. (19) Oliva, L.; Longo, P.; Zambelli, A. 13C-Enriched End Groups of Poly(3-methyl-1-pentene) Prepared in the Presence of Metallocene Catalysts. Macromolecules 1996, 29, 6383. (20) Borriello, A.; Busico, V.; Cipullo, R.; Chadwick, J. C.; Sudmeijer, O. Polymerization of 3-methyl-1-butene promoted by metallocene catalysts. Macromol. Rapid Commun. 1996, 17, 589. (21) Busico, V.; Cipullo, R.; Chadwick, J. C.; Modder, J. F.; Sudmeijer, O. Effects of Regiochemical and Stereochemical Errors on the Course of Isotactic Propene Polyinsertion Promoted by Homogeneous Ziegler-Natta Catalysts. Macromolecules 1994, 27, 7538. (22) Busico, V.; Cipullo, R.; Borriello, A. Regiospecificity of 1butene polymerization catalyzed by C2-symmetric group IV metallocenes. Macromol. Rapid Commun. 1995, 16, 269. (23) Grassi, A.; Zambelli, A.; Resconi, L.; Albizzati, E.; Mazzocchi, R. Microstructure of isotactic polypropylene prepared with homogeneous catalysis: stereoregularity, regioregularity, and 1,3 insertion. Macromolecules 1988, 21, 617. (24) Busico, V.; Cipullo, R. Growing chain isomerizations in metallocene-catalyzed Ziegler-Natta 1-alkene polymerization. J. Organomet. Chem. 1995, 497, 113. (25) Resconi, L.; Camurati, I.; Malizia, F. Metallocene Catalysts for 1-Butene Polymerization Macromol. Macromol. Chem. Phys. 2006, 207, 2257. (26) Corradini, P.; Busico, V.; Guerra, G. Monoalkene polymerization: stereospecificity. In Comprehensive Polymer Science; Pergamon Press: Oxford, UK, 1989, Vol. 4, Chapter 3, pp 29−50. (27) Ricci, G.; Sommazzi, A.; Masi, F.; Ricci, M.; Boglia, A.; Leone, G. Well Defined Transition Metal Complexes with Phosphorus and Nitrogen Ligands for 1,3-Dienes Polymerization. Coord. Chem. Rev. 2010, 254, 661. (28) Ricci, G.; Morganti, D.; Sommazzi, A.; Santi, R.; Masi, F. Polymerization of 1,3-dienes with iron complexes based catalysts. J. Mol. Catal. A: Chem. 2003, No. 204-205, 287. (29) Ricci, G.; Leone, G.; Boglia, A.; Boccia, A. C.; Zetta, L. Cis-1,4alt-3,4 Polyisoprene: Synthesis and Characterization. Macromolecules 2009, 42, 9263. (30) Boccia, A. C.; Leone, G.; Boglia, A.; Ricci, G. Novel stereoregular cis-1,4 and trans-1,2 poly(diene)s: Synthesis, characterization, and mechanistic considerations. Polymer 2013, 54, 3492. (31) Ricci, G.; Leone, G.; Boglia, A.; Bertini, F.; Boccia, A. C.; Zetta, L. Synthesis and Characterization of Isotactic 1,2-Poly(E-3-methyl1,3-pentadiene). Some Remarks about the Influence of Monomer Structure on Polymerization Stereoselectivity. Macromolecules 2009, 42, 3048. (32) De Rosa, C.; Auriemma, F.; Santillo, C.; Scoti, M.; Malafronte, A.; Zanchin, G.; Pierro, I.; Leone, G.; Ricci, G. Crystal Structure and

A high degree of disorder is present in the crystals so that the structure can be described by a disordered arrangement of parallel conformationally ordered 3/1 helical chains without long-range lateral correlations between the chains. Accordingly, the distribution of the diffracted intensities along the layer lines is well reproduced by the calculated Fourier transform of the single 3/1 helical chain of sP3MB.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01580.

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Procedure of calculations of the conformational energy (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.D.R.). ORCID

Claudio De Rosa: 0000-0002-5375-7475 Anna Malafronte: 0000-0002-7854-5823 Miriam Scoti: 0000-0001-9225-1509 Finizia Auriemma: 0000-0003-4604-2057 Giuseppe Leone: 0000-0001-6977-2920 Giovanni Ricci: 0000-0001-8586-9829 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from University of Napoli Federico II, Project Ricerca di Ateneo is gratefully acknowledged.

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REFERENCES

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K

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