Crystal Structure and Properties of Isotactic 1, 2-Poly (E-3-methyl-1, 3

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Crystal Structure and Properties of Isotactic 1,2-Poly(E‑3-methyl-1,3pentadiene) Claudio De Rosa,*,† Finizia Auriemma,† Chiara Santillo,† Miriam Scoti,† Anna Malafronte,† Giorgia Zanchin,‡,§ 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 § Dipartimento di Chimica, Università degli Studi di Milano, Via C. Golgi 19, I-20133 Milano, Italy ‡

S Supporting Information *

ABSTRACT: The resolution of the crystal structure and the physical properties of isotactic 1,2-poly(E-3-methyl-1,3-pentadiene) (iPE3MPD) are presented. This new polymer is one of the very few examples of known crystalline isotactic 1,2-polydienes. It does not crystallize by cooling from the melt but crystallizes by aging the amorphous samples at room temperature for several days and successive annealing. The stretching of amorphous or low crystalline compression-molded samples gives oriented fibers of a highly disordered mesomorphic form. Well-oriented fibers of the ordered crystalline form of iPE3MPD have been prepared by stretching amorphous or low crystalline compression-molded samples and successive annealing at 60−70 °C under tension. The crystalline structure of iPE3MPD is characterized by macromolecules in 7/2 helical conformation packed in an orthorhombic unit cell with parameters a = 17.4 Å, b = 16.5 Å, c = 15.3 Å according to the space group P21ab. iPE3MPD has shown interesting mechanical properties of high deformability with elastic behavior associated with the crystallization during deformation of the mesophase that melts upon relaxation.

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INTRODUCTION Isotactic 1,2-poly((E)-3-methyl-1,3-pentadiene) (iPE3MPD) is a new polymer which has been recently synthesized by some of us1 with new catalysts, composed of cobalt−phosphine complexes as CoCl2(PRPh2)2 (with R = methyl, ethyl, npropyl, i-propyl, cyclohexyl) associated with methylaluminoxane (MAO).2−4 These catalytic systems are active for the stereospecific 1,2-polymerization of various dienes (e.g., butadiene, 1,3-pentadiene, 1,3-hexadiene, 1,3-heptadiene, 1,3octadiene, and 5-methyl-1,3-hexadiene),5−7 with stereoselectivity that depends on the catalyst and monomer structures. It has been reported, indeed, that hindered phosphine ligands (R = ipropyl or cyclohexyl) and terminally substituted 1,3-dienes (e.g., 1,3-pentadiene, 1,3-hexadiene, 1,3-heptadiene, 1,3-octadiene, and 5-methyl-1,3-hexadiene) induce formation of 1,2syndiotactic polymers, while less hindered phosphines (R = methyl, ethyl, n-propyl) and internally substituted 1,3-dienes (e.g., 3-methyl-1,3-pentadiene) favor isotactic polymers.1−7 For 3-methyl-1,3-pentadiene, among the 12 different possible stereoregular polymers,1 the new isotactic 1,2-poly((E)-3methyl-1,3-pentadiene) (iPE3MPD) has been prepared with the catalysts CoCl2(PRPh2)2.1 The polymer obtained with the catalytic systems having minimally hindered ligands (e.g., PMePh2, PEtPh2, PnPrPh2) is highly isotactic (with isotactic triad mm content higher than 90%) and highly crystalline.1 This new polymer is only the third example of known © XXXX American Chemical Society

crystalline isotactic 1,2-polydienes. The other two cases are the isotactic 1,2-polybutadiene8,9 and isotactic 1,2-poly(4-methyl1,3-pentadiene),10−12 besides the isotactic 3,4-polyisoprene.13 In the structure of isotactic 1,2-polybutadiene macromolecules in 3/1 helical conformation are packed in a tetragonal unit cell with parameters a = b = 17.3 Å, c = 6.5 Å and space group R3c or R3c̅ .9 Isotactic 1,2-poly(4-methyl-1,3-pentadiene) is instead characterized by macromolecules in 18/5 helical conformation, arranged in a tetragonal unit cell with parameters a = b = 17.8 Å, c = 36.5 Å according to the space group I4̅c2.11 The different chain conformations and crystal packing of the two isotactic 1,2-polydienes highlight the influence of the size and structure of lateral groups on the chain conformation and packing of isotactic polydienes and in particular of polymers of substitute polubutadienes.14 In our recent papers we have shown that by hydrogenation of 1,2-poly((E)-3-methyl-1,3-pentadiene) a new poly((R,S)-3methyl-1-pentene) (iP(R,S)3MP) has been obtained,15,16 with formation of a chiral carbon atom on the side groups. This saturated polymer is, however, achiral since two chiral enantiomeric monomeric units (R)-3-methyl-1-pentene and (S)-3-methyl-1-pentene result randomly enchained. Received: May 1, 2017 Revised: June 29, 2017

A

DOI: 10.1021/acs.macromol.7b00897 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules In this paper we report the resolution of the crystal structure of isotactic 1,2-poly((E)-3-methyl-1,3-pentadiene) (iPE3MPD). A preliminary analysis of the physical and mechanical properties of this new polymer is also reported. We show that the conformation and the crystal structure of the precursor polydiene iPE3MPD are different from those of the hydrogenated polymer iP(R,S)3MP highlighting the influence of the structure of side groups and the presence of stereoisomeric centers on the lateral group on the chain conformation and crystal packing.

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EXPERIMENTAL SECTION

A sample of iPE3MPD was prepared with the catalyst obtained by combining the cobalt complex CoCl2(PnPrPh2)2 (nPr = n-propyl and Ph = phenyl) with methylaluminoxane (MAO),1−5 as described in ref 1. The sample presents mass average molecular mass Mw = 81 000 g/ mol and polydispersity index Mw/Mn = 1.2 and is highly isotactic (sample of run 3 of Table 1 in ref 1). Compression-molded amorphous samples were prepared by heating the as-prepared powder samples at ≈140 °C under a press and cooling to room temperature. The amorphous compression-molded sample was crystallized by aging at room temperature for 1 month. The crystallization was completed by successive annealing at 70 °C. Oriented fibers of iPE3MPD were prepared by stretching at high deformation (about 300% deformation) the compression-molded film aged at room temperature for 1 month and successive annealing of the stretched fibers at 60 °C for 18 h keeping the fibers under tension. DSC measurements were performed using a differential scanning calorimeter DSC Mettler 822 in a flowing N2 atmosphere at rate of 10 °C/min. X-ray diffraction patterns were obtained with Ni-filtered Cu Kα radiation (λ = 1.5418 Å) with an automatic PANalytical X’Pert diffractometer operating in the Bragg−Brentano θ/2θ reflection geometry. The bidimensional patterns were recorded on a BAS-MS imaging plate (FUJIFILM) using a cylindrical camera and processed with a digital imaging reader PerkinElmer Cyclone Plus (storage phosphor system). Details of the procedure for the analysis of the mechanical properties and of calculations of conformational and lattice energies and of diffracted intensities are reported in the Supporting Information.

Figure 1. X-ray powder diffraction profile of the as-prepared sample of iPE3MPD (A) and DSC thermograms (B), recorded at 10 °C/min of heating of as-prepared sample (a), cooling from the melt (b), and subsequent heating (c).

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RESULTS AND DISCUSSION X-ray Powder and Fiber Diffraction. The X-ray diffraction profile and the DSC thermograms of the as-prepared sample of iPE3MPD are reported in Figure 1. Intense reflections at 2θ = 10.1°, 16.2°, and 19.1° are present in the diffraction profile of Figure 1A, indicating that the as-prepared sample is crystalline with degree of crystallinity of nearly 30% and melting temperature of 93 °C (DSC curve a of Figure 1B). The as-prepared sample does not crystallize from the melt, as shown by the DSC cooling curve of Figure 1B (curve b) that does not present any exothermic peak of crystallization. The Xray diffraction profile of Figure 2 of a compression-molded sample prepared by heating the as-prepared sample at 140 °C and cooled to room temperature, shows, indeed, two broad halos centered at 2θ = 10° and 19°, indicating that the sample is amorphous (profile a of Figure 2). However, the amorphous sample crystallizes by aging at room temperature for long time. This is revealed by the X-ray diffraction profile b of Figure 2 of the amorphous compression-molded sample aged at room temperature for 1 month, which presents the same diffraction peaks at 2θ = 10.1° and 16.2° observed in the profile of the asprepared sample (Figure 1A), even though a lower degree of crystallinity of 19−20% is achieved. The crystallinty improves

Figure 2. X-ray diffraction profiles of a sample of iPE3MPD prepared by compression molding and cooling from the melt to room temperature (a), after aging at room temperature for 30 days (b), and of the aged sample in (b) annealed at 70 °C for 20 h (c).

after annealing of the crystalline aged sample. The diffraction profile of the compression-molded and aged sample annealed at 70 °C for ≈20 h of Figure 2c shows sharp reflections in the same position of the as-prepared samples with a high degree of crystallinity of nearly 50%. The DSC curves of the amorphous samples of iPE3MPD recorded during cooling or second heating show a glass transition temperature of nearly 30 °C (curves b and c of Figure 1B). Oriented fibers of iPE3PMD have been prepared by stretching at high deformation (about 300%) the sample of low crystallinity (19−20%) of Figure 2b obtained by aging of the compression-molded sample at room temperature for 1 B


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Figure 3. Bidimensional X-ray diffraction patterns (A−E), and corresponding profiles read along the equator (A′−E′), of the unoriented sample of iPE3MPD of low crystallinity (19−20%) prepared by compression molding and aging at room temperature for 30 days (A, A′) (sample of Figure 2b) and of fibers of iPE3MPD obtained by stretching at 300% deformation the compression-molded and aged sample A (B, B′) and after successive annealing at 60 °C for 18 h (C, C′), of the unoriented aged sample in A annealed at 70 °C for 20 h (D, D′) (sample of Figure 2c) and of fibers obtained by stretching at 300% deformation the annealed compression-molded sample in D (E, E′).

month (Figure 3A). The corresponding X-ray fiber diffraction patterns and the intensity profiles read along the equatorial layer line, shown in Figures 3B and 3B′, present a strong and broad reflection polarized on the equator and a weak reflection polarized on a layer line. The broadness of the reflections indicates that the stretching of the low crystalline sample has produced the development of the highly disordered crystalline mesomorphic form of iPE3PMD, well oriented with chain axes aligned along the stretching direction (Figure 3B). The diffraction pattern and the equatorial profile of the stretched fiber of this mesophase after successive annealing at 60 °C for 18 h, keeping the fibers under tension, are shown in Figure 3C,C′. It is evident that annealing of the stretched fiber at a temperature higher than the glass transition greatly improves crystallinity and the mesophase crystallizes in a highly crystalline and ordered phase. This ordered form corresponds to that of the as-prepared sample of Figure 1 and of the aged compression-molded sample annealed at 70 °C of Figure 2c. In

fact, the reflections observed in the pattern of Figure 3C,C′ are the same as those observed in the diffraction profiles of the asprepared sample of Figure 1 and of the annealed sample of Figure 2c. Therefore, while stretching of a low crystalline sample (sample of Figure 2b) produces a disordered crystalline mesophase, annealing of the stretched fibers gives the ordered crystalline phase. A different result has been obtained if the compressionmolded sample of low crystallinty (19%) of Figures 3A and 2b is first annealed at 70 °C, to improve crystallinity up to achieve a high crystallinity of 50% (Figure 3D, as in the sample of Figure 2c), and then stretched (Figure 3E). The diffraction pattern of Figure 3E of fibers obtained by stretching at 300% deformation the high crystalline annealed sample of Figure 3D (or Figure 2c) is indeed similar to the pattern of Figure 3B,B′, indicating that the stretching of the crystalline form produces transformation of the high crystalline and ordered phase in the disordered mesomorphic form. The formation of a mesophase C


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Table 1. Diffraction Angles (2θo), Bragg Distances (do), Reciprocal Cylindrical Coordinates (ξo and ζo), Layer Lines l, and Intensities (Io) of hkl Reflections Observed in the X-ray Fiber and Powder Diffraction Patterns of iPE3MPD of Figures 3C and 2c, Respectivelya fiber diffraction pattern (Figure 3C) −1

−1

powder diffraction profile (Figure 2c)

2θo (deg)

d0 (Å)

ξo (Å )

ζo (Å )

l

I0

10.1 11.0 12.4 13.6 18.1 19.1 20.4 23.1 29.1 30.8 18.3 19.9 12.8 15.9 19.2 21.4 23.3 21.3 24.6 23.5 24.7 27.2 30.5 32.2

8.76 8.05 7.16 6.49 4.91 4.65 4.36 3.85 3.07 2.90 4.85 4.45 6.92 5.56 4.63 4.16 3.81 4.17 3.62 3.78 3.60 3.28 2.93 2.78

0.114 0.124 0.140 0.154 0.204 0.215 0.229 0.260 0.326 0.345 0.195 0.214 0.068 0.126 0.174 0.204 0.229 0.120 0.183 0.079 0.116 0.172 0.230 0.256

0 0 0 0 0 0 0 0 0 0 0.067 0.067 0.128 0.128 0.128 0.128 0.128 0.207 0.207 0.252 0.252 0.252 0.252 0.252

0 0 0 0 0 0 0 0 0 0 1 1 2 2 2 2 2 3 3 4 4 4 4 4

vs vs vvw vvw vvw vvw m m vvw vvw vw vw vw vs w s vw m vvw vw m vw vw vvw

b

hkl

2θo (deg)

d0 (Å)

I0

hkl

200 020 120 220 320 230 400 420 440 600 131 231 112 212 222 132 402 123 133 114 204 314 134 334

10.1 13.1 16.2 19.1 21.5 23.1 24.5 27.4 30.7 32.8

8.76 6.70 5.48 4.65 4.14 3.84 3.63 3.25 2.91 2.72

43 3 30 14 7 3 4 3 3 2

200, 020 220 212 400, 231, 222 132 420 204 314 600, 134 144

a

The hkl indices are given for an orthorhombic unit cell with axes a = 17.4 Å, b = 16.5 Å, and c = 15.3 Å. bvs = very strong; s = strong; m = medium; w = weak; vw = very weak, vvw = very, very weak.

coordinates ζo (Table 1). From the values of ζo and of the ratio 1/ζo = c/l (Table 2), reasonable values of the chain axis c

by deformation starting from an unoriented ordered crystalline form is very common and it has been found in many different polymers (for example, in isotactic and syndiotactic polypropylenes; see Chapter 4 of ref 14). It is worth noting that while the formation of the mesophase in the stretched fibers of Figure 3B,E and its complete transformation in the crystalline form in the annealed fibers of Figure 3C are evident, it is not clear if the mesophase is also present in the powder samples of Figure 2b crystallized by aging. The presence of a certain amount of the mesomorphic form in the powder sample crystallized by aging (Figure 2b) and in the annealed sample (Figure 2c) may not be excluded. The data of Figures 1, 2, and 3C indicate that the peak of high intensity at 2θ = 10.1° in the diffraction profile of Figures 1A and 2b,c is an equatorial reflection and it is split in two reflections at values of 2θ = 10.1° and 10.9° in the fiber diffraction pattern of Figure 3C. This splitting agrees with the broadness of the diffraction peak at 2θ = 10.1° of Figures 1A and 2b,c, which presents a clear shoulder that is better resolved in the fiber diffraction pattern of Figure 3C. Moreover, Figure 3C clearly indicates that the peak at 2θ = 16° of high intensity in Figure 1A is not equatorial but is a layer line reflection. The values of Bragg angles and distances of the observed reflections in Figure 1A, 2c, and 3C are reported in Table 1. Moreover, a nearly meridional reflection at d ≈ 2.1 Å, corresponding to an approximate value of the reciprocal ζ coordinate of ζ = 0.46−0.48 Å−1, has also been observed in a fiber diffraction patter of the annealed fibers in tilted geometry. The diffracted intensity in Figure 3C is distributed over at least four visible layer lines at the observed reciprocal lattice

Table 2. Experimental Cylindrical Coordinates ζo, Their Reciprocal Values 1/ζo, Highest Intensity (Io) of the Reflections Observed on the Layer Lines l of the X-ray Fiber Diffraction Pattern of iPE3MPD of Figure 3C, and Absolute Values of the Lowest Order of the Bessel Functions n that Contribute to the Diffraction Intensity on the Layer Lines l for 7/1, 7/2, 7/3, 7/4, 7/5, and 7/6 Helical Conformations ζo (Å−1)

1/ζo = c/l (Å)

l

c (Å)

I0

|n| (7/1, 7/6)

|n| (7/2, 7/5)

|n| (7/3, 7/4)

0.067 0.128 0.208 0.252

14.925 7.813 4.831 3.968

1 2 3 4

14.93 15.62 14.50 15.87

vw vs m m

1 5 3 3

3 1 2 2

2 3 1 1

between 14.9 and 15.8 Å are obtained assuming indices l of the observed layer lines l = 1, 2, 3, and 4 (Table 2). From the data of Table 2 an average value of the chain axis c = 15.3 ± 0.5 Å has been evaluated. The isotactic configuration, the large size of the side group, and the high value of the identity period suggest a helical conformation with complex s(M/N) symmetry for the chains of iPE3MPD.14 The presence of a nearly meridional reflection with spacing d ≈ 2.1 Å indicates a periodicity p for structural unit of the helical conformation (unit height),14 p = c/M ≈ 2.1 Å. Since the chain axis is c = 15.3 ± 0.5 Å, M = 7 monomeric units should be included in the identity period. A D


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Figure 4. Graphical solution of equations li/ζi = c for indexing the observed layer lines of a fiber diffraction pattern of a helical structure in the case of iPE3MPD. For each observed layer line and experimental value of ζo, the possible values of the identity period c are plotted as a function of the trial values of the index l. The solutions are delineated by the dotted horizontal lines that correspond to a possible indexing of the observed layer lines for an identical value of c.

first approximation of the chain conformation is therefore the 7/N helix. A more accurate determination of the number of structural units M included in the period and of the numbers of turns N of the helix along which the structural units are arranged inside the period has been obtained applying the method proposed by Auriemma and De Rosa in refs 14 and 17. The method consists in performing an indexing of the observed layer lines by evaluating for each observed value of ζo trial values of the identity period c, as c = l/ζo with l an integer number corresponding to the trial value of the index l of the layer line. Solutions are selected among those that allow indexing all observed layer lines for identical values of c within the experimental error.17 The problem may be formally stated by solving a system of discrete equations, using the identity period c as a parameter: li/ζi = c. This system of equations is numerically solved admitting as solutions only the values of c for which all the values of li are integer numbers.14,17 The application of this method to the diffraction data of iPE3MPD of Table 2 is shown in Figure 4. For each observed layer line and experimental values of ζi, the possible values of the identity period c are plotted as a function of the trial values of the index l. Considering only the indexing schemes for which the weighted average value of the chain periodicity c is less than 100 Å, and the standard deviation from this average is below a threshold, the most likely solutions are delineated by the dotted horizontal lines in Figure 4. The simplest solution corresponds to the trial values of l = 1, 2, 3, and 4 for the observed values of ζo of Table 2 and allows indexing the observed layer lines for an identical value of c = 15.3 Å. The number of structural units M = 7 results as a consequence of indexing the meridional reflection at ζo = 1/p = 0.46 Å−1 as l = 7; that is, Mi is calculated

as the nearest integer number (nint) close to the ratio Mi = nint(ci/p) = 15.3 × 0.46 = 7. The numbers of turns N of the helix was established from a qualitative examination of the distribution of the diffraction intensities on the layer lines applying the selection rule l = mM + nN

that, according to the Cochran, Crick, and Vand theory,18 defines that the theoretical diffraction intensity on the layer line l in the X-ray fiber diffraction of a helical structure s(M/N) is related to the order n of the Bessel function Jn, where m is an arbitrary integer number. The intensities of reflections observed on the various layer lines are compared in Table 2 with the lowest order n of the Bessel functions that contribute to the diffraction intensity on each layer lines for hypothetical 7/N helices with N = 1 (or 6), 2 (or 5), and 3 (or 4) according to the selection rule. For all possible values of N of the helices s(7/ N), the value of N is determined as that corresponding to the lowest order n of the Bessel functions, considering that high intensity on a layer line l must corresponds to a low value of n. The best qualitative agreement with the experimental intensity distribution was found for N = 2 and the 7/2 helix (Table 2). The same solution was found applying the graphical method of Mitsui.19,20 As found for various isotactic polymers,14 the formation of s(M/N) complex helical symmetry, with M and N not corresponding to very small integers, is related to the presence of bulky lateral groups and corresponds to sequences of torsion angles of main chain bonds of the type (T′G′)n, with isodistortions of the torsion angles from the exact trans and gauche values (TG)n, typical of the 3/1 helix (T′ = T ± δ and G′ = G ± δ). The solution of 7/2 helix for the chain E


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Figure 5. Definition of the torsion angles θ1, θ2, and θ3 and of the bond angles τ1 and τ2 in the chain of iPE3MPD.

conformation of iPE3MPD is the first nontrivial approximation of a commensurable helix, with the lowest M and N integer number (Figure 4). Slight deviation of the ratio P/p from the value of 3.5, where P = c/N is the helical pitch, corresponding to the axial length of the helix in one turn, and p = c/M is the unit height, gives high values of M and N and/or incommensurable helices. Moreover, the 7/2 helix may also be an approximation of a nonuniform helix characterized by sequence of torsion angles ...T′G′T″G″T‴G‴..., as suggested for the nonuniform 7/2 helical conformation of form I of isotactic poly(4-methyl-1-pentene).21−23 Calculations of Conformational Energy and Geometrical Analysis. A model of the 7/2 helical conformation has been built by calculating the conformational energy on a segment of the chain of iPE3MPD of Figure 5 under the constrain of the equivalence principle14 and of the line repetition group s(M/N). The sequence of the dihedral angles in the backbone is therefore ...θ1θ2θ1θ2... (Figure 5). The conformational energy is function of only two variables θ1 and θ2 and is reported as a map in Figure 6. In this map for each pair of θ1 and θ2, the energy is minimized by scanning the torsion angle θ3, which defines the conformation of the branches. The map of Figure 6 presents two equivalent absolute minima in the region θ1 = 70°, θ2 = 180°, θ3 = −115°, and θ1 = 180°, θ2 = −70°, θ3 = −120°, which correspond to the left- and right-handed s(M/N) helix, respectively, and relative minima of higher energy corresponding to nonsignificant conformations. Pairs of θ1 and θ2 that correspond to the 7/2 (or 7/5) helical conformation with values of the unit twist t = 2πN/M = 102.8° (right-handed) or 257.2° (left-handed) are collected on the dashed curve in Figure 6. It is clear that the conformation with s(7/2) helical symmetry and value of t = 102.8° found by the analysis of the fiber diffraction pattern (Figure 4 and Table 2) is very close to the absolute minimum of the conformational energy of iPE3MPD. Among the possible 7/2 helical conformations with unit twist t = 102.8° of Figure 6 (the pairs of θ1 and θ2 on the dashed curve of Figure 6), the 7/2 helix having the exact periodicity of iP3EMPD and value of the unit height p = c/M = 15.3/7 = 2.18 Å has been found by calculating the exact pair of θ1 and θ2 by using the general equations that relate the conformational parameters (unit height p and unit twist t) and the internal coordinates.14,24−27 For example, using the general equations for a two-atoms helix,26 assuming fixed values of C−C bond lengths b1 and b2 and bond angles τ1 and τ2 (Figure 5)14,24−26 (see also Chapter 1 of ref 14, section 1.5.4):

Figure 6. Conformational energy map of iPE3MPD as a function of θ1 and θ2 with θ3 scanned every 5° under the constrain of the s(M/N) symmetry. The geometry of the chain is fixed at the values of bond lengths and bond angles reported in the Supporting Information. The energy levels are reported every 2 kJ/(mol of monomeric units). The values of the energy minima are indicated with asterisks. The lowest energy is set at E = 0. The dashed curve indicates pairs of θ1 and θ2 that correspond to the 7/2 (or 7/5) helical conformation and unit twist t = 102.8° (right-handed) or 257.2° (left-handed).

⎛ θ + θ2 ⎞ ⎛ τ1 ⎞ ⎛t ⎞ ⎟ sin⎜ ⎟ cos⎜ ⎟ = cos⎜ 1 ⎝2⎠ ⎝ 2 ⎠ ⎝2⎠ ⎛ θ − θ2 ⎞ ⎛ τ1 ⎞ ⎛ τ2 ⎞ ⎛τ ⎞ ⎟ cos⎜ ⎟ cos⎜ ⎟ × sin⎜ 2 ⎟ − cos⎜ 1 ⎝2⎠ ⎝ 2 ⎠ ⎝2⎠ ⎝2⎠

(1)

⎛ θ + θ2 ⎞ ⎛ τ1 ⎞ ⎛t ⎞ ⎟ sin⎜ ⎟ p sin⎜ ⎟ = (b1 + b2) sin⎜ 1 ⎝2⎠ ⎝ 2 ⎠ ⎝2⎠ ⎛ θ − θ2 ⎞ ⎛ τ1 ⎞ ⎛ τ2 ⎞ ⎛τ ⎞ ⎟ cos⎜ ⎟ cos⎜ ⎟ × sin⎜ 2 ⎟ − (b1 − b2) sin⎜ 1 ⎝2⎠ ⎝ 2 ⎠ ⎝2⎠ ⎝2⎠ (2)

or the general method of Tadokoro and Chatani,27 the maps of the values of the unit twist t = 2πN/M = |102.8°| and unit height p = c/7 = 2.18 Å of Figure 7 have been calculated. The continuous contours lines in Figure 7 join identical values of the unit twist t = 102.8° (7/2 right-handed helix) or −102.8° = 257.2° (left-handed 7/2 helix with 7/5 symmetry), whereas the dotted lines join identical values of unit height p = c/7 = 2.18 Å. F


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at θ3 = 50° (≈G+) and −115° (A−) have been obtained (Table 3). Table 3. Internal Coordinates θ1, θ2, and θ3 and Energy of the Models of the 7/2 Helical Conformation of iPE3MPDa models of iPE3MPD chains (A) iPE3MPD-R-A− (Figure 9A) (B) iPE3MPD-R-G+ (Figure 9B) (C) iPE3MPD-L-A− (D) iPE3MPD-L-G+

θ1 (deg)

θ2 (deg)

θ3 (deg)

189

−81

−120

189

−81

75

0.6

81 81

171 171

−115 50

0 0.7

E (kJ/(mol mu)) 0

a

The absolute minimum in Figure 8 is assumed as E = 0. In the name of the models, L and R indicate left- and right-handed, respectively, and A− and G+ indicate the value of the torsion angle θ3 (A− nearly anticlinal, close to −120°, and G+ = 50° and 75°, close to gauche+).

Figure 7. Geometrical curves of pairs of the torsion angles θ1 and θ2 for which the value of the unit twist is equal to t = 102.8° (M/N = 7/2 = 3.5) and t = −102.8° = 257.2° (M/(M − N) = 7/5 = 1.4) (continuous lines), corresponding to a 7/2 helix, and curves for which the value of the unit height is h = 2.18 Å (dashed lines), corresponding to the periodicity for monomeric unit of iPE3PMD (c = 15.3 Å). The curves are calculated assuming bond lengths b = 1.53 Å and bond angles τ1 = 113° and τ2 = 111°.14,26,27 The intersection points of the curves of the unit twist and unit height are indicated by white and black circles and provide the pairs of torsion angles (θ1, θ2) that correspond to the right-handed 7/2 helix (white circles) and the lefthanded 7/5 helix (black circles) having the unit height observed in iPE3MPD. The pair of torsion angles θ1 = 189°, θ2 = −81° (and that equivalent θ1 = −81°, θ2 = 189°) (white circles) corresponds to the right-handed 7/2 helix, whereas the pair θ1 = 81°, θ2 = 171° (and that equivalent θ1 = 171°, θ2 = 81°) (black circles) corresponds to the lefthanded 7/2 helix with symmetry s(7/5).

Two models of the 7/2 right-handed helical conformation of iPE3MPD (equivalent to those of the left-handed helices) for the two values of θ3 = 75° (≈G+) and −120° (A−) are shown in Figure 9. Unit Cell and Crystal Packing. All reflections present in the X-ray diffraction patterns of Figures 2 and 3 are indexed with hkl indices assuming an orthorhombic unit cell with axes a = 17.4 Å, b = 16.5 Å, and c = 15.3 Å (Table 1). The presence of

The pairs of values of θ1 and θ2 that satisfy the experimental values of unit twist (t = 102.8°) and unit height (p = 2.18 Å) can be easily found from the maps of Figure 7 by the intersection of the curves of the values of t and h, and are θ1 = 189°, θ2 = −81° (or that equivalent θ1 = −81°, θ2 = 189°), corresponding to the right-handed 7/2 helix, and θ1 = 81°, θ2 = 171° (or that equivalent θ1 = 171°, θ2 = 81°), corresponding to the left-handed 7/2 helix with symmetry s(7/5). The best conformations of the lateral groups have been determined by calculating the conformational energy for the constant values of θ1 = 189° and θ2 = −81° (right-handed 7/2 helix) changing the torsion angle θ3 (Figure 8). The energy curve shows two minima at θ3 = 75° (≈G+) and −120° (A−). From similar calculations for the left-handed 7/2 helix with constant values of θ1 = 81° and θ2 = 171°, two energy minima

Figure 8. Conformational energy for a chain of iPE3MPD in 7/2 righthanded helical conformation with θ1 = 189° and θ2 = −81° as a function of the torsion angle of the lateral groups θ3.

Figure 9. Two models of the chains of iPE3MPD in 7/2 right-handed helical conformation with θ1 = 189° and θ2 = −81° and values of the torsion angle θ3 = −120° (A) and θ3 = 75° (B). G


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Macromolecules two close equatorial reflections at 2θ = 10.1° and 11° allowed to exclude a possible tetragonal packing, which is a typical packing of complex helices,14 as for example in the case of 1,2poly(4-methyl-1,3-pentadiene) (iP4MPD)11 or isotactic poly(4-methyl-1-pentene).21−23 A space group P21ab is suggested in a first approximation by the observation in Table 1 that most of the hk0 and 0k0 reflections with k odd and h00 reflections with h odd are absent. The calculated crystalline density for the unit cell including four chains is 0.869 g/cm3, in good agreement with the measured densities of 0.882 g/cm3 of the sample of iPE3MPD having 51% crystallinity (annealed sample of Figure 2c) and of 0.884 g/cm3 of the amorphous sample of Figure 2a. According to this structure, the calculated crystalline density is lower than the density of the amorphous phase. A crystalline density lower than that of the amorphous phase was also found in the case of iP4MPD11 and form I of isotactic poly(4-methyl-1-pentene).21−23 Models of crystal packing have been obtained by calculating the lattice energy for the orthorhombic unit cell and space group P21ab as a function only of the angle of rotation ω of the 7/2 helical chain around its axis and the z coordinate along the c-axis of the unit cell (Figure 10), with the chain axis positioned at x/a = y/b = 0.25.

Figure 11. Lattice energy as a function of ω and z/c for models of chains of iPE3MPD in the 7/2 helical conformation of Figure 9A with θ3 = A− = −120° (A) and Figure 9B with θ3 = G+ = 75° (B) packed in the orthorhombic unit cell with axes a = 17.4 Å, b = 16.5 Å, c = 15.3 Å, and space group P21ab. The chain axis is at x/a = y/b = 0.25. The lowest energy minimum is set at E = 0 in (B), and energy levels every 10 kJ/(mol of monomeric unit) in (A) and 5 kJ/(mol of monomeric unit) in (B) are reported.

the experimental intensities of reflections in the powder and fiber diffraction patterns of Figures 2c and 3C. The best agreement has been obtained for the model of packing of Figure 12 that corresponds to the absolute minimum of the lattice energy of Figure 11B, that is, to the chain model iPE3MPD-R-G+ of Figure 9B placed in the unit cell at ω = 55° and z = 4.46 Å. The model of Figure 12 is characterized by packing of enantiomorphous helical chains, each right-handed helix being surrounded by four left-handed helices and vice versa. Moreover, neighboring enantiomorphous chains are related by glide planes a and b perpendicular to the b- and c-axes, respectively; therefore, they are isoclined (that is, they have the same orientation along the z-axis, all “up” or all “down”)14 along the a-axis and anticlined (“up” and “down”) along the baxis. As a consequence, rows of enantiomorphous isoclined helices are generated along a and rows of enantiomorphous anticlined helices are generated along b. The calculated structure factors for the model of Figure 12 are reported in Tables 4 and 5 and compared with the experimental structure factors (F0) and intensities of reflections observed in the X-ray powder and fiber diffraction patterns of Figures 2c and 3C. The agreement factor is R = 15%. The good agreement is also evident from the comparisons reported in Figures 13 and 14. The possible presence of a certain amount

Figure 10. Angle of rotation ω and height z of the 7/2 helical chain of iPE3MPD changed in the lattice energy calculations. z is the height of the filled carbon atom.

Results of the lattice energy calculations for the two models of chains of iPE3MPD of Figure 9A,B are shown in the maps of Figure 11. The maps present several equivalent energy minima that repeat for ω = |180° − t| = 77.2° and z = p = c/7 = 2.18 Å, t being the unit twist (t = 360 × 2/7 = 102.8°) and p the unit height. The deepest minima occur for the chain model of iPE3MPD having lateral group with θ3 ≈ G+ (Figures 9B) and for ω = 55° and z/c = 0.29 (Figure 11B). The lattice energy minima for the model of chains with θ3 ≈ A− of Figure 9A are higher than those of the model of chains with θ3 ≈ G+ by ≈20 kJ/mol (Figure 11A). Therefore, only chains of iPE3MPD with θ3 ≈ G+ (Figure 9B) may be present in the unit cell. The reliability of the packing models arising from the minima of the packing energy of Figure 11B has been evaluated comparing the structure factors calculated for the models and H


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Macromolecules

Table 4. Experimental and Calculated Diffraction Angles (2θo, 2θc) and Bragg Distances (do, dc), Observed Structure Factors Fo = (Io/LP)1/2, Evaluated from the Intensities Io of hkl Reflections Observed in the X-ray Powder Diffraction Profile of iPE3MPD of Figure 2c, and Calculated Structure Factors, Fc = (∑|Fi|2Mi)1/2, for the Model of Structure of Figure 12 in the Orthorhombic Unit Cell with Axes a = 17.4 Å, b = 16.5 Å, c = 15.3 Å, and Space Group P21aba

Figure 12. Limit ordered model for the crystal structure of iPE3MPD. Chains in 7/2 helical conformation (chain model iPE3MPD-R-G+ with θ3 = G+ of Figure 9B) are packed in the orthorhombic unit cell with axes a = 17.4 Å, b = 16.5 Å, c = 15.3 Å, and space group P21ab. R = right-handed and L = left-handed. Up and dw (down) are the relative orientation of the chains along the z-axis. The elements of symmetry of the space group are indicated.

of the mesophase in the powder annealed sample of Figures 2c and 13a, absent in the fiber of Figures 3C and 14A, may explain the better agreement of the calculated structure factors with the intensities of reflections in the fiber diffraction pattern (Figure 14). The coordinates of carbon atoms in the model of Figure 12 are reported in Table 6. It is worth noting that we have analyzed other models of the crystal structure corresponding to all the possible space groups compatible with the unit cell symmetry, the maximum number of chains included in the unit cell (given by the density), and the limited diffraction conditions. In particular, we have considered models of packing with chains in 7/2 helical conformation corresponding to different orthorhombic space groups, as for example the space groups P2aa, Pb21a, P21ca, Pbc21, and Pc21b. Moreover, a different possible monoclinic unit cell with axes a = 17.4 Å, b = 16.5 Å, c = 15.3 Å, and γ = 99.2° and models corresponding to different monoclinic space groups, as P21/a, P21/b, Aa, and Bb have been analyzed. For each model, calculations of the packing energy and of structure factors have been performed (see Supporting Information). Furthermore, models of packing in the orthorhombic or monoclinic unit cells and the mentioned space groups but with different possible conformations of the chains (the different solutions reported in Figure 4) have also been analyzed. The model of Figure 12 (space group P21ab) gives the lowest packing energy and the best agreement between calculated and experimental diffraction patterns. Finally, in the model of Figure 12 the possible presence of disorder in the up/down orientation of the chains has also been considered. Calculations of structure factors in the space group

a

Only the reflections with calculated structure factor higher than 10 are reported. I


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Macromolecules Table 5. Experimental and Calculated Diffraction Angles (2θo, 2θc) and Bragg Distances (do, dc), Experimental Intensities Io of hkl Reflections Observed in the X-ray Fiber Diffraction Pattern of iPE3MPD of Figure 3C, and Calculated Structure Factors, Fc = (Σ|Fi|2Mi)1/2, for the Model of Structure of Figure 12 in the Orthorhombic Unit Cell with Axes a = 17.4 Å, b = 16.5 Å, c = 15.3 Å, and Space Group P21aba

Figure 13. Diffraction patterns of iPE3MPD: experimental profile of Figure 2c after subtraction of the amorphous contribution (a) and calculated for the model of Figure 12 (b).

Figure 14. X-ray fiber diffraction patterns of iPE3MPD: experimental (A) and calculated for the model of Figure 12 (B).

P21ab including different amounts of up/down disorder have been performed. We did not observe clear improvement of the agreement. However, even though the best agreement has been obtained with the model of Figure 12 with rows of enantiomorphous isoclined helices along a and rows of enantiomorphous anticlined helices along b, the presence of a certain amount of up/down disorder cannot be excluded.

a

Only the reflections with calculated structure factor higher than 5 are reported. bvs = very strong; s = strong; m = medium; w = weak; vw = very weak, vvw = very, very weak. J


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Macromolecules Table 6. Fractional Coordinates of the Asymmetric Unit of the Model of the Crystal Structure of Figure 12 atom

x/a

y/b

z/c

occupancy

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 C33 C34 C35 C36 C37 C38 C39 C40 C41 C42

0.269 0.283 0.324 0.400 0.277 0.453 0.289 0.210 0.335 0.313 0.414 0.361 0.214 0.234 0.138 0.072 0.150 −0.001 0.226 0.296 0.213 0.265 0.129 0.249 0.297 0.245 0.378 0.421 0.402 0.502 0.253 0.205 0.231 0.160 0.304 0.141 0.201 0.274 0.130 0.118 0.074 0.046

0.202 0.285 0.140 0.146 0.065 0.081 0.280 0.276 0.350 0.427 0.319 0.495 0.284 0.203 0.316 0.276 0.404 0.310 0.205 0.244 0.121 0.061 0.113 −0.022 0.235 0.298 0.239 0.306 0.155 0.308 0.301 0.234 0.383 0.413 0.427 0.496 0.242 0.208 0.201 0.122 0.267 0.083

0.254 0.297 0.289 0.299 0.314 0.335 0.397 0.440 0.432 0.440 0.457 0.477 0.539 0.582 0.574 0.583 0.599 0.620 0.682 0.724 0.717 0.725 0.742 0.761 0.824 0.867 0.859 0.868 0.884 0.904 0.967 0.009 0.002 0.010 0.027 0.046 0.109 0.152 0.144 0.153 0.169 0.189

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Figure 15. Stress−strain curve of samples of iPE3MPD prepared by compression-molding and cooling from the melt to room temperature, corresponding to the amorphous sample of Figure 3a.

hardening. This maximum could be associated with a second yielding phenomenon due to the crystallization of the mesophase that occurs at the same degree of deformation, as demonstrated by the X-ray diffraction pattern of Figure 3B of the low crystalline sample stretched at 300% deformation. Therefore, this unusual deformation behavior may be associated with a succession of events occurring after the onset of plastic deformation at the yielding deformation εy = 11% that involve softening, beginning of orientation of the chains in the amorphous phase and successive (stress-induced) crystallization of the mesophase. It is worth noting that a double yielding behavior has been observed also for high density polyethylene (HDPE).28−32 In this case, whereas the first yield point has been associated with a fine chain slip mechanism, the second yield point has been ascribed to the occurrence of coarse slip movements, leading to lamellar fragmentation and martensitic transformation of part of the initial orthorhombic form into the monoclinic form.28−31 The observed elastic behavior of iPE3MPD is also associated with structural phase transformations occurring during mechanical cycles of stretching and relaxation. In fact, the mesomorphic form that crystallizes upon stretching at high deformations (higher than 200−300%) (Figure 3B) melts after removing the tension or after breaking, the orientation of the chains is lost and unoriented, amorphous samples are obtained upon relaxation, and correspondingly elastic recovery is observed. The elastic behavior of iPE3MPD is therefore associated with the reversible crystallization and melting of the mesophase, and it has been observed only in the case of the amorphous or low crystalline samples crystallized in the mesomorphic form. After annealing and crystallization of the stable and ordered crystalline form (Figure 3C or 3D) the elastic properties are lost.

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Mechanical Properties. The structural characterization and the phase transformations occurring during deformation discussed above (Figure 3) explain the mechanical properties of iPE3MPD. The stress−strain curve recorded on the compression-molded amorphous film of iPE3MPD of Figure 2a is reported in Figure 15. The sample shows high flexibility and deformability and strain hardening at high deformation, without viscous flow, with low modulus (70 MPa), deformation at break higher than 500%, and remarkable values of the stress at yielding (10 MPa) and at break (15 MPa), even though it is initially amorphous. Moreover, elastic behavior after breaking has been observed, as demonstrated by the value of the residual deformation after breaking of about 100% after more than 500% elongation. Furthermore, a maximum in the stress−strain curve is observed at deformation of nearly 200−300% after the classic yielding at deformation εy = 11% and before the strain

CONCLUDING REMARKS We have presented a characterization of the crystal structure of isotactic 1,2-poly(E-3-methyl-1,3-pentadiene) (iPE3MPD). This new polymer has been prepared with a catalyst composed of CoCl2(PRPh2)2, (with R = n-propyl) and is one of the very few examples of crystalline isotactic 1,2-polydienes described in the literature. The as-polymerized sample of iPE3MPD is crystalline with melting temperature of 93 °C and glass transition temperature of ≈30 °C. It does not crystallize by cooling from the melt but crystallizes by aging the amorphous samples at room temperature for several days. Crystallinity increases and improves by annealing at 60−70 °C for few hours. The stretching of amorphous or low crystalline aged compressionmolded samples gives oriented fibers of a highly disordered K


Article

Macromolecules mesophase. Well-oriented and crystalline fibers of iPE3MPD have been obtained by successive annealing at 60−70 °C under tension. The stretching of the high crystalline sample produces transformation of the ordered crystalline form into the mesomorphic form. The crystal structure of the ordered form has been resolved by analysis of the X-ray fiber diffraction pattern and by conformational and packing energy calculations. The structure of iPE3MPD is characterized by chains in 7/2 helical conformation, packed in an orthorhombic unit cell with axes a = 17.4 Å, b = 16.5 Å, c = 15.3 Å, according to the space group P21ab. The hydrogenation of iPE3MPD produces the saturated achiral polymer iP(R,S)3MP where the two chiral enantiomeric monomeric units (R)-3-methyl-1-pentene and (S)-3-methyl-1pentene are randomly enchained. In the hydrogenated polymer iP(R,S)3MP the chains are in 4/1 helical conformation and are packed in a monoclinic unit cell.15,16 Both the conformation and the crystal packing of iP(R,S)3MP are different from those of the precursor polydiene iPE3MPD, highlighting the influence of the structure of side groups and the presence of stereoisomeric centers on the lateral group on the chain conformation and crystal packing of polymers. It is worth mentioning that the crystallization of this low density crystalline form of iPE3MPD is difficult and is kinetically slow, but the crystalline form is stable at room temperature and it is obtained in the bulk, from solution directly from the polymerization and in fiber samples. However, the possible disorder in the conformation of the lateral groups, which may assume two isoenergetic conformations (Figure 8), and the selection of only one conformation in the crystals, may slow down the crystallization kinetics. iPE3MPD has shown interesting mechanical properties of high deformability and elastic properties when amorphous or low crystalline films are stretched. Unusual yielding phenomena have been observed during deformation, associated with the crystallization during deformation of the highly disordered crystalline mesophase that melts upon relaxation. After annealing with increase and improvement of crystallinity, the elastic properties are lost.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from “Ministero dell’Istruzione, dell’Università e della Ricerca” (project PON-DIATEME 2007-2013) and from CARIPLO Foundation (Crystalline Elastomers Project) is gratefully acknowledged.

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(1) Ricci, G.; Leone, G.; Boglia, A.; Bertini, F.; Boccia, A. C.; Zetta, L. Synthesis and Characterization of Isotactic 1,2-Poly(E-3-methyl-1,3pentadiene). Some Remarks about the Influence of Monomer Structure on Polymerization Stereoselectivity. Macromolecules 2009, 42, 3048. (2) Ricci, G.; Forni, A.; Boglia, A.; Motta, T.; Zannoni, G.; Canetti, M.; Bertini, F. Synthesis and X-ray Structure of CoCl2(PiPrPh2)2. A New Highly Active and Stereospecific Catalyst for 1,2 Polymerization of Conjugated Dienes When Used in Association with MAO. Macromolecules 2005, 38, 1064. (3) Ricci, G.; Forni, A.; Boglia, A.; Sommazzi, A.; Masi, F. Synthesis, structure and butadiene polymerization behavior of CoCl2(PRxPh3‑x)2 (R = methyl, ethyl, propyl, allyl, isopropyl, cyclohexyl; x = 1, 2). Influence of the phosphorous ligand on polymerization stereoselectivity. J. Organomet. Chem. 2005, 690, 1845. (4) Ricci, G.; Leone, G.; Boglia, A.; Boccia, A. C.; Zetta, L. cis-1,4-alt3,4 Polyisoprene: Synthesis and Characterization. Macromolecules 2009, 42, 9263. (5) Ricci, G.; Motta, T.; Boglia, A.; Alberti, E.; Zetta, L.; Bertini, F.; Arosio, P.; Famulari, A.; Meille, S. V. Synthesis, Characterization, and Crystalline Structure of Syndiotactic 1,2-Polypentadiene: The Trans Polymer. Macromolecules 2005, 38, 8345. (6) Ricci, G.; Boglia, A.; Motta, T.; Bertini, F.; Boccia, A. C.; Zetta, L.; Alberti, E.; Famulari, A.; Arosio, P.; Meille, S. V. Synthesis and structural characterization of syndiotactic trans-1,2 and cis-1,2 polyhexadienes. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5339. (7) 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. (8) Natta, G.; Porri, L.; Zanini, G.; Palvarini, A. Polimerizzazioni stereospecifiche di diolefine coniugate. Nota V: Preparazione e proprietà del polibutadiene 1,2 isotattico. Chem. Ind. (Milan) 1959, 41, 1163. Chem. Abstr. 1961, 55, 20906. (9) Natta, G.; Corradini, P.; Bassi, I. W. Sulla struttura cristallina del polibutadiene 1,2 isotattico. Rend. Fis. Accad. Lincei 1957, 23, 363. (10) Porri, L.; Gallazzi, M. C. Effect of substituents at position 4 on the stereospecific polymerization of 1,3-diolefins. Polymers with a 1,2 isotactic structure from 4-methyl-1,3-pentadiene. Eur. Polym. J. 1966, 2, 189. (11) Natta, G.; Corradini, P.; Bassi, I. W.; Fagherazzi, G. The crystal structure of 1, 2 isotactic poly-4-methyl-pentadiene-1,3. Eur. Polym. J. 1968, 4, 297. (12) Ricci, G.; Porri, L. Polymerization of 4-methyl-1,3-pentadiene with MAO/Ti(OnBu)4. The influence of preparation/ageing temperature upon the stereospecificity of the catalyst. Polymer 1997, 38, 4499. (13) Zhang, L.; Luo, Y.; Hou, Z. Unprecedented Isospecific 3,4Polymerization of Isoprene by Cationic Rare Earth Metal Alkyl Species Resulting from a Binuclear Precursor. J. Am. Chem. Soc. 2005, 127, 14562. (14) De Rosa, C.; Auriemma, F. Crystals and Crystallinity in Polymers; Wiley: Hoboken, NJ, 2014. (15) De Rosa, C.; Auriemma, F.; Santillo, C.; Di Girolamo, R.; Leone, G.; Ricci, G. Chirality, Entropy and Crystallization in Polymers: Isotactic Poly(3-methyl-1-pentene) as Example of Influence of Chirality and Entropy on the Crystal Structure. CrystEngComm 2015, 17, 6006.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00897. Details of polymer synthesis, solution NMR analysis, procedure of the analysis of mechanical properties and of calculations of conformational and packing energy and of structure factors; comparison between experimental Xray diffraction data and structure factors for other possible packing models characterized by different unit cells and space groups (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Claudio De Rosa: 0000-0002-5375-7475 Finizia Auriemma: 0000-0003-4604-2057 Giuseppe Leone: 0000-0001-6977-2920 Giovanni Ricci: 0000-0001-8586-9829 L


Article

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Crystal Structure and Properties of Isotactic 1, 2-Poly (E-3-methyl-1, 3

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