4-Methyl-1,3-pentadiene Alternating

Article pubs.acs.org/Macromolecules

Structure of Isotactic Ethylene/4-Methyl-1,3-pentadiene Alternating Copolymers Obtained from Postmetallocene Catalysts Finizia Auriemma,*,† Oreste Tarallo,† Claudio De Rosa,† Anna Malafronte,† and Rocco Di Girolamo† †

Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, complesso Monte Sant’Angelo, Via Cintia, 80126 Napoli, Italy

Antonio Proto,‡ Carmine Capacchione,*,‡ Rosa Ricciardi,‡ and Daniela Saviello‡ ‡

Dipartimento di Chimica e Biologia “Adolfo Zambelli”, Università degli Studi di Salerno, Via Giovanni Paolo II, 132 I-84084 Fisciano, SA, Italy ABSTRACT: The structure and thermal properties of a new class of isotactic copolymers of ethylene (E)/4-methyl-1,3-pentadiene (4MPD) with 4MPD content in the range 40−73 mol % and of isotactic poly-1,2-(4-methyl-1,3-pentadiene) (iP4MPD) homopolymer have been analyzed. The copolymers have been synthesized using 1,4-dithiabutanediyl-linked bis(phenolate) titanium complexes as catalyst precursors, activated by methylaluminoxane. Solution 13C NMR analysis reveals the tendency of the catalyst system to produce copolymers with alternating 4MPD/E constitution. All copolymers are crystalline and show melting temperatures in the range 90−100 °C. The samples with nearly equimolar concentration of units crystallize in a new crystalline form because of the alternating constitution. Structural analysis indicates that in this new form the alternating 4MPD-E sequences assume a nearly trans-planar conformation with periodicity c = 5.13 Å and are arranged in a monoclinic unit cell with parameters a = 5.70 Å, b (unique axis) = 14.95 Å, c (chain axis) = 5.13 Å, and β = 114.4°, according to the space group symmetry Pn.

■

17.80 Å, c = 36.5 Å, and space group symmetry I4̅c2 (two chains (36 monomeric units)/cell).2 More in general, the octahedral complexes such as 1 or 2 (Scheme 1), characterized by a tetra-dentate [OSSO]-type ligand having two phenoxo-units linked through a 1,ωdithiaalkanediyl bridge −S(CH2)2S−, have also been shown to produce to a high activity a new class of isotactic copolymers of 4-methyl-1,3-pentadiene (4MPD) with ethylene (E), in a wide composition range (8−70 mol % 4MPD).4 The resultant copolymers (4MPD/E) are characterized by a prevalent alternating constitution, in all explored composition range. The tendency of the catalysts 1/MAO and 2/MAO to produce copolymers with a prevalence of E-4MPD alternating sequences is indicated by solution 13C NMR analysis, presenting a non negligible resonance attributed to secondary methylene carbon atoms (S) in 4MPD-E-4MPD constitutional triads located in between tertiary carbon atoms (T) of 4MPD units in the β positions (Sββ; vide infra).4 As shown in ref 4, catalysts 1 and 2 show both good activity under the used polymerization conditions and no dramatic lowering of the catalyst activity by increasing the 4MPD concentration in the feed. In particular, the catalyst 2 incorporates quite efficiently the 4MPD in the chain resulting

INTRODUCTION Isotactic poly-1,2-(4-methyl-1,3-pentadiene) (iP4MPD) is a semicrystalline polymer synthesized earlier by Porri and Galazzi using heterogeneous titanium and vanadium catalysts activated by Al(C2H5)3.1,2 More recently, iP4MPD was synthesized also resorting to a homogeneous catalytic precursor, namely, the complex 1 of Scheme 1, dichloro{1,4-dithiabutanediyl-2,2′Scheme 1

bis(4,6-di-tert-butyl-phenoxy)}titanium activated with MAO.3 Two crystalline polymorphs have been identified for iP4MPD, the metastable form I, which is the characteristic form of assynthesized samples normally obtained from solution2 and the more stable form II that is obtained by crystallization from the melt.1,2 Only the crystal structure of form II has been studied to date.2 It corresponds to chains in 18/5 helical conformation packed in a body centered tetragonal unit cell with axes a = b = © 2015 American Chemical Society

Received: July 25, 2015 Revised: September 5, 2015 Published: September 28, 2015 6931

DOI: 10.1021/acs.macromol.5b01660 Macromolecules 2015, 48, 6931−6940

Article

Macromolecules

Table 1. Type of Catalyst, Concentration of 4-Methyl-1,3-pentadiene in the Feed, and in the Copolymer Chains, Number Average Molecular Mass (Mn), Polydispersity Index (Mw/Mn), Glass Transition Temperature (Tg), and Melting Temperature (Tm) of Isotactic Ethylene/4-Methyl-1,3-pentadiene Alternating Copolymers and Isotactic Poly-1,2-(4-methyl-1,3-pentadiene) Homopolymera sample

catalyst

4MPDfeed (mol %)

4MPDchain (mol %)

Tgb (°C)

Tmb (°C)

Mn (Kg/mol)c

Mw/Mnb

iP4MPD-2 4MPD69/E31-2 4MPD66/E34-2 4MPD49/E51-2 4MPD46/E54-2 4MPD45/E55-1 4MPD41/E59-2

2 2 2 2 2 1 2

78 71 60 59 74 40

100 69 66 49 46 45 41

13.1 4.2 −8.7 −17.0 −14.3 −22.3 −25.2

128.1 100.0; 112.8 105.3 101.3 102.0 96.93 109.4

610 125 40.0 11.6 15.4 6.90 10.4

≈1 1.4 2.1 1.8 1.9 1.6 1.7

a All samples have been dissolved in chloroform, coagulated in ethanol, acidified with aqueous HCl, and then recovered by filtration and drying under vacuum at room temperature in order to eliminate the impurities. bDetermined by DSC analysis at a heating rate of 10 °C/min, during the second heating scan. cDetermined by GPC analysis.

■

in copolymers with high content of 4MPD (up to 83 mol%). Furthermore, the mass average molecular mass of the copolymers synthesized with catalyst 1 is ≈7 × 103 g/mol, regardless of compositions, while in the case of catalyst 2 it is ≈1 × 104 g/mol for 4MPD/E copolymers with 40 mol % 4MPD, and increases with 4MPD concentration up to reach values of 1 × 106 g/mol for 4MPD/E copolymers with ≈80 mol % 4MPD. In all cases, the molecular mass distributions are monomodal and are comprised in between 1 and 2, indicating a single-site catalyst and the effective formation of copolymers. In addition, efforts to fractionate the polymer in common organic solvents (acetone, hexane, ethyl acetate, and THF) were unsuccessful.4 A preliminary X-ray diffraction analysis was also performed, revealing that, in agreement with the stereoregular configuration, as prepared samples were crystalline and that the type of crystalline form was dependent on 4MPD concentration.4 In particular, it was pointed out that the copolymers containing ≈50 mol % 4MPD crystallize in a new crystalline form, different from the crystalline forms of polyethylene5 and iP4MPD.1,2 This strongly suggests that this new form is most likely due to the crystallization of long alternating sequences.4 In this paper, the structure and thermal properties of 4MPD/ E copolymers with 4MPD concentration ranging from 40 to 70 mol % are investigated in detail and compared with the properties of iP4MPD homopolymer. In particular, in the present paper, a more thorough NMR analysis of the ethylene4MPD copolymers is performed, and a full characterization via DSC and X-ray-diffraction is done. In addition, a possible model for the crystal structure of the new crystalline form in 4MPD/E copolymers with nearly equimolar composition is suggested. It is worth noting that iP4MPD homopolymer and the isotactic copolymers 4MPD/E are characterized by the presence of pendant double bonds in the side chains. The possibility to easily convert the double bonds into functional groups, makes iP4MPD and 4MPD copolymers with ethylene of high potential interest, because a broad range of polymers containing polar functionalities may be obtained while maintaining the isotactic main chain configuration, whose properties can be finely controlled by 4MPD concentration. Moreover, the discovery that the alternating copolymers crystallize in a new crystalline form assesses a new, unique feature of these polymeric materials that can lead to undisclosed applications.

EXPERIMENTAL SECTION

The synthesis of iP4MPD homopolymer and 4MPD/E isotactic copolymers were performed according to the literature method3,4 using the catalyst precursors 1 and 2 of Scheme 1 activated with MAO. The 4MPD/E copolymers are characterized by a 4MPD concentration ranging from 41 to 69 mol % and narrow molecular mass distributions Mw/Mn = 1−2, typical of single-site metallorganic catalyst systems (Mw and Mn being the mass average and number-average molecular mass, respectively). In Table 1, the properties of the investigated copolymer and homopolymer samples are summarized. It is worth reminding that both catalysts 1/MAO and 2/MAO have good activity under the polymerization conditions. In particular, as 4MPD content in the feed increases, the activity does not vary greatly by using 1/MAO, while it increases in the case of 2/MAO.4 The catalyst 2/MAO promotes the insertion of 4MPD more efficiently than 1/MAO. As an example, the samples 4MPD45/E55-1 and 4MPD46/E54-2 having identical compositions, have been synthesized using the two different catalytic systems, and therefore present different molecular mass, corresponding to values of ≈7 and 15 Kg/mol, respectively.4 Materials. All manipulations of air- and water-sensitive compounds were performed under a nitrogen atmosphere using standard Schlenk techniques or a MBraun glovebox. Commercial grade toluene (Carlo Erba) was dried over calcium chloride, refluxed 48 h under a nitrogen atmosphere over sodium, and distilled before using. Polymerization grade ethylene, purchased from Società Ossigeno Napoli (S.O.N.), was dried by passing through a column filled with activated molecular sieves (4 Å). 4-Methyl-1,3-pentadiene was synthesized according to a literature procedure6 and purified by distillation over calcium hydride under a nitrogen atmosphere. Methylalumoxane (MAO), purchased from Aldrich as a 10 wt % solution in toluene, was dried in vacuo at 50 °C to remove toluene and “free” AlMe3 and used as a solid after washing with hexane. The titanium complexes 1 and 2 were prepared according to the literature procedures.7,8 Homopolymerization of 4-Methyl-1,3-pentadiene. Polymerization of 4MPD was carried out under a nitrogen atmosphere in a 100 mL glass flask, equipped with a magnetic stirrer and immersed in thermostated water bath at 25 °C for 1 h. The glass flask was sequentially filled with 4.0 mL of toluene, 300 mg (5.0 mmol) of solid MAO, 1.80 g (22 mmol) of 4-methyl-1,3-pentadiene, and 1.0 mL of the catalyst 2 toluene solution (1.0 × 10−2 M). The run was stopped by injecting ethanol. The polymer sample, coagulated with acidified ethanol, was washed several times with ethanol, recovered by filtration, and dried in a vacuum at 60 °C. Copolymerization of Ethylene and 4-Methyl-1,3-pentadiene. The copolymerization runs were carried out following a standard procedure. A 100 mL flask equipped with a magnetic bar was charged with 0.70 g of MAO (0.01 mol, Al/Ti = 1200) and the proper amount of a 4-methyl-1,3-pentadiene solution in toluene to reach a total volume of 10 mL. The flask was then evacuated and filled with ethylene (1 atm); after equilibration of the solution at target temperature, the reaction was started by injection of a toluene 6932


Article

Macromolecules

Figure 1. Aliphatic regions of the solution 13C NMR spectra of isotactic 4MPD/E copolymer samples of Table 1. Spectra are recorded at 363 K in tetrachloroethane-d2 as solvent. Chemical shifts are referenced to TMS. Symbols Sxy and Txy denote resonances belonging to secondary (S) and tertiary (T) carbon atoms in the backbone, located in between tertiary carbon atoms in x and y positions (with x,y = α, β, γ, δ), as shown in Scheme 2.13 The peaks marked with an asterisk are relative to secondary carbon atoms separated by more than four bonds from a tertiary carbon, and those marked with double asterisk at ≈31 and 35 ppm (red) are probably due to impurities. The 4MPD content of copolymers is also indicated. solution (1 mL) of the catalyst (10 μmol). The run was terminated after the desired time by introducing ethanol (15 mL) and antioxidant (Wingstay K; 0.5−0.75 phr). The polymer was coagulated in ethanol (200 mL), acidified with aqueous HCl, recovered by filtration, washed with an excess of ethanol, and dried in vacuo at room temperature until constant weight. Characterization of the Polymers. The 13C NMR spectra of the 4MPD/E copolymer samples were recorded with a Bruker AVANCE 300 spectrometer (300 MHz for 1H and 75 MHz for 13C). Using 5 mm (o.d.) NMR tubes, polymer samples (30 mg) were dissolved in tetrachloroethane-d2 (0.7 mL) and analyzed at 363 K. Chemical shifts were referenced to TMS and calculated by using the residual isotopic impurities of the deuterated solvent (74.26 ppm for tetrachloroethaned2). The average molecular mass of the polymer samples were determined at 30 °C with a 150C Waters GPC equipped with JASCO 875-UV (254 nm) and WGE-DR BURES ETA1002 refractive index detectors and three PSS columns set consisting of 105, 104, and 100 Å (pore size), 5 μm (particle size) column. CHCl3 was used as the carrier solvent, with a flow rate of 1.0 mL/min. The calibration curve was established with polystyrene standards. All samples presented some impurities due to the presence of the catalyst remnants. In order to eliminate the impurities, before the thermal and structural characterization, the samples have been dissolved in chloroform, coagulated in ethanol, acidified with aqueous HCl, kept under stirring, and then at rest overnight, and finally recovered by filtration and drying under vacuum at room temperature. DSC scans have been performed using a differential scanning calorimeter DSC Mettler 822 in a flowing N2 atmosphere at a scanning rate of 10 °C/min. Melt-pressed films with a uniform thickness of ≈300 μm have been prepared by heating as precipitated samples at a temperature of 20 °C higher than melting for 10 min under a press and by slowly cooling to room temperature (average cooling rate 0.5 °C/min). X-ray diffraction patterns have been obtained with Ni-filtered Cu Kα radiation (λ = 1.5418 Å). The powder diffraction patterns have been obtained with an automatic Philips diffractometer. The crystallinity index (xc) of the samples has been evaluated from the X-ray powder diffraction profiles, after subtraction of a baseline approximating the background contribution, as the ratio between the

area of the crystalline phase Ac and the area subtending the whole diffraction profile of the sample Atot = Ac + Aa (eq 1):

xc =

Ac A − Aa = tot A tot A tot

(1)

With Aa being the area under the diffraction profile of amorphous phase. The diffraction profile of the amorphous phase has been constructed by collecting X-ray powder diffraction profiles of the melted samples (temperature range 130−160 °C) and by successive extrapolation of these data to room temperature to account for the thermal expansion of the amorphous/molten state. Calculated structure factors have been obtained as Fcalc = (∑| Fi|2Mi)1/2, where Fi is the structure factor and Mi is the multiplicity factor of the reflection i (Miller indices (h k l)i) in powder diffraction profiles, and the summation is taken over all reflections included in the 2θ range of the corresponding diffraction peak observed in the X-ray powder diffraction profile. A thermal factor B = 8 Å2 and atomic scattering factors, as in ref 9, have been assumed. The observed structure factors, Fobs, have been evaluated from the intensities Iobs of the reflections observed in the powder diffraction profiles as Fobs = (Iobs/LP)1/2, where LP is the Lorentz-polarization factor for X-ray powder diffraction.10 The experimental intensities Iobs have been evaluated by measuring the area of the peaks in the X-ray powder diffraction profile, after subtraction of a straight baseline approximating the background and of the amorphous contribution. Simulated X-ray powder diffraction profiles have been obtained with the software package11 CERIUS2, using the isotropic thermal factor B = 8 Å2. For the calculation of powder diffraction data, profile functions having a half-height width regulated by the average crystallite size along a, b, and c axes, La = Lb = 150 Å and Lc = 100 Å, respectively, have been used. These values correspond to a coherence length along a, b, and c and are not true crystallite sizes.

■

RESULTS AND DISCUSSION

Solution 13C NMR Analysis. The 13C NMR spectra of the 4MPD/E isotactic copolymers are shown in Figure 1. The peak assignment has been performed according to previous work4,12 and is shown in Table 2. 6933


Article

Macromolecules Table 2. 13C NMR Assignements for the 4MPD/E Copolymersa carbon typeb

chemical shift (ppm)

sequencec

CH3 Sββ CH3′ Sβδ Sδδ Sγδ Tβδ Tββ Sαδ Sαγ Tδδ Sαα C CH Sαδ+d Sβδ+d

18.42−18.47 25.21 25.86−26.23 27.55 29.89 30.38 32.68 34.12 36.36 36.57 38.30 42.44 129.0−130.0 131.1−132.1 33.89, 34.36 27.16

DDD/EDE DED DDD/EDE DEED DEEED DEEED DDED DDD DEED DED DEDED DDD DDD/EDE EDE/DDD DEED/DEEED DEEEED/DEEED

Then the monomer compositions of the copolymer ethylene(4MPD) samples were determined by using the following formulas: D = DD + 1/2DE = I(Sαα) + 1/2I(Sαγ ) + 1/2I(Sαδ)

E = EE + 1/2DE = 1/2(I(Sβδ) + I(Sδδ)) + 1/4I(Sγδ) + 1/2I(Sαγ ) + 1/2I(Sαδ)

(6)

The results of this analysis are summarized in Table 3. Table 3. Percentage of DD, DE, and EE Sequences in 4MPD/E Copolymers

a

For details of assignment, see ref 4. bSee Scheme 2. cD = 4-methyl1,3-pentadiene unit; E = ethylene unit. D = inverted 4-methyl-1,3pentadiene unit. dSymbol “δ+” denotes tertiary carbon atoms separated from the atom in question by more than four bonds.

In Figure 1 and Table 2 the symbols Sxy and Txy identify secondary (S) and tertiary (T) carbon atoms in the backbone, located in between tertiary carbon atoms in the x and y positions (with x,y = α, β, γ, δ), as shown in Scheme 2.13

D = 4MPD, E = Ethylene.

The dyad distribution was evaluated by comparing the integrals of the 13C methylene resonances by the following equations:14,15

DD = I(Sαα)

(2)

DE = I(Sαγ ) + I(Sαδ)

(3)

EE = 1/2(I(Sβδ) + I(Sδδ)) + 1/4I(Sγδ)

(4)

sample

DD (%)

DE (%)

EE (%)

D (%)

E (%)

4MPD69/E31-2 4MPD66/E34-2 4MPD49/E51-2 4MPD46/E54-2 4MPD45/E55-1 4MPD41/E59-2

52.9 40.7 2.6 16.7 3.6 4.7

32.9 51.4 92.3 59.5 82.7 71.6

14.2 7.9 5.1 23.8 13.7 23.7

69.4 66.4 48.8 46.5 44.9 40.5

30.6 33.6 51.2 53.5 55.1 59.5

The data of Table 3 denote the tendency of catalysts 1/MAO and 2/MAO to incorporate alternating E-4MPD sequences along the chain at any composition. In particular, the copolymers with nearly equimolar content of E and 4MPD units 4MPD49/E51-2 and 4MPD45/E55-1 show a percentage of DE sequences of ≈92 and ≈83 mol %, respectively, and therefore approach a nearly complete alternating constitution. The sample 4MPD46/E54-2, instead, shows ≈60% of alternating DE sequences, and a non-negligible fraction of EE and DD homosequences, indicating a more random distribution of constitutional units along the chain. Finally, the copolymers having a 4MPD content of 66−70 mol % (4MPD66/E34-2 and 4MPD69/E31-2) have comparable concentration of DE and DD sequences (in the range 33− 53%), whereas the sample 4MPD41/E59-2 (4MPD = 41%mol) has a prevalence of DE sequences (72%), a significant percentage of EE sequences (24%mol), and only a negligible concentration of DD sequences (5%). X-ray Diffraction and Thermal Analysis. The X-ray powder diffraction profiles of 4MPD/E copolymers are shown in Figure 2, both in the case of samples obtained by precipitation from solution (Figure 2A) and films slowly crystallized from melt by compression molding (Figure 2B). For comparison, the diffraction profiles of iP4MPD homopolymer samples crystallized in the same conditions are also reported in Figure 2. All samples show Bragg peaks the position and relative intensity of which depend on 4MPD composition and crystallization conditions. In particular, in the case of the homopolymer, the X-ray diffraction profile of the solution precipitated sample (curve a of Figure 2A) shows broad peaks at d ≈ 7.8, 5, and 3.8 Å (2θ ≈ 11, 18, and 23°, respectively) due to the crystallization of form I of iP4MPD.1 The diffraction profile of the melt crystallized film (curve a of Figure 2B), instead, shows reflections at d ≈ 9, 6, 5, and 4 Å (2θ ≈ 10, 14, 16, and 22°, respectively) due to the crystallization of form II of iP4MPD.2 Therefore, in agreement with literature, iP4MPD crystallizes from solution in the metastable form I1 and from

Scheme 2. Secondary (S) and Tertiary (T) Carbon Atoms in the Relevant Constitutional Sequence of 4MPD/E Copolymers Synthesized with 1/MAO and 2/MAOa,13

a

(5)

6934


Article

Macromolecules

Figure 2. X-ray powder diffraction profiles of isotactic 1,2-poly(4-methyl-1,3-pentadiene) (a) and isotactic 4-methyl-1,3-pentadiene/ethylene copolymers (b−g) in the case of samples crystallized from solution (A) and slowly crystallized from melt (B). For the samples 4MPD69/E31-2 and 4MPD66/E34-2 (b, c), the reflections of forms I and II of iP4MPD are indicated, whereas the reflections of form III observed in profiles (d) and (e) are marked with an asterisk on the profiles b-c.

the melt in the more stable form II.2 In both cases, a high crystallinity index xc is achieved, equal to 50−60% (Figure 2). In the case of the 4MPD/E copolymers with 40−50 mol % 4MPD content, the diffraction profiles (curves d−f of Figure 2) present peaks in positions completely different from those of forms I and II of iP4MPD. In fact, the main reflections occur at d ≈ 15, 7, 4.3, and 3.7 Å (2θ ≈ 6, 12, 21 and 24°, respectively), regardless of crystallization conditions. The absence of the diagnostic reflections of form I at d ≈ 7.8 Å (2θ ≈ 11) and form II at d ≈ 9 Å (2θ ≈ 10) of iP4MPD1,2 indicate that a new crystalline form, defined form III, is obtained in these samples. Due to the nearly equimolar composition and in agreement with solution 13C NMR data, the obtainment of form III can be attributed to the prevailing alternating constitutions of these samples and therefore to the crystallization of long E-4MPD alternating sequences. Accordingly, a high crystallinity index of ≈40−45% is achieved by copolymers 4MPD49/E51-2 and 4MPD45/E55-1 with a concentration of ED diads higher than 80 mol %, whereas the crystallinity index achieved by the sample 4MPD46/E54-2, characterized by a more random distribution of monomers, is only xc ≈ 35%. Also the sample 4MPD41/E59-2 (xc ≈ 44%) crystallizes in this new form, as indicated, in the X-ray diffraction profiles g of Figure 2, by the presence of peaks at d ≈ 15, 7, 4.3, and 3.7 Å (2θ ≈ 6, 12, 21 and 24°, respectively), characteristic of form III, and the absence of the diagnostic reflections of forms I and II of iP4MPD.1,2 It is worth remarking that this new form shows diffraction peaks at d ≈ 4.3 and 3.7 Å (2θ ≈ 21 and 24°, respectively), that is at interplanar distances identical to those of 110 and 200 reflections of the orthorhombic form of polyethylene (PE).5 However, whereas for the copolymers having nearly equimolar composition of E and 4MPD units, the relative intensity of these reflections is similar, in the case of the copolymer 4MPD41/E59-2 with 41 mol % 4MPD, the ratio between the intensity of the reflections at d ≈ 4.3 (2θ ≈ 21°) and at d ≈ 3.7 Å (2θ ≈ 24°) is higher than 1 (curve g of Figure

1), and similar to that one between the 110 and 200 reflections of PE.5 This suggests that the copolymer 4MPD40/E60-2 crystallizes partly in form III and partly forming PE-like crystals. The possible crystallization of PE-like crystals is in agreement with solution 13C NMR data (Table 3). In fact, the copolymer 4MPD40/E60-2 contains not only a prevalence of alternating E-4MPD diads (≈72 mol %), but also a remarkable concentration of EE diads (≈24 mol %). Therefore, this copolymer not only contains long alternating sequences crystallizing in form III, but also ethylene sequences long enough to crystallize in the orthorhombic form of PE. Finally, the 4MPD/E copolymers with ≈64−70 mol % of 4MPD crystallize as mixtures of crystals of form III of the alternating copolymers and of forms I and II of iP4MPD.1,2 This is indicated by the fact that, regardless of crystallization conditions, the diffraction profiles of these samples (curves b and c of Figure 2) show not only reflections at d ≈ 9 and 6 Å (2θ ≈ 10 and 14°, respectively) diagnostic of form II of iP4MPD1 and at d ≈ 7.8 Å (2θ ≈ 11°), typical of form I of iP4MPD,2 but also reflections (indicated with an asterisk in Figure 2) at d ≈ 15, 7, 4.3, and 3.7 Å (2θ ≈ 6, 12, 21, and 24°, respectively), characteristic of the new crystalline form III of the alternating 4MPD/E copolymers. The mixture of three different crystalline forms in 4MPD/E copolymers with ≈64−70 mol % of 4MPD is obtained, regardless of crystallizing conditions. The main difference between samples crystallized from solution (Figure 2A) and melt (Figure 2B) consists in a small increase of the crystallinity index xc from values close to 25−30% to values close to 30−35% and in the formation of better developed crystals in melt crystallized samples. According to solution 13C NMR analysis, these copolymers are characterized by a high concentration of both DD homosequences and ED heterosequences (>32 mol %) and only a low concentration of EE homosequence (25−30%) achieved by 4MPD/E copolymers regardless of 4MPD concentration indicates that they are not only of a prevailing alternating constitution, but are also highly regio- and stereoregular. The DSC thermograms of the melt crystallized samples are reported in Figure 3. They have been recorded during the second heating scan on solution precipitated samples first heated from room temperature up to 150 °C and successively cooled to −50 °C, at scanning rate of 10 °C/min. The values of the melting peaks are reported in Table 1, whereas the glass transition temperatures are reported in Table 1 and Figure 3C. The DSC curves of Figure 3A show a glass transition at temperatures lower than 10 °C, followed by a melting endotherm at temperatures higher than 90 °C. In particular, the glass transition temperature (Figure 3C) of iP4MPD homopolymer is about 10 °C and decreases almost linearly with an increase of ethylene content. The DSC curve of the homopolymer sample iP4MPD shows a narrow endothermic peak at ≈128 °C due to the melting of form II crystals obtained during the cooling step (curve a of Figure 3A). The melting temperature of form I, instead (data not shown), is ≈90 °C. The sample 4MPD69/E31-2 with 69 mol % 4MPD (curve b of Figure 3A) shows a cold crystallization peak around 72 °C followed by a double melting endotherm with peaks at ≈100 and 113 °C, probably due to the simultaneous presence of crystals in forms I, II, and III of different thermal stabilities formed during the cooling step and to the crystals formed by cold crystallization. The copolymers with nearly equimolar composition crystallizing in the pure form III (curves d,e of Figure 3A), instead, show a single broad endothermic peak around 100 °C. However, whereas the samples approaching a perfectly alternating constitution 4MPD49/E51-2 and 4MPD45/E55-1

to the tendency of alternating sequences to crystallize in the form III and of long 4MPD sequences to crystallize in form I and form II of iP4MPD. Therefore, contrary to iP4MPD homopolymer, in the case of 4MPD/E copolymers with ≈70 mol % of 4MPD, form I may be easily obtained not only from solution, but also from the melt. The 2θ position and corresponding interplanar distances of the main reflections relative to forms I and II of iP4MPD,1,2 form III of isotactic 4MPD/E copolymers with alternating constitution, and the orthorhombic form of PE17 are shown in Table 4. Table 4. Diffraction Angles (2θ) and Interplanar Distances (d) for the Main Reflections of Forms I and II of iP4MPD,1,2 Form III of Isotactic Copolymers 4MPD/E with Alternating Constitution, and the Orthorhombic Form of Polyethylene5 4MPD/E-form III

iP4MPD-form I1

iP4MPD-form II2

polyethylene5

2θ (deg)

d (Å)

2θ (deg)

d (Å)

2θ (deg)

d (Å)

2θ (deg)

d (Å)

5.9 11.85 18.1 19.9 20.85 23.8 24.7 29.9 34.7

15.0 7.47 4.90 4.46 4.26 3.74 3.60 2.99 2.59

11.3 17.6 23.2

7.83 5.04 3.83

9.98 14.12 16.48 21.88

8.86 6.27 5.38 4.06

21.7 24

4.09 3.71

The crystallinity index evaluated from diffraction data of Figure 2 are reported in Figure 3B. The crystallinity index of the homopolymer is around 60%, and drops to values of ≈25− 35% in the case of copolymers 4MPD/E with ≈64−70 mol % 4MPD, up to increase again to values of 40−45% for the copolymers with nearly equimolar composition, because of the

Figure 3. (A) DSC thermograms (10 °C/min scanning rate) of the homopolymer sample iP4MPD-2 (a) and samples of isotactic 4MPD/E copolymers (b−g), recorded during the second heating scan after crystallization from the melt by cooling the melt to room temperature, (B) corresponding values of crystallinity index evaluated from the X-ray diffraction profiles of Figure 2B (red dots) and 2A (blue squares), and (C) values of glass transition temperature of the copolymers as a function of concentration of 4MPD. 6936


Article

Macromolecules (curves d,f of Figure 3A) show an almost symmetric endotherm, the sample 4MPD46/E54-2 (curve e of Figure 3A) shows a broader endotherm because of the less regular alternating constitution (see Table 3). Finally, the samples 4MPD66/E34-2 with 66 mol % of 4MPD crystallizing as a mixture of forms I, II and III (curve b of Figure 3A), 4MPD41/E59-2 with 41 mol % of 4MPD crystallizing as a mixture of form III, and the orthorhombic form of PE (curve g of Figure 3A) show broad melting peaks centered at 105−110 °C, characterized by long tails in the low temperature range due to the gradual melting of the crystals in the different polymorphs and of different degrees of perfection. Structural Analysis of Form III in Alternating 4MPD-E Copolymers. The melt crystallized sample 4MPD45/E55-1, characterized by an almost completely alternating constitution, has been annealed for 24 h at 80 °C. The corresponding X-ray diffraction profile is reported in Figure 4, before (curve a of

Table 5. Values of the Observed (2θobs, dobs) and Calculated (2θcalc, dcalc) Diffraction Angles and Interplanar Distances, and Miller Indices of Reflections of Form III in Isotactic 4Methyl-1,3-pentadiene/Ethylene Copolymers with Alternating Constitution reflection number

2θobs (deg)

dobs (Å)

2θcalc (deg)

dcalc (Å)

hkl

1 2 3 4 5 6 7 8 9 10 11 12 13

5.90 11.85 18.10 19.90 20.85 22.44 23.79 24.70 29.90 34.65 35.65 38.54 39.85

14.98 7.47 4.90 4.46 4.26 3.96 3.74 3.60 2.99 2.59 2.52 2.34 2.26

5.91 11.84 18.09 19.91 20.83 22.44 23.81 24.77 29.88 34.62 35.65 38.54 39.85

14.95 7.48 4.90 4.46 4.26 3.96 3.74 3.60 2.99 2.59 2.52 2.34 2.26

010 020 110 011 120 021 040 130 050 150 051 002 2̅02

Scheme 3. Scheme of Isotactic 4-Methyl-1,3-pentadiene/ Ethylene Alternating Copolymers

fifth diffraction orders at d ≈ 7.5, 3.7, and 3 Å, respectively (2θ ≈ 12, 24, and 30°, respectively), the diffraction pattern of form III in alternating 4MPD/E copolymers (Figure 4) can be indexed according to a monoclinic unit cell with parameters a = 5.70 ± 0.03 Å, b = 14.95 ± 0.05 Å, c (chain axis) = 5.13 ± 0.07 Å, and β = 114.4 ± 0.2° (b unique axis). As shown in Table 5, the values of the observed interplanar distances are in good agreement with the calculated values. The theoretical value of crystalline density of 0.92 g/cm3 has been evaluated assuming two chains (two ethylene and two 4MPD units) per unit cell, in accordance with the experimental value of 0.89 g/cm 3 determined by flotation on a melt crystallized sample of the copolymer 4MPD45/E55-1 with a crystallinity index xc of ≈40%. Similar results would be obtained with the sample 4MPD49/E51-2, which shows a diffraction profile (curves d,f of Figure 2B) and thermal behavior (curves d,f of Figure 3A) identical to those of the sample 4MPD45/E55-1 within the experimental error. In fact, in both samples, only the alternating sequences are able to crystallize. Conformational Model. In Scheme 3A, the relevant internal variables for each 4MPD/E constitutional unit are indicated, that is the backbone torsion angles θ1, θ2, θ3, and θ4 and the torsion angle defining the position of the 2-methyl-1propenyl (isobutenyl) side groups θ5. Conformational models of low internal energy with chain periodicity close to 5.1 Å, suitable for the crystalline form III of the alternating copolymers, may be easily built up assuming the identical repetition of a single 4MPD/E constitutional diad according to

Figure 4. X-ray powder diffraction profile of the sample of an isotactic alternating copolymer 4MPD45/E55-1 crystallized from melt and then annealed at 80 °C for 24 h before (curve a of A) and after subtraction (curve a of B) of amorphous contribution. In A, the profile of the amorphous phase is indicated with the dashed line (a′). In B, curve a′ represents the high 2θ regions of the experimental diffraction profile a, plotted on an enlarged y-scale.

Figure 4A) and after subtraction for the amorphous contribution (curve a of Figure 4B). The values of diffraction angles (2θobs) and interplanar distances (dobs) of the reflections numbered in Figure 4B are reported in Table 5. As shown by diffraction analysis (Figure 2) and DSC measurements (Figure 3A), this sample crystallizes in form III and melts at temperatures of ≈100 °C. The diffraction profile of Figure 4B shows exclusively the diffraction peaks of form III (Table 4), indicating that form III is stable and does not transform into another form upon prolonged annealing at 80 °C. The alternating constitution of the copolymer along with the isotactic conformation of the chains (Scheme 3A) suggests a nearly trans-planar conformation of the backbone in the crystals, corresponding to the identical repetition of a single ethylene-4MPD constitutional diad according to a periodicity c of ≈5.1 Å. Based on the presence of an intense reflection at d ≈ 15 Å (2θ ≈ 6°) and of the corresponding second, fourth, and 6937


Article

Macromolecules the sequence of backbone dihedral angles (θ1θ2θ3θ4)n, with θ1, θ2, θ3, and θ4 close to trans-planar, and fixing the position of the lateral groups in a plane normal to the average plane of the backbone. In Scheme 3B, the configurational sign of the backbone bonds adjacent to the tertiary carbon atoms are also indicated.16 Minimum energy conformations correspond to setting θ5 equal to θ5 = −120 ± 30° when it is measured with respect to the (−) backbone bond (i.e., in the sequence 2−3··· 1′−2′ of Scheme 3B) or, equivalently, equal to θ5 = +120 ± 30°, when it is measured with respect to the (+) backbone bond (i.e., in the sequence 4−3···1′−2′ of Scheme 3B). A model of low internal energy for the conformation of the chains of isotactic 4MPD/E copolymers having an alternating constitution suitable the crystalline form III is shown in Figure 5. The backbone torsion angles θ1, θ2, θ3, and θ4 in consecutive

h0l reflections with h + l odd (Table 5) suggests that the space group symmetry is Pn. A packing model for the 4MPD/E chains in form III is shown in Figure 6.

Figure 6. Packing model for the form III of isotactic alternating 4MPD/E copolymer in which the trans-planar chain are arranged in a monoclinic unit cell with parameters a = 5.70 Å, b = 14.95 Å, c (chain axis) = 5.13 Å and β = 114.4°, according to the symmetry of the space group Pn. Projection perpendicular (A) and parallel (B) to the chain axes. Two kinds of interfaces between adjacent ac layers of chains are evidenced, the s−s interface between the side chains and the b−b interface between the backbone atoms.

In this model the asymmetric unit coincides with the chain repetition unit and includes a 4MPD residue covalently linked to an ethylene unit. The corresponding atomic fractional coordinates are reported in Table 6. In the model of Figure 6 the chains are organized in the unit cell forming ac layers, in which the backbone atoms belonging to adjacent layers face each other along b (interface b−b) alternating with a second type of interface where the side group atoms belonging to adjacent ac layers face each other along b (interface s−s). In the packing model of Figure 6 the degree of freedom allowed by the space group symmetry Pn are the azimuthal setting of the chains and the relative positions of the chain axes along b. Distances between nonbonded carbon atoms in adjacent chains higher than 3.9, and distances between hydrogen atoms higher than 2.4 Å are obtained for the model of Figure 6. The X-ray powder diffraction profile relative to the structural model of Figure 6 has been calculated using the utility “Diffraction-Crystal” of Cerius2 program and it is compared in Figure 7 with the experimental diffraction profile of form III of isotactic alternating copolymers (Figure 4). The main features of the experimental diffraction data (curve a of Figure 7) are reproduced quite well in the simulated pattern (curve b of Figure 7). The comparison between the structure factors Fc calculated for the model of Figure 6 and those observed, Fobs, for form III, evaluated from the experimental values of intensity in the X-ray powder diffraction profile of Figure 4, is shown in Table 7.

Figure 5. Low energy conformal model for the isotactic chain of alternating ethylene/4-methyl-1,3-pentadiene copolymers, in the crystalline form III. Projection parallel (A) and perpendicular (B) to the chain axis. The chain periodicity c = 5.13 Å is indicated. The configurational sign close to the backbone bonds adjacent to the tertiary carbon atoms16 are indicated. The value of backbone torsion angles θ1, θ2, θ3, and θ4 are close to 180°. The value of the torsion angle defining the position of the lateral groups θ5 ≈ −115° is defined with respect to the backbone bond with sign (−).

repeating units are close to |179°|, whereas the value of θ5, defined with respect to the (−) bond in the backbone, is ≈−115°. Packing Model and Structure Factor Calculations. As shown in the conformational model of Figure 5, the shape of the alternating 4MPD/E copolymer chains in the trans-planar conformation is rather elongated in the y-direction, because of the intrinsic steric encumbrance of the side groups. This anisotropic shape suggests a packing mode of the chains in the monoclinic unit cell (a = 5.70 Å, b = 14.95 Å, c = 5.13 ± 0.07 Å, β = 114.4°), with the side groups pointing in b-axis direction, thus accounting for the large length value of b. The absence of 6938


Article

Macromolecules

Table 6. Atomic Fractional Coordinates in the Asymmetric Unit for the Packing Model of Isotactic Alternating 4MPD/E Copolymers in the Form III of Figure 6, According to the Space Group Symmetry Pn C atoms

x/a

y/b

z/c

H atoms

x/a

y/b

z/c

C1 C2 C3 C4 C5 C6 C7 C8

0.819 0.705 0.714 0.787 0.634 0.343 0.743 0.819

0.109 0.149 0.153 0.252 0.324 0.321 0.417 0.109

0.624 0.320 0.824 0.859 0.777 0.625 0.831 0.121

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14

0.779 1.031 0.737 0.494 0.503 0.995 0.265 0.278 0.954 0.694 0.778 1.031 0.256 0.666

0.037 0.116 0.222 0.139 0.144 0.263 0.253 0.355 0.417 0.454 0.037 0.116 0.356 0.456

0.610 0.716 0.334 0.223 0.728 0.965 0.584 0.416 0.945 0.628 1.098 0.223 0.752 0.961

Table 7. Comparison between the Observed (Fobs) and Calculated (Fcalc) Structure Factors for the Isotactic Alternating 4MPD/E Copolymers in the Form IIIa

Figure 7. Comparison between the experimental (a) and calculated (b) X-ray powder diffraction profiles for form III of isotactic alternating 4MPD/E copolymers. Profile b has been calculated for the structural model of Figure 6.

A discrepancy factor R of 13% is obtained for the only observed reflections, whereas the discrepancy factor R′ calculated for both observed and nonobserved reflections is 26%. This confirms that the structural model of Figure 6 in the space group symmetry Pn is a good description for the arrangement of the chains of isotactic alternating 4MPD/E copolymers in form III, even though the structure factors of some reflections are calculated too high with respect to the observed ones. In particular, discrepancies are observed for the structure factors of the reflections in the 2θ region higher than 36.5° and for the 1̅31, 111, and 2̅11 reflections at d ≈ 3.3, 2.9, and 2.8 Å, respectively (2θ ≈ 27, 31, and 32°, respectively), which are not observed. Although further improvement of the experimental and calculated diffraction data can be easily achieved by finely adjusting the relative arrangement of the chains in the unit cell, and introducing structural disorder for the azimuthal setting of the chains and conformational disorder for the rotation of the side groups around the torsion angle θ5 (Scheme 3B), the agreement obtained for the limit ordered model of Figure 6 with the experimental data may be considered satisfactory.

■

CONCLUDING REMARKS The structure and thermal properties of isotactic copolymers of 4-methyl-1,3-pentadiene with ethylene having 4MPD content in the range 41−69 mol and of isotactic poly(4-methyl-1,3-

a

Calculations are relative to the packing model of Figure 6, in the space group symmetry Pn. Only reflections having a calculated intensity ≥8 are reported.

6939


Article

Macromolecules

(10) (a) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley-Interscience: New York, 1974. (b) Buerger, M. J. Crystal Structure Analysis; John Wiley & Sons: New York, 1960. (c) De Rosa, C.; Auriemma, F. Crystals and Crystallinity in Polymers; Wiley: Hoboken, NJ, 2014. (11) Cerius2 Modeling Environment; Molecular Simulations Inc.: San Diego, CA, 1999. (12) Proto, A.; Senatore, D. Macromol. Chem. Phys. 1999, 200, 1961− 1964. (13) Carman, C. J.; Wilkes, C. E. Rubber Chem. Technol. 1971, 44, 781−804. (14) Ray, G. J.; Johnson, P. E.; Knox, J. R. Macromolecules 1977, 10, 773−778. (15) Kakugo, M.; Naito, Y.; Mizunuma, K.; Miyatake, T. Macromolecules 1982, 15, 1150−1152. (16) (a) Meille, S. V.; Allegra, G.; Geil, P. H.; He, J.; Hess, J.; Jin, J.-I.; Kratochvíl, P.; Mormann, W.; Stepto, R. Pure Appl. Chem. 2011, 83, 1831−1871. (b) Allegra, G.; Corradini, P.; Elias, H. G.; Geil, P. H.; Keith, H. D.; Wunderlich, B. Pure Appl. Chem. 1989, 61, 769−785.

pentadiene) homopolymer have been analyzed. These samples have been synthesized resorting to homogeneous organometallic complexes characterized by a tetradentate [OSSO]type ligand having two phenoxo-units linked through a 1,ωdithiaalkanediyl bridge −S(CH2)2S−, activated with MAO. Solution 13C NMR analysis reveals that these copolymers have a prevailing alternating constitution and are regioregular. Moreover, all the copolymers are crystalline and show melting endotherms in the temperature range 90−100 °C. In particular, the samples with nearly equimolar concentration of E and 4MPD units show diffraction profiles different from those of forms I and II of iP4MPD homopolymer. They indeed crystallize in a new crystalline form, called form III, which has been attributed to the crystallization of alternating E-4MPD sequences. The copolymer with 4MPD concentration around 65−70 mol %, instead, crystallize as mixtures of forms I and II of iP4MPD and form III. A structural model for the form III of the alternating isotactic 4MPD/E copolymer is proposed. The alternating sequences assume a nearly trans-planar conformation with periodicity c = 5.13 Å, and are arranged in a monoclinic unit cell with axes a = 5.70 Å, b (unique axis) = 14.95 Å, c (chain axis) = 5.13 Å, and β = 114.4° in the space group symmetry Pn. The 4MPD/E isotactic copolymers represent a new class of materials of potential interest as they bear double bonds in the side chains. The possible functionalization of these groups may allow obtaining new materials with tailored properties that can be easily controlled by changing the 4MPD concentration.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: ++39 081 674341. Fax ++39 081 674090. E-mail: finizia. [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from MIUR (Project Prin 2010-2011), Pirelli Tyre S.p.A, and Fondazione Cariplo (Cariplo Project 2013 “Crystalline Elastomers”) is acknowledged.

■ ■

DEDICATION Dedicated to the late memory of Prof. Adolfo Zambelli. REFERENCES

(1) Porri, L.; Gallazzi, M. C. Eur. Polym. J. 1966, 2, 189−198. (2) Natta, G.; Corradini, P.; Bassi, I. W.; Fagherazzi, G. Eur. Polym. J. 1968, 4, 297−298. (3) Proto, A.; Capacchione, C.; Venditto, V.; Okuda, J. Macromolecules 2003, 36, 9249−9251. (4) Proto, A.; Avagliano, A.; Saviello, D.; Capacchione, C. Macromolecules 2009, 42, 6981−6985. (5) Bunn, C. W. Trans. Faraday Soc. 1939, 35, 482−491. (6) Bachman, G. B.; Goebel, C. G. J. Am. Chem. Soc. 1942, 64, 787− 790. (7) Capacchione, C.; Proto, A.; Ebeling, H.; Mulhaupt, R.; Spaniol, T. P.; Möller, K.; Okuda, J. J. Am. Chem. Soc. 2003, 125, 4964−4965. (8) Capacchione, C.; Manivannan, R.; Barone, M.; Beckerle, K.; Centore, R.; Oliva, L.; Proto, A.; Tuzi, A.; Spaniol, T. P.; Okuda, J. Organometallics 2005, 24, 2971−2982. (9) Cromer, D. T.; Mann, J. B. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1968, 24, 321. 6940


4-Methyl-1,3-pentadiene Alternating

Recommend Documents