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Isotactic and Syndiotactic Alternating Ethylene/Propylene Copolymers Obtained Through Non-Catalytic Hydrogenation of Highly Stereoregular cis-1,4 Poly(1,3-diene)s

The homogeneous non-catalytic hydrogenation of cis-1,4 poly(isoprene), isotactic cis-1,4 poly(1,3-pentadiene) and syndiotactic cis-1,4 poly(1,3-pentadiene) with diimide, formed by thermal decomposition of para-toluenesulfonylhydrazide, is examined. Perfectly alternating ethylene/propylene copolymers having different tacticity (i.e., isotactic and syndiotactic), which are difficult to synthesize by stereospecific copolymerization of the corresponding monomers, are obtained. Both isotactic and syndiotactic alternating ethylene/propylene copolymers are amorphous, with very low glass transition temperatures.


Introduction
Nowadays ethylene/propylene (E/P) copolymers are one of the most important families of polymeric materials, with endless applications. The performance of E/P copolymers can be easily tuned by varying the comonomer composition, comonomer distribution and chain stereoregularity. When the comonomers are randomly distributed, amorphous polymers (E/P rubbers and E/P/diene rubbers), emerging as a new class of thermoplastic elastomers, can be obtained [1]. If the comonomer is isolated, crystalline copolymers result, which are widely used as impact-strength modifiers in blends with isotactic poly(propylene) [2].
The synthesis of alternating E/P copolymers has been one of the challenging subjects of both practical and fundamental interest in recent years [3]. The development of metallocene [4], and post-metallocene complexes [5] for olefin polymerization has opened new opportunities for the synthesis of alternating copolymers with controlled microstructure and properties. Alternating E/P copolymers have been obtained by the copolymerization of ethylene with propylene with some ansa-zirconocenes-based catalysts [6][7][8], and by living polymerization of 1-pentene with an α-diimine Ni(II) catalyst through a controlled chain-walking mechanism [9].

Scheme 2.
Saturated α-olefin (co)polymers which, at least in principle, can be obtained by hydrogenation of cis-1,4 poly(1,3-diene)s. In bold the alternating E/P copolymer object of the present investigation. Poly(E-alt-2-butene) can be also obtained with R 1 = R 2 = Me and R 3 = H.

Results and Discussion
The highly stereoregular poly(1,3-diene)s on which this study focused were: syndiotactic cis-1,4 poly(1,3-pentadiene) [hereafter named sP(EP)], isotactic cis-1,4 poly(1,3-pentadiene) [iP(EP)] and cis-1,4 poly(isoprene) (PI). iP(EP) and PI were obtained by polymerizing (E)-1,3-pentadiene and isoprene, respectively, with the ternary catalyst system AlEt 2 Cl/Nd(OCOC 7 H 15 ) 3 /AliBu 3 , as described in [21,32,33], respectively. The sP(EP) was synthesized with catalyst CoCl 2 (P t Bu 2 Me) 2 /MAO as described in [34]. The polymerization data are summarized in Table 1. The hydrogenation of the above polymers was then examined. There are different methods to hydrogenate poly(1,3-diene)s which involve both catalytic and non-catalytic ways [35]. Catalytic hydrogenation is the conventional one, but it has some drawbacks such as: (i) the high cost of the equipment, (ii) the use of expensive hydrogenation conditions, which are mostly associated with high pressure reactors and expensive catalyst, and (iii) the low efficiencies resulting from limited solubility of the reagents. Alternatively, the hydrogenation of unsaturated polymers by a non-catalytic way, in which the reaction is promoted by diimide (diazene, NH=NH), has been shown to be an attractive process and extremely efficient in the case of 1,3-diene polymers [36,37].
In this work, we exploited the non-catalytic hydrogenation of the target stereoregular poly(1,3-diene)s with diimide in homogeneous conditions at 120 • C in o-xylene (Scheme 3). Diimide was generated in situ through the thermolysis of p-toluenesulfonic acid (TSH) [38]. The hydrogenation process converted iso-and syndiotactic cis-1,4 poly(1,3-pentadiene)s and cis-1,4 poly(isoprene) into perfectly alternating E/P copolymers (Scheme 3), having iso-or syndiotactic structures depending on the starting unsaturated polymer. Hereafter, we will name the alternating E/P copolymers from the hydrogenation of the corresponding 1,3-diene polymers as H-sP(EP), H-iP(EP) and H-PI, respectively.
The complete hydrogenation of the diene polymers was confirmed by comparison of FTIR and 1 H-NMR spectra of the starting cis-1,4 poly(1,3-diene)s and of the corresponding hydrogenated products (Figures 1 and 2, respectively).
The typical bands observed at 746 cm −1 in the FTIR spectra of cis-1,4 poly(1,3-pentadiene)s and 840 cm −1 in the FTIR spectra of cis-1,4 poly(isoprene), ascribed to the out-of-plane vibration of the hydrogen atoms adjacent to the double bond in a cis-1,4 unit, are completely absent in the FTIR spectra of the corresponding saturated polymers; besides, a new band at 735 cm −1 was observed in the FTIR spectra of the hydrogenated polymers, ascribed to the vibration of a -CH 2 -unit, typical of saturated polyolefins ( Figure 1).          The 1 H-NMR spectra of sP(EP), iP(EP), PI and those of the corresponding hydrogenated products H-sP(EP), H-iP(EP) and H-PI, are shown in Figure 2. As it is clearly evident the peaks in the olefinic region (from 5.2 to 5.4 ppm), observed in the 1 H-NMR spectra of the diene polymers, and due to the olefinic hydrogen atoms, are not observed in the 1 H-NMR spectra of the hydrogenated polymers, confirming indeed the complete hydrogenation of the diene polymers.
The structure and tacticity of the resulting E/P copolymers were investigated by means of 1 H and 13 C-NMR, 2D NMR experiments [i.e., Heteronuclear Single Quantum Correlation (HSQC)] and X-ray diffraction analysis. Figure 3 shows the 13 C-NMR spectra of the diene polymers; the peaks were assigned as already reported [39]. The 1 H-NMR spectra of sP(EP), iP(EP), PI and those of the corresponding hydrogenated products H-sP(EP), H-iP(EP) and H-PI, are shown in Figure 2. As it is clearly evident the peaks in the olefinic region (from 5.2 to 5.4 ppm), observed in the 1 H-NMR spectra of the diene polymers, and due to the olefinic hydrogen atoms, are not observed in the 1 H-NMR spectra of the hydrogenated polymers, confirming indeed the complete hydrogenation of the diene polymers.
The structure and tacticity of the resulting E/P copolymers were investigated by means of 1 H and 13 C-NMR, 2D NMR experiments [i.e., Heteronuclear Single Quantum Correlation (HSQC)] and X-ray diffraction analysis. Figure 3 shows the 13 C-NMR spectra of the diene polymers; the peaks were assigned as already reported [39].  The 13 C-NMR spectra of the hydrogenated polymers (E/P copolymers) are shown in Figure 4 and exhibit four major resonances around 17.9 (C5), 22.6 (C2), 31.0 (C4), and 35.7 (C1,C3) ppm likely corresponding to the four unique signals of a perfectly alternating E/P copolymer structure. The chemical shifts are very close for the three hydrogenated polymers so that, on the basis of the 13 C-NMR spectra only, it is not possible to distinguish the tacticity of the copolymers obtained with the hydrogenation reaction. In principle, however, since the hydrogenation reaction in the case of poly(1,3-pentadiene)s does not lead to the formation of new asymmetric carbon atoms, it is reasonable to assume that the tacticity of the diene polymer precursors is maintained in the alternating resulting E/P copolymers, that is isotactic for H-iP(EP) and syndiotactic for H-sP(EP). This assumption was confirmed by means of the two-dimensional correlation spectroscopy, HSQC The 13 C-NMR spectra of the hydrogenated polymers (E/P copolymers) are shown in Figure 4 and exhibit four major resonances around 17.9 (C5), 22.6 (C2), 31.0 (C4), and 35.7 (C1,C3) ppm likely corresponding to the four unique signals of a perfectly alternating E/P copolymer structure. The chemical shifts are very close for the three hydrogenated polymers so that, on the basis of the 13 C-NMR spectra only, it is not possible to distinguish the tacticity of the copolymers obtained with the hydrogenation reaction. In principle, however, since the hydrogenation reaction in the case of poly(1,3-pentadiene)s does not lead to the formation of new asymmetric carbon atoms, it is reasonable to assume that the tacticity of the diene polymer precursors is maintained in the alternating resulting E/P copolymers, that is isotactic for H-iP(EP) and syndiotactic for H-sP(EP). This assumption was confirmed by means of the two-dimensional correlation spectroscopy, HSQC experiment, as the presence of a cross peak is indicative of protons and carbons directly linked through 1 J CH , and was able to give information about the different tacticity of the E/P copolymers. Results of these experiments are shown in Figure 5. Specifically, the signal at about 22.6 ppm, corresponding to the Sββ methylene carbon, has proved to be diagnostic of the tacticity of the copolymer, as it is influenced by the arrangement of the methyl substituents, although not adjacent to the carbon atom bearing the methyl. experiment, as the presence of a cross peak is indicative of protons and carbons directly linked through 1 JCH, and was able to give information about the different tacticity of the E/P copolymers. Results of these experiments are shown in Figure 5. Specifically, the signal at about 22.6 ppm, corresponding to the Sββ methylene carbon, has proved to be diagnostic of the tacticity of the copolymer, as it is influenced by the arrangement of the methyl substituents, although not adjacent to the carbon atom bearing the methyl.  In the HSQC spectrum of the product of the hydrogenation of isotactic cis-1,4 poly(1,3-pentadene) (Figure 5a, iP(EP)), the C2 carbon at δ = 22.59 ppm correlates with two cross peaks at δH = 1.25 and 1.14 ppm, respectively. This is indicative of a methylene to which two magnetically non-equivalent protons are linked, as they display two different resonance frequencies "like in a different environment", thus suggesting that the E/P copolymer obtained by hydrogenation of iP(EP) has an isotactic structure [40,41].
In the case of the E/P copolymer (Figure 5b) obtained by hydrogenation of sP(EP), the methylene at δ = 22.61 ppm showed a single correlation peak at δH = 1.19 ppm, meaning a linking with two magnetic equivalent protons, in agreement with the syndiotactic structure of H-sP(EP). Finally, for the E/P copolymer obtained by hydrogenation of PI, the 2D HSQC experiment suggests the coexistence of isotactic and syndiotactic stereoregularities (Figure 5c). The syndiotacticity (associated to the single cross peak for δC = 22.6 ppm, δH = 1.19 ppm) seems to be slightly preferred with respect to the isotacticity (two cross peaks for δC = 22.6 ppm, δH = 1.13 and 1.27 ppm), being the single cross peak intensity more pronounced in the 2D spectrum. Once the tacticity was established, the peaks multiplicity observed for the 13    In the HSQC spectrum of the product of the hydrogenation of isotactic cis-1,4 poly(1,3-pentadene) (Figure 5a, iP(EP)), the C2 carbon at δ = 22.59 ppm correlates with two cross peaks at δ H = 1.25 and 1.14 ppm, respectively. This is indicative of a methylene to which two magnetically non-equivalent protons are linked, as they display two different resonance frequencies "like in a different environment", thus suggesting that the E/P copolymer obtained by hydrogenation of iP(EP) has an isotactic structure [40,41].
In the case of the E/P copolymer (Figure 5b) obtained by hydrogenation of sP(EP), the methylene at δ = 22.61 ppm showed a single correlation peak at δ H = 1.19 ppm, meaning a linking with two magnetic equivalent protons, in agreement with the syndiotactic structure of H-sP(EP). Finally, for the E/P copolymer obtained by hydrogenation of PI, the 2D HSQC experiment suggests the coexistence of isotactic and syndiotactic stereoregularities (Figure 5c). The syndiotacticity (associated to the single cross peak for δ C = 22.6 ppm, δ H = 1.19 ppm) seems to be slightly preferred with respect to the isotacticity (two cross peaks for δ C = 22.6 ppm, δ H = 1.13 and 1.27 ppm), being the single cross peak intensity more pronounced in the 2D spectrum. Once the tacticity was established, the peaks multiplicity observed for the 13   The X-ray powder diffraction profiles of the hydrogenated polymers H-iP(EP), H-sP(EP), and H-PI are shown in Figure 6. It is apparent that all samples show broad diffraction profiles with an absence of Bragg reflections, indicating that all samples are amorphous. The diffraction profiles do not change upon thermal treatments and the samples do not crystallize even after annealing at relatively high temperatures or upon aging at room and low temperatures. Moreover, in the case of sample iP(EP), that can be stretched at relatively high deformations, crystallization does not take place by stretching up to high degrees of deformation and even keeping the sample under tension at low temperature for long time. The NMR data of Figures 4 and 5 as well as the diffraction data of Figure 6 indicate that the E/P alternating copolymers have a regular stereochemical structure, isotactic and syndiotactic for H-iP(EP) and H-sP(EP), respectively, and a statistical atactic structure for H-PI; the regular copolymers are however not able to crystallize.
The DSC heating curves of samples H-iP(EP), H-sP(EP) and H-PI are shown in Figure 7. According to the X-ray diffraction profiles of Figure 6, all DSC curves show only a glass transition at about −60 °C and the absence of any endothermic signal ( Table 2). Only the glass transition at the same temperatures is observed in the successive cooling scans with absence of exothermic signals. The X-ray powder diffraction profiles of the hydrogenated polymers H-iP(EP), H-sP(EP), and H-PI are shown in Figure 6. It is apparent that all samples show broad diffraction profiles with an absence of Bragg reflections, indicating that all samples are amorphous. The diffraction profiles do not change upon thermal treatments and the samples do not crystallize even after annealing at relatively high temperatures or upon aging at room and low temperatures. Moreover, in the case of sample iP(EP), that can be stretched at relatively high deformations, crystallization does not take place by stretching up to high degrees of deformation and even keeping the sample under tension at low temperature for long time. The NMR data of Figures 4 and 5 as well as the diffraction data of Figure 6 indicate that the E/P alternating copolymers have a regular stereochemical structure, isotactic and syndiotactic for H-iP(EP) and H-sP(EP), respectively, and a statistical atactic structure for H-PI; the regular copolymers are however not able to crystallize.
The DSC heating curves of samples H-iP(EP), H-sP(EP) and H-PI are shown in Figure 7. According to the X-ray diffraction profiles of Figure 6, all DSC curves show only a glass transition at about −60 • C and the absence of any endothermic signal (Table 2). Only the glass transition at the same temperatures is observed in the successive cooling scans with absence of exothermic signals.        The mechanical properties under tensile deformation of the three E/P alternating copolymers of different stereochemical structure have also been studied. The stress-strain curves recorded at room temperature of compression-molded films of the hydrogenated polymers are shown in Figure 8. According to the absence of crystallinity, all samples show low values of the Young modulus (0.6-1.0 MPa) and of the stress at any strain and viscous flow already at relatively low deformations. The differences in the stress-strain curves are mainly related to the different molecular masses of the three alternating copolymers. Higher values of the stress are, indeed, shown by the sample H-PI of higher molecular mass (Figure 8). The mechanical properties under tensile deformation of the three E/P alternating copolymers of different stereochemical structure have also been studied. The stress-strain curves recorded at room temperature of compression-molded films of the hydrogenated polymers are shown in Figure 8. According to the absence of crystallinity, all samples show low values of the Young modulus (0.6-1.0 MPa) and of the stress at any strain and viscous flow already at relatively low deformations. The differences in the stress-strain curves are mainly related to the different molecular masses of the three alternating copolymers. Higher values of the stress are, indeed, shown by the sample H-PI of higher molecular mass (Figure 8).

Hydrogenation Procedure
The hydrogenation was carried out in a round-bottom flask equipped with a reflux condenser, a nitrogen inlet port, and a temperature controller. Typically, the specified amount of the diene polymer was dissolved in o-xylene. The mixture was continuously stirred at room temperature until the polymer was completely dissolved. TSH was then added and the mixture was refluxed by slowly heating to 120 °C. After 3 days the mixture was allowed to cool spontaneously to room

Hydrogenation Procedure
The hydrogenation was carried out in a round-bottom flask equipped with a reflux condenser, a nitrogen inlet port, and a temperature controller. Typically, the specified amount of the diene polymer was dissolved in o-xylene. The mixture was continuously stirred at room temperature until the polymer was completely dissolved. TSH was then added and the mixture was refluxed by slowly heating to 120 • C. After 3 days the mixture was allowed to cool spontaneously to room temperature and TSH was added. This operation is repeated once again. Upon completion of the reaction, the hydrogenated sample was hot-filtered, the volume of the filtered solution was reduced under vacuum, and the dissolved polymer precipitated with methanol and collected by filtration. The polymer was dried under vacuum at room temperature, and then it was extracted with acetone through a Soxhlet method for 10 h in order to remove any excess TSH and by products originating from TSH decomposition. The residual polymer was finally dried under vacuum, dissolved in toluene, precipitated into methanol, and dried again under vacuum at room temperature to constant weight. Specifically, for each polymer sample the conditions were: Two benchmarks have been placed on the test specimens and used to measure elongation. The ratio between the drawing rate and the initial length was fixed equal to 0.1 mm/(mm × min) for the measurement of Young's modulus and 10 mm/(mm × min) for the measurement of stress-strain curves. The reported stress-strain curves and the values of the mechanical properties are averaged over at least five independent experiments.