Ethylene-alt-α-Olefin Copolymers by Hydrogenation of Highly Stereoregular cis-1,4 Polydienes: Synthesis and Structural Characterization

The homogeneous non-catalytic hydrogenation of several types of iso- and syndiotactic cis-1,4 poly(1,3-diene)s with diimide, formed by thermal decomposition of p-toluene-sulfonyl-hydrazide, was examined. Perfectly alternating ethylene/1-alkene copolymers having different tacticity (i.e., isotactic and syndiotactic), which in some cases are difficult to synthesize by simple stereospecific co-polymerization of the corresponding monomers, were obtained. All the copolymers synthesized were fully characterized from a structural, morphological, and rheological point of view through different analytical techniques (FT-IR, NMR, GPC, DSC, RX).

We have already reported on the synthesis and characterization of isotactic and syndiotactic perfectly alternating ethylene/2-butene [16] and ethylene/propylene [17] copolymers.
We have already reported on the synthesis and characterization of isotactic and syndiotactic perfectly alternating ethylene/2-butene [16] and ethylene/propylene [17] copolymers.
The hydrogenation of the above polymers was then examined.The hydrogenation is a type of chemical modification of polymers, allowing to reduce the amount of unsaturation, and leading to polymers with modified and improved properties and to the production of new materials [28].Nevertheless, the hydrogenation may also be seen as a possible route to provide polymers which can be hardly prepared by a simple conventional polymerization of the corresponding monomers.
There are different methods to hydrogenate poly(1,3-diene)s, which involve catalytic and non-catalytic processes.The hydrogenation of unsaturated polymers in a non-catalytic way, in which the reaction is promoted by diimide (diazene, NH=NH), turned out to be an attractive process and extremely efficient in the case of 1,3-diene polymers [28,29,[35][36][37].Diimide hydrogenation is a non-catalytic process in which the hydrogenated polymers are formed through the reduction reaction between the diimide and the olefinic group (-C=C-).The diimide molecule (N 2 H 2 ), as the hydrogenation agent, is generated in situ from the thermal decomposition of p-toluenesulfonyl hydrazide (TSH).The diimide molecule (N 2 H 2 ) can then release a hydrogen molecule directly to the carbon-carbon double bonds, thus allowing the hydrogenation process.
Data concerning tacticity, molecular weight and molecular weight distribution, and glass transition temperature for all the hydrogenated polymers, are summarized in Table 2.The hydrogenation process of the isotactic cis-1,4 poly(1,3-hexadiene) was illustrated in Scheme 2.
The successful complete hydrogenation of the diene polymer was confirmed by comparison of the Fourier transform infrared (FT-IR) (Figure S1) and 1 H and 13 C NMR spectra (Figure 1) of the starting cis-1,4 poly(1,3-diene) and of the corresponding hydrogenated product.

Polymer
Mw
The successful complete hydrogenation of the diene polymer was confirmed by comparison of the Fourier transform infrared (FT-IR) (Figure S1) and 1 H and 13 C NMR spectra (Figure 1) of the starting cis-1,4 poly(1,3-diene) and of the corresponding hydrogenated product.alternating ethylene/1-butene isotactic copolymer H(cis1,4 iso PHX) The typical band observed at 751 cm −1 in the FTIR spectrum of cis-1,4 poly(1,3hexadiene), ascribed to the out-of-plane vibration of the hydrogen atoms adjacent to the double bond in a cis-1,4 unit, is completely absent in the FT-IR spectrum of the corresponding hydrogenated polymer; besides, a new band at 735 cm −1 is observed in the FT-IR spectrum of the hydrogenated polymer, ascribed to the vibration of a -CH 2 -unit, typical of saturated polyolefins (Figure S1) The 1 H and 13 C NMR spectra of the isotactic cis-1,4 poly(1,3-hexadiene) and of the corresponding hydrogenated product are shown in Figure 1.As it is clearly evident, peaks in the olefinic region observed in the 1 H NMR (from 5.2 to 5.4 ppm) and 13 C NMR (from 120 to 140 ppm) spectra of the diene polymers, and due to the olefinic hydrogen and carbon atoms, are not observed in both the NMR spectra of the hydrogenated polymer, confirming indeed the complete hydrogenation of the diene polymer.
The structure of the ethylene /1-butene copolymer in Figure 1 was determined by means of 1 H, 13 C, and two-dimensional NMR experiments, such as 1 H- 13 C heteronuclear experiments (HSQC and HMBC in Figure S3) and two-dimensional 1 H-1 H homonuclear experiments (COSY and TOCSY Figure S4).
The X-ray diffraction profile of the alternating isotactic ethylene/1-butene copolymer is shown in Figure 2A, and the DSC curves recorded during the first heating, successive cooling from the melt, and second heating of the melt crystallized samples are reported in Figure 2B.Both X-ray diffraction and DSC data indicate the absence of crystallinity in this sample.The alternating isotactic ethylene/1-butene copolymer does not crystallize after annealing or cooling from high temperatures, as confirmed by the absence of exothermic peaks or endothermic peaks in the DSC cooling and heating curves.The DSC shows only a glass transition temperature of −66 • C (Figure 2B and Table 2).As in the case of alternating stereoregular ethylene/propene copolymers prepared from hydrogenation of isotactic and syndiotactic cis-1,4 poly(1,3-pentadiene) [17], the alternating isotactic ethylene/1-butene copolymer is not able to crystallize notwithstanding the regular stereochemical structure.
double bond in a cis-1,4 unit, is completely absent in the FT-IR spectrum of the corresponding hydrogenated polymer; besides, a new band at 735 cm −1 is observed in the FT-IR spectrum of the hydrogenated polymer, ascribed to the vibration of a -CH2unit, typical of saturated polyolefins (Figure S1) The 1 H and 13 C NMR spectra of the isotactic cis-1,4 poly(1,3-hexadiene) and of the corresponding hydrogenated product are shown in Figure 1.As it is clearly evident, peaks in the olefinic region observed in the 1 H NMR (from 5.2 to 5.4 ppm) and 13 C NMR (from 120 to 140 ppm) spectra of the diene polymers, and due to the olefinic hydrogen and carbon atoms, are not observed in both the NMR spectra of the hydrogenated polymer, confirming indeed the complete hydrogenation of the diene polymer.
The structure of the ethylene /1-butene copolymer in Figure 1 was determined by means of 1 H, 13 C, and two-dimensional NMR experiments, such as 1 H- 13 C heteronuclear experiments (HSQC and HMBC in Figure S3) and two-dimensional 1 H-1 H homonuclear experiments (COSY and TOCSY Figure S4).
The X-ray diffraction profile of the alternating isotactic ethylene/1-butene copolymer is shown in Figure 2A, and the DSC curves recorded during the first heating, successive cooling from the melt, and second heating of the melt crystallized samples are reported in Figure 2B.Both X-ray diffraction and DSC data indicate the absence of crystallinity in this sample.The alternating isotactic ethylene/1-butene copolymer does not crystallize after annealing or cooling from high temperatures, as confirmed by the absence of exothermic peaks or endothermic peaks in the DSC cooling and heating curves.The DSC shows only a glass transition temperature of −66 °C (Figure 2B and Table 2).As in the case of alternating stereoregular ethylene/propene copolymers prepared from hydrogenation of isotactic and syndiotactic cis-1,4 poly(1,3-pentadiene) [17], the alternating isotactic ethylene/1-butene copolymer is not able to crystallize notwithstanding the regular stereochemical structure.
To verify the successful hydrogenation of the diene polymers, FT-IR and 1 H and 13 C NMR spectra (Figures 3 and 4) of the starting cis-1,4 poly(1,3-diene)s were acquired and compared to the corresponding hydrogenated products.
As for the stereoregular E/1-B copolymer, the DSC thermograms and the X-ray diffraction of E/1-P copolymers indicate the both iso-and syndiotactc E/1-P copolymers are amorphous with a T g of −65 and −63 • C, respectively (Table 2).
To verify the successful hydrogenation of the diene polymers, FT-IR and 1 H and 13 C NMR spectra (Figures 3 and 4) of the starting cis-1,4 poly(1,3-diene)s were acquired and compared to the corresponding hydrogenated products.The peaks attribution of the isotactic and syndiotactic cis-1,4 poly(1,3-heptadiene)s and their saturated E/1-P isotactic and syndiotactic copolymers, are shown in Figures 3  and 4, respectively.
As for the stereoregular E/1-B copolymer, the DSC thermograms and the X-ray diffraction of E/1-P copolymers indicate the both iso-and syndiotactc E/1-P copolymers are amorphous with a Tg of −65 and −63 °C, respectively (Table 2).

Iso-and Syndiotactic Alternating Ethylene/1-Hexene Copolymers (H(cis1,4 iso PO) and H(cis1,4syndioPO))
The successful complete hydrogenation of the isotactic and syndiotactic cis-1,4 poly(1,3octadiene)s to alternating E/1-H copolymers, as reported in Scheme 4, was confirmed by FT-IR (Figure S2) and by 1 H and 13 C NMR spectra of the starting cis-1,4 poly(1,3-diene)s and of the corresponding hydrogenated products (Figures 5 and 6).The peaks in the olefinic regions due to the olefinic hydrogen atoms (from 5.0 to 5.6 ppm) observed in the 1 H NMR spectra (Figures 5 and 6) and of olefinic carbon atoms observed in the carbon spectra (at ≈126 and 133 ppm) of the diene polymers are not observed in the spectra of the hydrogenated polymers (Figures 5 and 6), thus confirming indeed the complete hydrogenation reaction.The peaks in the olefinic regions due to the olefinic hydrogen atoms (from 5.0 to 5.6 ppm) observed in the 1 H NMR spectra (Figures 5 and 6) and of olefinic carbon atoms observed in the carbon spectra (at ≈126 and 133 ppm) of the diene polymers are not observed in the spectra of the hydrogenated polymers (Figures 5 and 6), thus confirming indeed the complete hydrogenation reaction.  1C NMR (right) spectra of isotactic cis-1,4 poly(1,3-octadiene) (cis1,4 iso PO) (top) and its saturated E/1-H isotactic copolymer (H(cis1,4 iso PO)) (bottom).Figure 5. 1 H (left) and 13 C NMR (right) spectra of isotactic cis-1,4 poly(1,3-octadiene) (cis1,4 iso PO) (top) and its saturated E/1-H isotactic copolymer (H(cis1,4 iso PO)) (bottom).Also in that case, the polymers microstructure was determined by two dimensional NMR experiments, reported in Figures S9 and S10.
The X-ray diffraction profile of the alternating isotactic ethylene/1-hexene copolymer is shown in Figure 7A, and the DSC curves recorded during the first heating, successive cooling from the melt, and second heating of the melt crystallized samples are reported in Figure 7B.Both X-ray diffraction and DSC data indicate the E/1-H isotactic copolymers are amorphous and are not able to crystallize upon slow cooling from high temperatures or annealing.Similar behavior has been observed for the syndiotactic E/1-H copolymer.The DSC curves show that for both isotactic and syndiotactic E/1-H copolymers, only glass The peaks in the olefinic regions due to the olefinic hydrogen atoms (from 5.0 to 5.6 ppm) observed in the 1 H NMR spectra (Figures 5 and 6) and of olefinic carbon atoms observed in the carbon spectra (at ≈126 and 133 ppm) of the diene polymers are not observed in the spectra of the hydrogenated polymers (Figures 5 and 6), thus confirming indeed the complete hydrogenation reaction.
Also in that case, the polymers microstructure was determined by two dimensional NMR experiments, reported in Figures S9 and S10.
The X-ray diffraction profile of the alternating isotactic ethylene/1-hexene copolymer is shown in Figure 7A, and the DSC curves recorded during the first heating, successive cooling from the melt, and second heating of the melt crystallized samples are reported in Figure 7B.Both X-ray diffraction and DSC data indicate the E/1-H isotactic copolymers are amorphous and are not able to crystallize upon slow cooling from high temperatures or annealing.Similar behavior has been observed for the syndiotactic E/1-H copolymer.The DSC curves show that for both isotactic and syndiotactic E/1-H copolymers, only glass transition temperatures are nearly −60 • C (Figure 7B and Table 2).Also in that case, the polymers microstructure was determined by two dimensional NMR experiments, reported in Figures S9 and S10.
The X-ray diffraction profile of the alternating isotactic ethylene/1-hexene copolymer is shown in Figure 7A, and the DSC curves recorded during the first heating, successive cooling from the melt, and second heating of the melt crystallized samples are reported in Figure 7B.Both X-ray diffraction and DSC data indicate the E/1-H isotactic copolymers are amorphous and are not able to crystallize upon slow cooling from high temperatures or annealing.Similar behavior has been observed for the syndiotactic E/1-H copolymer.The DSC curves show that for both isotactic and syndiotactic E/1-H copolymers, only glass transition temperatures are nearly −60 °C (Figure 7B and Table 2).

Hydrogenation Procedure
The hydrogenation was carried out in a round-bottom flask equipped with a magnetic stirring, reflux condenser, nitrogen inlet port, and temperature controller.Typically, the specified amount of the diene polymer was dissolved in o-xylene.The reaction 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 byproducts originating from TSH decomposition.The residual polymer was finally dried under a vacuum, dissolved in toluene, precipitated into methanol, and dried again under a vacuum at room temperature to constant weight.

Polymer Characterization
1 H and 13 C NMR measurements were carried out on a Bruker (Billerica, MA, USA) Avance 400 spectrometer.The spectra were obtained in C 2 D 2 Cl 4 at 103 • C (hexamethyldisiloxane, HMDS, as internal standard).The concentration of polymer solutions was about 10 wt %. 13 C parameters were: spectral width 17 kHz; 90 • pulse 11.0 µs PL1 −5.0 dB, with a delay of 16 s.
The molecular weight averages (M w ) and the molecular weight distribution (M w /M n ) were obtained by a high-temperature Waters (Milford, MA, USA) GPCV2000 size exclusion chromatography (SEC) system using two online detectors: a differential viscometer and a refractometer.The experimental conditions consisted of three PL Gel Olexis columns, o-DCB as the mobile phase, 0.8 mL min −1 flow rate, and 145 • C temperature.Universal calibration of the SEC system was performed using eighteen narrow M w /M n polystyrene standards with molar weights ranging from 162 to 5.6 × 10 6 g mol −1 .For the analysis, about 12 mg of the polymer was dissolved in 5 mL of o-DCB with 0.05% of BHT as the antioxidant.
X-ray powder diffraction profiles were obtained with Ni-filtered Cu Kα radiation by using an Empyrean diffractometer by Malvern Panalytical (Malvern, UK) operating in the reflection geometry with continuous scans of the 2θ angle and scanning rate of 0.02 degree/s Thermal analysis was performed with a differential scanning calorimeter Mettler-DSC30/2285 (Zurich, Switzerland), equipped with a liquid nitrogen cooling system for measurements at low temperature.The scans were recorded in flowing nitrogen atmosphere at heating or cooling rates of 10 • C/min.