Keto-Polyethylenes with Controlled Crystallinity and Materials Properties from Catalytic Ethylene–CO–Norbornene Terpolymerization

Recent advances in Ni(II) catalyzed, nonalternating catalytic copolymerization of ethylene with carbon monoxide (CO) enable the synthesis of in-chain keto-functionalized polyethylenes (keto-PEs) with high-density polyethylene-like materials properties. Addition of norbornene as a bulky, noncrystallizable comonomer during catalytic polymerization allows tuning of the crystallinity in these keto-PE materials by randomly incorporated norbornene units in the polymer chain, while molecular weights are not adversely affected. Such crystallinity-reduced keto-PEs are characterized as softer materials with better ductility and may therefore be more suited for, e.g., potential film applications.


■ INTRODUCTION
Polyethylene (PE), representative of a semicrystalline polymer, exhibits a flexible molecular structure and is distinguished by a high crystallinity, yielding a strong and ductile material at room temperature.Since several applications require improved impact properties or enhanced transparency and deformability, precise control over materials properties by tailored modifications of the molecular structure or molecular weight distribution can further expand the range of PE's potential applications.−3 For example, low-density polyethylene (LDPE) from free radical polymerization exhibits a highly branched molecular structure compared to high-density polyethylene (HDPE) from transition metal-catalyzed ethylene polymerization, which consists of linear hydrocarbon chains devoid of branches. 2These different microstructures impact crystallization of the hydrocarbon chains, resulting in either crystalline, therefore more rigid, HDPE materials or soft LDPE materials with low crystallinity due to inhibited −CH 2 − alignment in folded chain crystallites. 2,4However, free-radical polymerization offers only limited control over branch formation. 5−13 In particular, the addition of a small amount of randomly distributed, noncrystallizable units into the PE main chain, similar to the branches found in LDPE, can alter the crystallization ability and decrease its melting temperature, as well as crystallinity.Common examples include ethylene copolymers with linear α-olefins to yield linear low-density polyethylene (LLDPE), 6−15 which is applied extensively in, e.g., packaging films for commodities or structural components.
Especially, the class of cyclic olefin copolymers (COCs) employing norbornene as a cyclic, noncrystallizable unit has gained particular attention due to their versatility.−31 However, the majority of these literaturereported COCs exclusively focuses on amorphous copolymers with high norbornene contents (>30 mol %), which display high glass transition temperatures and properties that differ completely from those of PE.
Materials properties of PE beyond crystallinity can also be influenced by the introduction of functional groups to the otherwise highly apolar and hydrophobic polymer.−43 In contrast to vinyl comonomers, the copolymerization of ethylene with carbon monoxide (CO) can yield keto groups directly in the polyethylene backbone.−55 Such materials have only recently been enabled by nonalternating copolymerization of ethylene and CO. 52,56−60 Advanced neutral phosphinophenolate Ni(II) catalysts 61−65 have been particularly suitable for this direct catalytic copolymerization of ethylene and CO, yielding photodegradable keto-PE materials with high molecular weights (up to M w 400 000 g mol −1 ) and virtually uncompromised HDPE-like properties. 42,51,66,67owever, these keto-PE materials are highly crystalline, which may limit their possible applications if softer materials are required.We now report the use of norbornene as a noncrystallizable, second comonomer in nonalternating Ni(II)catalyzed ethylene-CO copolymerization.This enables control over crystallinity in the obtained keto-PEs and can improve their physical and mechanical materials properties, while not adversely affecting molecular weights of the obtained terpolymers, unlike in ethylene-CO acrylate terpolymerization. 42

■ RESULTS AND DISCUSSION
The exposure of a state-of-the-art neutral phosphinophenolate Ni(II) catalyst 1, previously reported for efficient co-and terpolymerizations of ethylene and CO, 42,51,67 to a mixed feed of gaseous ethylene and CO (10 atm total pressure) with a low ratio of CO (0.8%) in the presence of norbornene at different concentrations resulted in the formation of solid PE-like polymers (Scheme 1 and Table 1).
Polymerization activities and polymer yields are reduced by the combined presence of both comonomers norbornene and carbon monoxide compared to the respective ethylene copolymerization employing only one respective comonomer (Table 1, entries 1 and 2).However, the decrease in the yield and activity is much less pronounced compared to previously reported terpolymerizations of ethylene, CO, and acrylates. 42,68urthermore, polymerization yields are not largely influenced by the variation of norbornene in the initial reaction mixture.The analysis of the obtained polymers by attenuated total reflectance (ATR)-IR spectroscopy revealed the incorporation of predominantly isolated in-chain keto groups by the presence of an C�O absorption peak at 1715 cm −1 , which is characteristic for the latter (Figure 1).analysis of IR spectra allowed calculation of the C�O incorporation ratios, which are around the target value of approximately 1 mol % (0.7−1.1 mol %).The analysis by 1 H and 13 C NMR spectroscopy confirmed the presence of in-chain keto groups as well as their incorporation in a largely isolated fashion (Figures 2 and S4−S10).For enhanced sensitivity in 13 C NMR spectroscopy, 13 CO instead of 12 CO was employed as a comonomer to conveniently introduce isotopic labeling in a representative copolymer (Table 1, entry 6).The carbonyl microstructure was found to be comparable to our previously reported keto-PEs 46,51,67 with the majority of in-chain carbonyl groups incorporated as isolated units in the polymer backbone, as expected from observations by IR spectroscopy (Figure 2a).
Due to the lack of IR-sensitive functional groups in norbornene, incorporation of the second comonomer was visible only in 1 H and 13 C NMR spectroscopy, which was also used for simultaneous quantification of the norbornene content in the obtained terpolymers.Norbornene contents in a range of 0.5−3.3mol % could be obtained, which were found to be controllable by the initial norbornene concentration applied in the reaction mixture (cf.Table 1).Ethylene incorporation is favored over norbornene incorporation and in fact, only a small amount of the initially present norbornene is reacted, corresponding to near steady-state conditions of norbornene monomer concentration (<10% conversion as determined from the composition of the initial reaction mixture, and the amount and composition of polymer formed).Note that the other monomers, ethylene and CO, are replenished by the automated feed system.Thorough investigation by 1D and 2D NMR spectroscopy allowed for a complete assignment of all signals observed in NMR spectroscopy (cf. Figure 2 and Supporting Information).
This revealed the incorporation of norbornene in an exclusively isolated fashion, and neither alternating nor block-like motifs could be observed.−72 The occurrence of additional polynorbornene  resonances was not related to the formed ethylenenorbornene-CO terpolymer but was referred to a small and variable fraction of polynorbornene formation by a ROMP mechanism upon the exposure of norbornene to the Ni catalyst or impurities in the setup at the elevated polymerization temperatures. 73These amorphous polynorbornene fractions could be removed from the desired terpolymer by washing with toluene without affecting any materials properties of the crystallinity-reduced keto-PEs (cf.Supporting Information) (Figure S17 and Table S3).In addition to the characteristic norbornene resonances, the observation of new carbonyl signals in 13 C NMR spectroscopy on 13 CO-labeled samples showed the presence of a carbonyl group adjacent to a norbornene unit, as well as a carbonyl group separated by likely one −(C 2 H 4 )− unit from ethylene insertion between a respective CO and a norbornene insertion event (Figure 2a).Nevertheless, no preference for promoted insertion of either comonomer after the other could be observed, similar to previous reports on ethylene-CO terpolymerization with vinylic monomers. 42,68Contrary to previously reported ethylene co-and terpolymerizations with other vinyl monomers, 42,62,68,74 only end groups from expected chain termination by β-H elimination after an ethylene incorporation and no enhanced chain termination by the presence of norbornene were observable.In fact, terpolymer molecular weights are only slightly lowered by the presence of norbornene as a second comonomer and are accessible in a similar range compared to the keto-PE without norbornene (cf.Table 1, entry 2 vs 6, and Figure S14).
Wide angle X-ray powder diffraction (WAXS) profiles collected on melt-crystallized samples indicated that all ethylene−CO−norbornene terpolymers crystallize in an orthorhombic solid-state structure characteristic of PE (Figure 3a).The incorporation of the small amounts of carbon monoxide and norbornene, even in the terpolymer with the highest norbornene content of 3.3 mol % (Table 1, entry 9), does not have a significant effect on the crystalline packing of the polyethylene chains in the range of explored comonomer contents.Nevertheless, a significant reduction of crystallinity with comonomer incorporation was observed.In particular, the degree of crystallinity, evaluated from WAXS diffraction profiles (cf.Supporting Information) (Figure S15), is rather high [xcWAXS = 60%, xcDSC = 63%] in the neat keto-PE containing only ethylene and CO comonomers and gradually decreases in the terpolymers with increasing norbornene content, from nearly 48% of sample 3 with 0.5 mol % NB to about 31% of sample 9 with 3.3 mol % NB (Figure 3b).The influence of the presence of norbornene counits on PE crystallinity has been thoroughly investigated by Alamo et al. 13 These studies included several samples of random ethylenenorbornene copolymers in a compositional range similar to the terpolymers investigated in this work (1−5 mol % NB).Alamo et al. further demonstrated that, for low comonomer concentrations, the impact of norbornene on the PE crystallinity is virtually identical to those reported for ethylene−1-alkene (1-butene, 1-hexene, 1-octene) copolymers. 13Our results are consistent with those reported in ref 13, again confirming that the presence of in-chain carbonyl groups does not affect crystallinity and, hence, the decrease in crystallinity is solely due to the incorporated norbornene.WAXS data also confirm the predominantly isolated nature of the in-chain carbonyl groups as evident by the absence of the (110) reflection 2θ ≈ 22.5°related to alternating polyketone crystals (Figure 3a). 75SC thermograms (Figure 4) clearly confirm the incorporation of norbornene into all synthesized terpolymer samples (Figure 4), in line with NMR spectroscopic analysis.In fact, crystallization (Figure 4a) and melting (Figure 4b) points gradually decrease as the NB contents increase.However, it is worth noting that both melting and crystallization temperatures are only mildly affected by the presence of the two comonomers retaining relatively high values, only slightly lower than those of HDPE, even for norbornene contents above 3 mol %.Such thermal properties are a particular prerequisite for the potential processing of the obtained materials employing established methods.Only for the sample with the highest concentration of NB units (1.1 mol % CO and 3.3 mol % NB), the DSC curves show two crystallization peaks (Figure 4a) and two melting peaks (Figure 4b), which can be attributed to a slightly heterogeneous microstructure with consequent crystallization and successive melting at high temperatures of chain segments poorer in NB units and crystallization and melting at the lower temperature of chain segments richer in NB units.
The mechanical properties of all ethylene−CO−norbornene terpolymer samples were studied on compression-molded films (Figure 5).All terpolymers, as well as ethylene-CO (keto-PE) and ethylene-NB reference copolymers, exhibit deformation with necking and are characterized by remarkable strength and deformability with high values of strain at break, higher than 600−800% (Figures 5, S16, and Table S2).
The copresence of carbon monoxide and norbornene resulted in a moderate enhancement of ductility in terpolymers compared with keto-PE (Figures 5, S16, and Table S2).Moreover, the simultaneous incorporation of both defects in the polyethylene backbone results in a decrease in the stress at yield (σ y ) (Figure 6a) and Young's modulus (E) (Figure 6b) that both progressively decrease with increasing norbornene content, in accordance with the reduction in crystallinity.In particular, E ranges from ≈610 MPa for the neat keto-PE to ≈200 MPa in the case of the terpolymer with the highest NB content (Figure 6a and Table S2).Considering the typical values of HDPE and LDPE Young's moduli (≈900 and ≈240 MPa, respectively), 2 our data indicate that the precise norbornene incorporation enables a controlled modification of stiffness and yield stress while keeping high deformability to achieve a broad spectrum of material properties.

■ CONCLUSIONS
The addition of norbornene to the established Ni-catalyzed nonalternating copolymerization of ethylene and CO can yield terpolymers with isolated norbornene units in the polymer chain.Contrary to the previously reported terpolymerization of ethylene-CO and polar vinyl monomers, 42 no involvement of norbornene in enhanced chain transfer rates could be detected and molecular weights are largely retained compared to neat ethylene-CO copolymerization.The bulky norbornene groups act as noncrystallizable units in these high molecular weight keto-PEs and degrees of crystallinity are substantially reduced compared to neat, highly crystalline keto-PE.Nevertheless, crystallinity-reduced keto-PEs retain the basic thermal and crystallization behavior of polyethylene, which allows for melt processing.Such melt-processed, crystallinity-reduced keto-PEs showed improved ductility and lower stress at yield in tensile tests.Therefore, the inclusion of bulky norbornene units as noncrystallizable units might be used as a straightforward tool to tailor materials properties of otherwise highly crystalline and thus mostly rigid keto-HDPEs.This can enable, for example, film applications.

Figure 1 .
Figure 1.ATR-IR spectra (left) with details of the carbonyl region (right) of ethylene-norbornene-CO terpolymers.Carbonyl absorption bands at 1714 cm −1 (1673 cm −1 for 13 CO-labeled samples) show the mainly isolated nature of in-chain carbonyl groups.

Figure 3 .
Figure 3. WAXS traces of melt-crystallized ethylene−CO−norbornene terpolymers of the indicated CO and NB content (a).Values of the degree of crystallinity (x c WAXS ) of the melt-crystallized samples as a function of the NB content (b).In (a), the diffraction profiles of ethylene-CO and ethylene-norbornene copolymer samples prepared with the same catalyst are also reported.The 110 and 200 reflections of the orthorhombic form of PE at 2θ ≈ 21.4°and 23.8°, respectively, are indicated.Traces are vertically shifted for clarity.Note that the interpolation line in (b) is just a guide for the eye.

Figure 4 .
Figure 4. DSC thermograms of samples reported in Table 1 recorded at 10 °C min −1 during cooling from the melt (a) and successive heating (b).In (b), values of the degree of crystallinity (x c DSC) of the melt-crystallized samples, evaluated as reported in the Supporting Information, are also given.

Figure 5 .
Figure 5. Stress−strain curves of selected samples: reference ethylene-CO and ethylene-norbornene copolymers (black and red, respectively), and different ethylene−CO−norbornene terpolymers with a comonomer content of 0.9 mol % CO and 0.5 mol % NB (green), 0.7 mol % CO and 1.3 mol % NB (blue), and 1.1 mol % CO and 3.3 mol % NB (orange).Stress−strain curves of commercial HDPE and LDPE samples are also reported for comparison.

Table 1 .
45,50,69Quantitative Scheme 1. Terpolymerization of Ethylene with CO and Norbornene as a Bulky Comonomer to Crystallinity-Reduced Keto-Functionalized Polyethylene Catalyzed by a State-of-the-Art Phosphinophenolate Ni(II) Catalyst (1) Results of the Catalytic Terpolymerization of Ethylene with CO and Norbornene (NB) 1oncentration of norbornene in the initial reaction solution.cDeterminedbyATR-IRspectroscopy (cf.Supporting Information for details).In brackets: Incorporation determined by 1 H NMR spectroscopy by integration of the 1 H signals of α-carbonyl CH 2 (CO) in relation to the overall integral.dDeterminedby1HNMRspectroscopy by integration of 1 H signals of norbornene H1 and H4 protons at 2.00 ppm.In brackets: Incorporation determined by quantitative13C NMR spectroscopy.e Determined by SEC in 1,2-dichlorobenzene at 160 °C (1.0 mL min −1 ) via linear calibration with narrow PE standards.f Determined by DSC, second heating cycle (10 K min −1 ).g13 CO employed as a comonomer.