A different polynorbornene backbone by combination of two polymer growth pathways: vinylic addition and ring opening via β-C elimination

Adifferent polynorbornene backbone by combination of two polymer growth pathways: vinylic addition and ring opening via b-C elimination† Ignacio P´ erez-Ortega and Ana C. Alb´ eniz * A new polynorbornene skeleton has been found that contains bicyclic norbornane units and cyclohexenyl methyl linkages. The polymers have been synthesized using a nickel catalyst in the presence of a controlled amount of ligands with low or moderate coordination ability. The backbone structure is the result of a vinylic addition polymerization, via sequential insertions of norbornene into a Ni–C bond (bicyclic units) combined with an unusual ring opening of the norbornene structure by a b-C elimination (cyclohexenyl methyl units) to give a new Ni–C(alkyl) bond that continues the polymerization. The ring opening events are favored when the rate of propagation of the vinylic addition polymerization decreases, and this can be modulated by making the coordination of norbornene to the metal center less favorable using additional ligands. Introduction Bicyclo[2.2.1]hept-2-ene or norbornene (NB) can be polymerized in several ways to give different polymer structures (Scheme 1). 1 Radical or cationic routes lead to low molecular weight polymers that show a 2,7-linkage of the bicyclic moieties (Scheme 1a). Norbornene and many norbornene derivatives are amenable to ring opening metathesis polymerization, which produces unsaturated polymeric structures (ROMP-PNB, Scheme 1b). 2 The vinylic addition polynorbornenes (VA-PNBs) result from a metal-catalyzed double bond insertion polymerization leading to an aliphatic backbone where the bicyclic units are preserved and show an exo-2,3-enchainment (Scheme 1c). 3 Both ROMP-PNBs and VA-PNBs can be obtained as high molecular weight polymers with a wide range of applications and commercial availability. 4,5 It has been shown that using suitable catalysts the ROMP route can be controlled to synthesize cyclic unsaturated polynorbornenes via what is called ring expansion metathesis polymerization or REMP. 6 Few exceptions to these polymeric structures have been found and the reported examples combine two types of the known polymeric arrangements in Scheme 1a–c. For example, a few metal catalysts are capable of bringing about the ROMP and VA-polymerization of norbornene by switching during the process between a metal carbene species and a metal alkyl through an a-elimination–readdition process. The outcome is a polymer structure that mixes 2,3-bicyclic moieties and ring opened olenic units as shown in Scheme 2b. 7 A norbornene oligomer was reported by Fink et al. that shows 2,3and 2,7enchained bicyclic units. This structure is the result of a s-bond metathesis between a Zr–C and a C(7)–H bond of the growing polymer chain in a VA-polymerization with a Zr metallocene (Scheme 2a). 8 We report here a new type of polynorbornene structure (Scheme 1d) that includes in the polymer backbone both 2, 3bicyclic and cyclohexenylmethyl fragments via vinylic addition polymerization and a ring opening of norbornene (NB) by b-C elimination, and therefore can be labeled as VA/RO-PNB. Results and discussion Complexes [Ni(R F ) 2 L 2 ](R F ¼C 6 F 5 ,C 6 Cl 2 F 3 ), where L is a labile ligand, are very efficient catalysts in the VA-polymerization of Scheme 1 Types of norbornene polymerization. IU CINQUIMA/Qu´ ımica Inorg´ anica, Universidad de Valladolid, 47071 Valladolid, Spain. E-mail: [email protected] †Electronic supplementary information (ESI) available: Experimental details and characterization data. See DOI: 10.1039/d1sc07028b Cite this: Chem. Sci., 2022, 13,1823 All publication charges for this article have been paid for by the Royal Society of Chemistry Received 17th December 2021 Accepted 19th January 2022 DOI: 10.1039/d1sc07028b rsc.li/chemical-science © 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2022, 13,1823–1828 | 1823 Chemical Science EDGE ARTICLE Open Access Article. Published on 20 January 2022. Downloaded on 2/9/2022 3:35:58 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue norbornene. 9 We have been using for some time [Ni(C 6 F 5 ) 2 (- SbPh 3 ) 2 ](1) which is very active in the VA-polymerization of norbornene and some norbornene derivatives such as haloalkylnorbornenes, 10b generally much more reluctant to this type of polymerization, as well as in the copolymerization of functionalized norbornenes such as haloalkyl-, 10b alkenyl-, 10c and stannylated norbornenes, 10a with norbornene. 4b In the course of these studies, we observed that when the activity of the catalyst was low as, for example, in the copolymerization of substituted norbornenes with carbonyl groups, the polymers obtained in low yields showed an olenic signal at about 5.7 ppm in the 1 H NMR. 11 We decided to look into the origin of this unsaturation and we started by carrying out the polymerization of norbornene with complexes [Ni(C 6 F 5 ) 2 L 2 ] with ligands of different coordination ability and low NB : Ni mol ratios. Some of the polymers obtained showed the abovementioned unsaturation (Fig. 1a) and, as it is shown in Fig. 1, this signal is not consistent with the presence of ROMP units in the polymer (cf. spectra, Fig. 1a and c). The chemical shis in the 1 H and 13 C NMR are in agreement with an endocyclic cyclohexenyl double bond that could result from b-C elimination in a Ni–norbornyl complex during the polymerization as shown in eqn (1). b-C (or b-alkyl) elimination is not as common for group 10 metals as it is for early transition metals. 12 However, examples of C–C cleavage by group 10 metal complexes can be found, 13 and the process is gaining great importance in the context of norbornene-mediated palladium catalyzed regioselective reactions, that rely on a reversible norbornene insertion into a Pd–C bond. 14 In the latter processes the norbornene cyclic structure remains intact and the type of ring opening of norbornene shown in eqn (1) is very scarce in the literature. Catellani et al. reported in 1983 the ring opening by bC elimination of norbornene aer two sequential insertions of norbornene into a Pd–Ar bond. 15 Milstein also reported a similar process when studying the Heck reaction using norbornene. 16 In the context of the Pd-catalyzed VA-polymerization of an ester derivative of norbornene, Rhodes et al. proposed the occurrence of the ring opening process in eqn (1) followed by bH elimination to explain the chain termination observed. 17 For Ni complexes, only the dimerization of norbornene mediated by an in situ generated nickel hydride has been reported and it leads to 4, shown in Fig. 1b. 18 As can be seen when comparing the spectra in Fig. 1a and b, the polymer unsaturation is consistent with the endocyclic double bond of a cyclohexenyl fragment similar to that in dimer 4synthesized independently. In the polymerization, the cyclohexenyl methyl nickel complex (eqn (1)) undergoes a new monomer insertion instead of a b-H elimination and the exocyclic double bond is not formed. (1) Table 1 shows the polymerization experiments with complexes 1–3. The polymerization of norbornene requires Fig. 1 1 H NMR (CDCl 3 ) of: (a) VA/RO-PNB (NB VA /NB RO ¼7.6/1). (b) dimer 4; (c) ROMP-PNB. *Signal corresponding to the residual solvent. Table 1 Formation of VA-PNB or VA/RO-PNB with catalysts 1,2and 3 Entry [Ni] m[NB] 0a NB VA /NB ROb %NB ROc Yield M wd Đ d 1175 0.34 No NB RO 0% 90% 162090 4.4 2175 0.061 67/1 1.5% 75% 49065 1.9 3275 0.34 No NB RO 0% 95% 138411 4.2 4275 0.061 14.3/1 6.5% 67% 14071 2.3 52225 0.061 No NB RO 0% 74% 87947 1.9 6375 0.34 ————— a Initial molar concentration of norbornene (NB). b The mol ratio NB VA / NB RO was calculated by comparison of the integral of the 1 HNMR signals (olenic vs. aliphatic) of the polymer (see ESI). c The mol% of NB RO was calculated from the mol ratio NB VA /NB RO . d M w (Da) and Đ (M w /M n ) determined by GPC in CHCl 3 using polystyrene standards. Scheme 2 Examples of two type of linkages in a polynorbornene. 1824 |Chem. Sci.,2022,13,1823–1828 © 2022 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article Open Access Article. Published on 20 January 2022. Downloaded on 2/9/2022 3:35:58 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online a weakly coordinating ligand, i.e. SbPh 3 or AsPh 3 . The more donating PPh 3 renders the complex inactive (cf. entries 1, 3 and 6, Table 1). The presence of ring-opened norbornene units (NB RO ) was observed when the reactions were carried out at low initial concentration of monomer (entries 2 and 4, Table 1). This effect is more important for the more coordinating AsPh 3 vs. SbPh 3 (cf. entries 2 and 4, Table 1). The amount of cyclohexenylmethyl units (or ring-opened norbornene, NB RO ) can be calculated by comparison of the intensity of the olenic signal at about 5.7 ppm with the aliphatic region in the 1 H NMR spectra of the polymers (for details see page S4, ESI†). The composition of the polymers is given in the tables as the ratio of bicyclic units (NB VA ) to ring-opened units (NB RO ) as well as the percentage of NB RO in the polymers. Table 1 (entry 5) also shows that a decrease in the catalyst amount, using the same initial NB concentration, disfavors the ring opening process. The use of a lower concentration of the nickel complex implies that the concentration of free ligand L, dissociated during the polymerization, is also lower. Therefore, all these results show that the formation of NB RO units is favored by a low initial monomer (NB) concentration and the presence of free AsPh 3 , dissociated from the nickel complex. The polymers were characterized by NMR (see below and ESI for details†) and they show unimodal distributions in GPC, indicating that they are not mixtures of two types of polymers. A higher amount of NB RO units is associated with a lower yield of the polymerization as well as a lower M w for the polymer (Table 1), therefore a less efficient polymer growth. Thus, we tested the polymerization in the presence of small amounts of compounds that have shown to hamper the VA-polymerization of norbornene when used as solvents. 9b,19 These are coordinating solvents, such as ketones, acetonitrile or amides. Table 2 shows that polymers with up to 12% of NB RO units can be obtained in good or moderate yields using a controlled amount of ketones (acetone or acetophenone) as additives in a polymerization of norbornene with complex 2(entries 1–5, Table 2). The more coordinating DMA can also be used but a smaller amount is required (entries 8–11, Table 2). The presence of MeCN halts the polymerization. The experiments with the ketones show that there is a direct correlation between the amount of additive and the percentage of NB RO units (entries 2–4, Table 2). Also, a larger amount of NB RO is observed when the coordinating ability of the additive increases (cf. entries 2 and 5, Table 2). 20 The effect of the temperature in the polymerization was tested in the small range allowed by the signicant catalyst decomposition (T>45 C) and the very low conversion at 0 C. The relative amount of ring opened units (% NB RO ) decreases when lowering the reaction temperature (entries 6 and 7, Table 2). The data in Table 2 show that the VA/RO-PNBs have molecular weights in the range 1–3x10 4 Da and T g s between 150–230 C. As the percentage of NB RO units increases, a trend toward lower M w s (entries 2–4, Table 2) and lower T g s is observed (cf. entries 1 and 5, Table 2, for polymers with similar size). The thermogravimetric analysis shows the decomposition of the polymers at about 430 C with no signicant differences between VA/ROPNBs with different percentage of NB RO units (Fig. S32, ESI†). In order to learn about the distribution of NB RO units in the polymer, experiments were carried out in the conditions of entry 5, Table 2, quenching the polymerization at different reaction times, i.e. at different conversions (Table S1, ESI†). Fig. 2a, shows that at short reaction times, the % of NB RO units is low but it increases with time, as the conversion increases and norbornene is consumed. This is consistent with the results in Table 1 (entries 3 and 4) and the experiments shown in Fig. 2b, Table 2 Formation of VA/RO-PNBs with catalyst 2in the presence of coordinating solvents Entry [NB] 0a msolvent NB VA /NB ROb %NB ROc Yield M wd Đ d T ge (C) 1 0.34 160 Me 2 CO 15.7/1 6.0% 70% 18769 2.4 227 2 0.061 160 Me 2 CO 12.1/1 7.6% 65% 14302 2.2 3 0.061 320 Me 2 CO 9.4/1 9.6% 63% 11770 2.6 169 4 0.061 640 Me 2 CO 8.5/1 10.5% 50% 10900 2.3 164 5 0.061 160 PhMeCO 7.6/1 11.6% 64% 17391 1.5 159 6 f 0.061 160 PhMeCO 8.1/1 10.9% 50% 12141 2.0 7 g 0.061 160 PhMeCO 16.1/1 5.8% 43% 19297 1.8 8 0.061 20 DMA 7.0/1 12.5% 34% 12570 1.4 9 0.34 20 DMA 13.9/1 6.7% 74% 33605 1.6 10 0.34 40 DMA 8.4/1 10.6% 55% 22366 1.7 11 0.061 160 DMA ——0% —— 12 0.34 20 MeCN ——0% —— a Initial molar concentration. b The mol ratio NB VA /NB RO was calculated by comparison of the integral of the 1 H NMR signals (olenic vs. aliphatic) of the polymer (see ESI). c The molar% was calculated from the mol ratio NB VA /NB RO . d M w (Da) and Đ(M w /M n ) determined by GPC in CHCl 3 using polystyrene standards. e Determined by DSC. f T¼45 C; some catalyst decomposition was observed. g T¼12.5 C. © 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2022, 13,1823–1828 | 1825 Edge Article Chemical Science Open Access Article. Published on 20 January 2022. Downloaded on 2/9/2022 3:35:58 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online where low initial NB concentrations favor the ring opening for polymerizations quenched at the same reaction time (Table S2, ESI†). As the reaction progresses the NB concentration also decreases leading to a higher relative ratio of NB RO . A short polymer was synthesized in order to gather some information about the end-groups of the materials. Using a lower NB : Ni mol ratio in the presence of acetophenone (NB : Ni : PhCOMe ¼5 : 1 : 15) a polymer with M w ¼2270 Da was prepared, which contains pentauorophenyl groups as clearly shown in its 19 F NMR spectrum (Fig. S8, ESI†). This indicates that the polymerization initiates by insertion of a norbornene into a Ni–C 6 F 5 bond, as it has been observed for this type of complexes before. 9 The 1 H NMR spectrum of this polymer also shows a signal corresponding to the CH–C 6 F 5 group at 3.25 ppm, as well as two singlets at about 4.7 ppm characteristic of a terminal exocyclic methylenecyclohexenyl double bond (Fig. 3, S23 and S24, ESI†). This fragment is the result of a b-H elimination in a cyclohexenylmethyl nickel growing polymer chain and it is a termination pathway for the polymerization. However, when comparing the integral value of the CH–C 6 F 5 (chain initiation) in Fig. 3 and the C]CH 2 resonances (chain termination) it is clear that the terminal olenic signal is less intense than expected, and therefore the b-H elimination route is important but not the only termination pathway. We could not identify any other end-group that could shed light into it.‡In these polymerizations the metal stays attached to the growing chain long enough to be trapped by a carbonylation process, indicating that the termination step is slow. 21 We quenched a polymerization under the same conditions used for the synthesis of the polymer mentioned above, by bubbling CO through the solution and adding NaOMe in MeOH. The obtained polymer showed characteristic -COOMe resonance in the 1 H NMR (3.68 ppm, Fig. S11, ESI†) and a carbonyl resonance at 174 ppm as a cross peak in a long range 1 H– 13 C HMBC NMR experiment. This ester group was also found when a VA-polymerization (conditions of Table 1, entry 1, 15 min) was quenched in the same way (Fig. S16, ESI†). According to all these data a plausible polymerization scheme can be drawn as depicted in Scheme 3. The polymerization initiates by insertion of norbornene into a Ni–C 6 F 5 bond, as supported by the presence of pentauorophenyl groups in the polymer. The initiation is slower than the propagation of the polymerization as shown by the high molecular weight of the polymers obtained when compared to the amount of catalyst used. For example, most of the polymerizations were carried out with a mol ratio Ni : NB ¼1 : 75; if every Ni atom starts Fig. 2 Plot of the amount or ring opened units (% of NB RO )vs. the polymerization reaction time (a) or the initial norbornene concentration (b). Fig. 3 1 H NMR (CDCl 3 ) of a short VA/RO-PNB (NB VA /NB RO ¼2.3/1, M w ¼2270 Da). Scheme 3 Formation of VA/RO-PNB. 1826 |Chem. Sci., 2022, 13,1823–1828 © 2022 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article Open Access Article. Published on 20 January 2022. Downloaded on 2/9/2022 3:35:58 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online a polymer chain the maximum M w of the polymer should be 7062 Da (i.e. 75 M WNB ,M WNB ¼94.16). In all cases the M w found is higher than expected, so only a fraction of the Ni catalyst, which undergoes the slower initiation, is responsible for the polymer growth. The equilibrium between intermediates Band Cdetermines the pathway for the polymer growth. 2,3-Insertion of NB (NB VA units) is favored by a high concentration of intermediate Cand this occurs when L is labile and the amount of free ligand is low (i.e. no additive added) as well as when there is a high concentration of NB. If the equilibrium is shied to intermediate Bthe insertion is disfavored and the b-C elimination with concomitant ring opening occurs (NB RO units). This scenario is favored for more coordinating L, higher L concentration and low NB concentration. The latter inevitably occurs as the polymerization progresses and the relative number of NB RO events increases accordingly as we have observed (Fig. 2a). At the end of the polymerization the probability of b-C elimination vs. a new NB insertion is highest so this pathway is a viable termination in the VA-polymerization of norbornene with group 10 metal complexes. 17 We have carried out the monitorization of a polymerization reaction in the conditions described above for the synthesis of a low molecular weight VA/RO-PNB (mol ratio NB : Ni : PhCOMe ¼5 : 1 : 15). The kinetic experimental data conforms to the scenario described above as shown by microkinetic modeling using the COPASI soware. 22 All the details can be found in the ESI (Section 1.10†). An increase in the percentage of NB RO units was observed upon increasing the polymerization temperature (entries 6 and 7, Table 2) and this could be rationalized considering the entropic effects affecting the VA and RO routes. The ring opening (b-C elimination) is a unimolecular process and it is reasonable to assume that it will have a small entropy of activation. In contrast the formation of Cis a ligand substitution reaction, which in an associative scenario would imply a more ordered transition state (negative entropic term). Thus, an increase in temperature would disfavor the VA route while little affecting the ring opening and therefore increasing the relative amount of NB RO units. The b-H elimination in the Ni–cyclohexenylmethyl complex Dis not very fast and the insertion of a new norbornene molecule in the primary Ni–alkyl bond competes efficiently leading to the incorporation of the ring-opened moiety in the polymer. Because of the sluggish b-H elimination and comparably faster olen insertion, 23 nickel complexes are more convenient to obtain these type of VA/RO-PNBs than palladium complexes. Eventually, at the end of the polymerization the formation of an exocyclic double bond by b-H elimination to give a methylene cyclohexene end group can occur, as we have observed. Conclusions A new type of polynorbornene backbone has been found that results from a typical vinylic addition polymerization, leading to bicyclic norbornyl units (NB VA ) combined with an unusual ring opening via ab-C elimination, which forms cyclohexenylmethyl units (NB RO ). This mixed VA/RO-PNB skeleton appears when the coordination of the monomer to the Ni center used as catalyst is disfavored by lowering the norbornene concentration or by using competing ligands of moderated coordination ability. These results show that the b–g–C–C cleavage of a nickel bound norbornyl group is facile whereas the b-H elimination in the primary Ni-cyclohexenylmethyl moiety formed is not too fast and further olen insertions into the Ni–alkyl bond occur. This leads to the incorporation of the ring-opened norbornene fragments into the polymer, and to a new VA/ROpolynorbornene structure. The C–C cleavage in the norbornyl-M fragment is more prevalent at the end of the polymerization and therefore this is a probable termination pathway in the conventional vinylic addition polymerization of norbornene or norbornene derivatives where some of the common termination pathways, such as a direct b-H elimination, are not possible. 21 Data availability Data supporting this article have been uploaded as ESI.† Author contributions I. P. O. conducted the investigation under A. C. A. supervision. A. C. A. wrote the manuscript and I. P. O prepared the ESI.†All authors contributed to the conceptualization of the project and the review and editing of the manuscript. Conflicts of interest There are no conicts to declare. Acknowledgements We acknowledge the nancial support of the Spanish MICINN (PID2019-111406GB-I00) and the Junta de Castilla y Le´ onFEDER (VA224P20). Notes and references ‡A termination by involvement of the MeOH used to quench the polymerization could be plausible, via methoxy coordination, b-H elimination to give a Ni–H, and reductive elimination. We carried out several experiment using CD 3 OD but could not detect the incorporation of deuterium in the polymer. A chain transfer to the monomer via sigma bond metathesis has also been suggested and this aliphatic termination would not be detected in the polymer. 1 R. Madan, A. Srivastava, R. C. Anand and I. K. Varma, Prog. Polym. Sci., 1998, 23, 621–663. 2 C. W. Bielawski and R. G. Grubbs, Prog. Polym. Sci., 2007, 32, 1–29. 3(a) F. Blank and C. Janiak, Coord. Chem. Rev., 2009, 253, 827– 861; (b) M. V. Bermeshev and P. P. Chapala, Prog. Polym. Sci., 2018, 84,1–46. 4(a) M. Eo, D. Han, M. H. Park, M. Hong, Y. Do, S. Yoo and M. H. 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Diao, Organometallics, 2017, 36, 4099–4102. 1828 |Chem. Sci., 2022, 13,1823–1828 © 2022 The Author(s). Published by the Royal Society of Chemistry Chemical Science Edge Article Open Access Article. Published on 20 January 2022. Downloaded on 2/9/2022 3:35:58 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online