Synthesis of norbornene–cyclooctene copolymers by the cross-metathesis of polynorbornene with polyoctenamer

Abstract

Copolymers of norbornene and cyclooctene were synthesized for the first time by the cross-metathesis of polynorbornene with polyoctenamer. This strategy made it possible to use the 1^st generation Grubbs catalyst, which exhibits low activity toward copolymerization of those monomers. Statistical multiblock copolymers with average block lengths varying from 200 to 2 units were obtained.

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Norbornene (NB) and cyclooctene (COE) are well-known objects in ring-opening metathesis polymerization (ROMP). Their homopolymers synthesized by ROMP are important commercial products.¹ Polynorbornene (PNB) available under the trademark Norsorex meets various specific applications, mainly as oil and solvent solidifier for complete absorption of oil or other hydrocarbons.² Polyoctenamer (PCOE), known as Vestenamer, is a semicrystalline rubber with low melting point and extremely low viscosity. Possessing some properties, unusual for an elastomer, it is applied as a polymer processing aid for tire manufacturing, extrusion, injection molding, etc.^1,3 At the same time, the ROMP synthesis of NB–COE copolymers is rather difficult because of the considerable difference in polymerization activities of the comonomers, which stems from much higher strain in the NB structure (the NB strain energy is 100 kJ mol⁻¹, −ΔG^o ROMP = 47 kJ mol⁻¹) in comparison to COE (the COE strain energy is 16 kJ mol⁻¹, −ΔG^o ROMP = 13 kJ mol⁻¹).^1,4

It was reported⁵ that copolymerization of NB and COE mediated by different catalytic systems (RuCl₃/PhOH, WCl₆/Sn(CH₃)₄, Cl₂(PCy₃)₂Ru = CHPh, [(CH₃)₃CO]₂NMesMo = CH(CH₃)₂Ph) under a large excess of COE led to the mixtures of COE-rich and NB-rich copolymers. However, the ¹³C NMR spectra of those copolymers did not contain alternating COE–NB dyad signals. Moreover, it was noted in the article⁶ that the 1st generation Grubbs catalyst, Cl₂(PCy₃)₂Ru=CHPh (Gr-1), produced a norbornene homopolymer until NB was exhausted, and then initiated homopolymerization of COE.

NB–COE copolymers with high degree of alternation were obtained with a range of Ru–carbene catalysts specially synthesized for that purpose.⁷ Another way to highly alternating NB–COE copolymers is a monomer modification, namely using NB with substituents decreasing its activity.⁸

We propose a new approach to the synthesis of NB–COE copolymers via the interchain cross-metathesis of corresponding homopolymers. In this way, the multiblock copolymers of different structure can be obtained using Gr-1 catalyst, though it is not suitable for effective ring-opening copolymerization of NB and COE.

It is noteworthy that cross-metathesis reactions involving polymers were mostly studied with regard to intramolecular (via polymer – catalyst interactions^9,10) or intermolecular (polymer cometathesis with different olefins^1,11) degradation. As far as we are aware, there are only a few examples of cross-metathesis between polymers.^12,13 Polynorbornenes have not been investigated in this respect at all.

In this study, we carried out the cross-metathesis between PNB and PCOE using Gr-1 catalyst. Homopolymers of similar molecular weight with prevailing trans carbon–carbon double bonds were synthesized by ROMP mediated by the same catalyst. They were characterized by GPC, DSC, and NMR (PNB: M_n = 1 × 10⁵, M_w/M_n = 3.1, T_g = 39 °C, trans-88%; PCOE: M_n = 1.4 × 10⁵, M_w/M_n = 1.8, T_g = −79 °C, T_m = 44 °C, trans-68%).

Chloroform was chosen as the best solvent for PNB/PCOE mixtures compared to toluene, THF, CH₂Cl₂, and PhCl. The cross-metathesis reaction was performed in solution at the total polymer concentration [PNB + PCOE] = 4 wt%, equal molar ratio of PNB and PCOE, and the Gr-1 concentration of 0.33–5.00% mol (relative to the molarity of C=C bonds). The polymer concentration was as high as possible to minimize the impact of intrachain metathesis.¹⁰ In the course of the reaction, aliquots were withdrawn and mixed with a large excess of ethyl vinyl ether for deactivation of the metathesis catalyst. The solution was concentrated in vacuum and then analyzed by ¹H and ¹³C NMR. The polymeric products from the aliquots and final reaction mixture were precipitated in ethanol, dried until constant weight, and analyzed by GPC and DSC. The polymer yield was 70–80%.

The mixture viscosity was considerably reduced at the early stage of the reaction (10–20 min) indicating a decrease in the molecular weight of macromolecules due to their interaction with Gr-1 and appearance of new active centers. Later on, alternating NB–COE dyads were detected in the ¹³C NMR spectra (Fig. 1) thus revealing the formation of copolymers as a result of interchain interactions. Following the ref. 7, the signal at δ = 128.3 ppm was assigned to C^b atom of the alternating dyad (Scheme 1). The set of signals around 135.0 ppm generally consisting of five peaks at 135.0, 134.76, 134.73, 134.70, and 134.58 ppm was assigned to C^a atom.

Fig. 1 ¹³C NMR spectra (150.9 MHz, CDCl₃) of the multiblock NB–COE copolymer prepared by the cross-metathesis of PNB and PCOE with the 1^st generation Grubbs catalyst (5 mol% of all C=C bonds).

Scheme 1

The intensities of those signals were not equal, possibly because of high trans double bond fraction in both NB (132.9 ppm, 80%) and COE (130.2 ppm, 78%) copolymer blocks (Fig. 1), which was changed in the course of the cross-metathesis.

The copolymer chain structure can be controlled by altering the reaction time and catalyst concentration. As is seen from Table 1, the use of more Gr-1 catalyst substantially increased the alternating NB–COE dyad fraction.

Table 1 Cross-metathesis of PNB and PCOE with the 1st generation Grubbs catalyst^a

[Gr-1], % mol	Mn × 10^−3	Mw/Mn	Tg, °C	Tm, °C	L		NB–COE dyads, %
					LNB	LCOE
a T = 20 °C; CHCl3; PNB : PCOE = 1 : 1 (mol); 24 h; [PNB + PCOE] = 4 wt%; L – the average block length; abs – absent.
0.33	37	2.9	−32	41	14.0	15.0	7
1.0	16	2.8	−35	36	6.8	7.3	14
2.5	9.2	2.2	−45	32	4.2	5.0	22
5.0	6.0	1.8	−44	abs	2.0	2.3	49

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The content of alternating dyads was gradually increased during the course of the reaction. It means that the cross-metathesis leads to the formation of statistical multiblock copolymers with various lengths of NB and COE blocks. Fig. 2 demonstrates time dependences of the average block lengths L_NB and L_COE, which were calculated from the integral ratio of the initial (NB–NB and COE–COE) and alternating (NB–COE) dyad signals in the NMR spectra:

L_NB = [I(C^c,d) + I(C^a)]/I(C^a), L_COE = [I(C^e,f) + I(C^b)]/I(C^b)

where I(C^c,d) is the intensity of the peaks from cis-(133.75 ppm) and trans-(132.68–132.97 ppm) double bonds in NB–NB dyads; I(C^e,f) is that from cis-(129.71 ppm) and trans-(130.17 ppm) double bonds in COE–COE dyads; I(C^a) and I(C^b) are the peak intensities related to the alternating dyads.

Fig. 2 Time dependences of the average block lengths in the reacting system initially containing equimolar amounts of PNB and PCOE and (a) 0.33 and (b) 1 mol% of Gr-1 catalyst.

The average block lengths decreased with conversion and asymptotically approached the value of 2, characteristic of a completely random (Bernoullian) copolymer. However, this does not necessarily mean the absence of Ru–carbene complex selectivity with respect to NB or COE units. According to the theory of interchange reactions involving chain ends,¹⁴ the fully random chain structure is attained when k_NB–NBk_COE–COE = k_NB–COEk_COE–NB, where k_α–β is the elementary rate constant describing the attack of an α terminal unit on a β internal unit. As is seen from Fig. 2, it is possible to get copolymers with a prescribed average block length in the range from 200 to 2 monomer units by the proper choice of the reaction time and Gr-1 catalyst concentration.

Fig. 3 represents the DSC thermograms of the NB–COE multiblock copolymers with different average COE block lengths. In the case of a PNB/PCOE blend without Gr-1, an endothermic melting peak related to the crystalline structure of PCOE was observed at 44 °C, as for the pure PCOE. With decreasing the average block length, the endothermic melting peak shifted to lower temperatures (32 °C for L_COE = 5) and disappeared when L_COE was as small as 2 units. Not only the position, but also the shape and size of the melting peak depend on the average block length. The sample with L_COE = 15 exhibited a rather broad endotherm (Fig. 3, curve 1). DSC thermograms for the copolymers with L_COE = 7 and 5 contained two peaks (curves 2, 3). It is known that PCOE crystallinity is determined by the trans double bond content.¹⁵ In our case, all the obtained copolymers contained an approximately equal amount of such bonds (76–78%). Therefore, it should be another factor also affecting DSC curves. Presumably, it is the block length distribution. We can speculate that short COE blocks contain less trans C=C bonds than long ones. If so, the lower temperature peak resembling the melting of PCOE with low trans content could be related to the crystallites formed by short blocks, while the higher temperature peak would describe the melting of long crystalline blocks that behave as PCOE with high trans content. A more detailed study of crystallinity in the NB–COE copolymers based on the X-ray data will be reported in the near future.

Fig. 3 DSC thermograms of the NB–COE multiblock copolymers with different average COE block lengths. L_COE = (1) 15, (2) 7.3, (3) 5, (4) 2.3 units. Arrows indicate the glass transition temperatures.

The glass transition temperature also depends on the average length of COE blocks, being increased from −79 °C for the PCOE homopolymer to −44 ÷ −32 °C for the copolymers. Unfortunately, it was not possible to determine how T_g is changed with the average length of NB blocks, because their glass transition coincides with the melting of crystalline COE blocks. For the amorphous copolymer with the average lengths of COE and NB blocks close to 2 units, a single T_g = −44 °C was observed (Fig. 3, curve 4).

Conclusions and outlook

Copolymers of NB and COE containing up to 50% of alternating dyads were synthesized via the cross-metathesis of PNB and PCOE mediated by the 1^st generation Grubbs catalyst. The obtained copolymers possess a random multiblock chain structure. At the equimolar composition of the initial system, the resulting average block lengths can be varied from more than 200 to 2 units by adjusting the reaction time and catalyst concentration. The copolymer crystallinity (of the PCOE type) is lost when the average COE block length approaches 2 units.

Our study opens broad perspectives for the controlled synthesis of statistical multiblock copolymers via the cross-metathesis reactions between polymers with carbon–carbon double bonds in their backbone. This can be considered as an alternative to other copolymerization techniques, which gives an opportunity to circumvent their longstanding problems and obtain novel copolymers with desired properties.

In particular, it would be interesting to employ widely available catalytic systems for synthesizing practically useful copolymers of norbornene and its derivatives with other monomers that are much less active in polymerization.

Acknowledgements

The work was supported by RFBR (project 14-03-00665). The authors thank Dr A. S. Peregudov (INEOS RAS) for the NMR measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details of the homopolymer synthesis and cross-metathesis reaction as well as NMR-spectra of PNB, PCOE, and NB–COE copolymer. See DOI: 10.1039/c4ra12001a

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