One Step Block-like Copolymers from an Upcycled Monomer Using Ring-Opening Metathesis Polymerization

Abstract

Block-like copolymers can be readily synthesized in one step using monomers with widely separated reactivity ratios. Here, we show that trans,trans,trans-1,5,9-cyclododecatriene (CDT), produced from polybutadiene via cyclodepolymerization (CDP), undergoes gradient copolymerization via ring-opening metathesis polymerization (ROMP) with a selected norbornene monomer to produce a block-like copolymer that exhibits bulk nanophase separation and excellent thermomechanical performance. These data highlight a new ROMP copolymerization system with a sharp compositional drift and introduce a simple pathway to convert waste plastic into a valuable building block for high-value thermoplastic materials.

Introduction

Diblock copolymers are a unique class of materials with applications ranging from bulk thermoplastic materials to compatibilizers to templates for nanoscale fabrication. − Their distinct behavior originates from the covalent attachment of two immiscible polymer blocks, driving their phase separation into discrete domains that exhibit thermomechanical properties resembling their constituent homopolymers. These structured materials are typically synthesized through multistep strategies, for example via sequential monomer additions, chain extension of macroinitiators or macro-chain-transfer agents, or polymer–polymer coupling. Multistep syntheses require living polymerization behavior, while polymer coupling involves the isolation and purification of polymer intermediatesfactors that increase the overall complexity and cost of material production.

Block-like copolymers, featuring gradient topological profiles, can be accessed in a one-step approach by copolymerizing monomers with highly different reactivity ratios. , Through appropriate monomer design, copolymerizations with sharp compositional gradients can be achieved such that a “fast” monomer is preferentially consumed in the early stages of the reaction and a “slow” monomer later as the “fast” monomer becomes depleted. Provided a sufficiently sharp compositional drift, block-like copolymers can exhibit behavior similar to traditional diblock copolymers, including bulk microphase separation and solution self-assembly.

Monomer systems for gradient copolymer synthesis have been developed using radical, anionic, and cationic mechanisms. Beyond these powerful chemistries, ring-opening metathesis polymerization (ROMP) represents a highly oxygen- and functional group-tolerant approach for copolymer synthesis. − However, reports on simple ROMP comonomer systems with sharp gradient compositional profiles are scarce. Cheng et al. showed that differences in ROMP reactivity ratios between macromonomers and small molecule reactive diluents could be maximized through tuning diluent stereochemistry or steric bulk. The obtained reactivity ratios for these copolymerization systems were not sufficient to produce block-like copolymers but instead were leveraged to synthesize bottlebrush copolymers with controllable grafting densities. The Choi group achieved a sharp compositional gradient during ROMP by using endo-tricyclo­[4.2.2.0]­deca-3,9-diene (TD) derivatives and cyclooctatetraene, demonstrating that the resulting block-like copolymers underwent self-assembly to form stable 0D and 1D nanoscale assemblies. However, these monomers were derived from expensive building blocks and may be unsuitable for producing bulk polymer materials. Finally, Kilbinger and co-workers successfully achieved a one-step synthesis of block-like copolymers by using sterically encumbered oxanorbornene monomers in combination with a fast-propagating exonorbornene imide. Their reported comonomer system was notably simpler and thus more commercially feasible compared with the previous examples, but the preparation of high molecular weight (MW) polymers was not investigated, and bulk material properties were not explored.

In this contribution, we detail the synthesis of block-like copolymers by a one-step ROMP copolymerization approach using N-benzyl-exo-norbornene carboximide (NI) and trans,trans,trans-1,5,9-cyclododecatriene (CDT). This combination of monomers exhibited widely separated reactivity ratios and produced block-like copolymer materials with hard and soft segments resembling styrene–butadiene block copolymers. We found that CDT could be readily prepared in high isolated yield via cyclodepolymerization (CDP) of polybutadiene (PB), , creating a direct line of sight from an underutilized plastic waste feedstock to a useful polymer building block. Kinetic analysis of NI/CDT copolymerization revealed a sharp compositional drift and linear scaling of MW with total monomer conversion. Bulk polymer samples prepared via solvent casting exhibited microphase separation and excellent tensile properties, with enhanced strength and ductility compared with an analogous homopolymer. These data highlight a unique approach for repurposing plastic waste, extend our knowledge of ROMP copolymerization behavior, and provide a roadmap for the simple synthesis of a new class of block-like copolymers.

Experimental Section

Materials and Methods

Polybutadiene, cis (PB, M _w ∼ 200 kg mol^–1, 98% cis-1,4-butadiene) was purchased from Millipore Sigma. All other chemicals were purchased from commercial sources and used as received unless otherwise stated. Monomer NI was synthesized according to a literature procedure.

Synthesis of Monomer trans,trans,trans-1,5,9-Cyclododecatriene via Cyclodepolymerization of Polybutadiene (CDT)

A 500 mL round-bottom flask was charged with PB (2.03 g, 37.5 mmol) and 220 mL of CH₂Cl₂. The flask was capped with a rubber septum, and the suspension was stirred overnight at room temperature to dissolve the polymer. The PB solution was then purged with Ar for 20 min. A solution of HGII (0.117 g, 0.187 mmol) in a minimal amount of CH₂Cl₂ was added via syringe. The reaction mixture was heated at 35 °C in an oil bath under Ar for 16 h. After cooling, ca. 5 mL of ethyl vinyl ether was added to the flask and the reaction mixture was stirred for an additional 20 min. The solvent was removed in vacuo. CDT was recovered from the crude residue via distillation at reduced pressure (oil bath temperature = 100 °C) as a colorless oil that crystallized upon cooling (1.51 g, 74% yield). ¹ H NMR (400 MHz, CDCl₃): δ 5.00 (s, 6H), 2.04 (s, 12H). ¹³ C NMR (400 MHz, CDCl₃): δ 131.41, 32.29. HRMS (ESI) calcd for C₁₂H₁₀ ⁺: m/z = 163.1481; [M + H]⁺; found: 163.1482.

Synthesis of P­(NI-grad-CDT)

A 20 mL glass vial was charged with CDT (0.644 g, 3.95 mmol), NI (1.00 g, 3.95 mmol), and 7.6 mL of CH₂Cl₂. The suspension was stirred until all solids dissolved (ca. 5 min). To the vial was added 0.287 mL of a 50 mg/mL GIII stock solution in CH₂Cl₂ (0.0197 mmol added). The vial was capped and the reaction mixture stirred at room temperature for 8 h. The polymerization was quenched via addition of a few drops of ethyl vinyl ether (EVE). A crude sample was removed for analysis by ¹H NMR spectroscopy. The polymer was isolated and purified via precipitation from CH₃OH (3×) and was dried under vacuum at room temperature for 24 h.

Copolymer Characterization

¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III 400 NMR spectrometer operating at 400 MHz. Chemical shifts are reported in parts per million (ppm) relative to residual protonated solvent for ¹H (CHCl₃ = δ 7.26) and relative to carbon resonances of the solvent for ¹³C (CDCl₃ = δ 77.16). Peak multiplicities are annotated as follows: app = apparent, br = broad, s = singlet, d = doublet, dd = doublet of doublets, t = triplet, q = quartet, p = quintet, m = multiplet. Size exclusion chromatography (SEC) analysis was conducted in THF using a Tosoh EcoSEC Elite system equipped with two Tosoh TSKgel Super AWM-H columns (15 cm × 6 mm ID, 9 μm pore size) with a differential RI detector and a light scattering detector. Number-average molecular weights (M_n ), weight-average molecular weights (M _w), and dispersities (Đ = M _w/M_n ) were calculated based on a calibration curve using narrow PMMA standards (Agilent EasiVial PMMA Tri-Pack, M _p = 500–1,500,000 g mol^–1). Thermal transitions were determined for the various polymer samples (1–10 mg) using a TA Instruments Discovery Series DSC 250 by heating the sample from −120 to 250 °C with different rates of 10 °C/min and 5 °C/min under N₂ atmosphere over three subsequent heating/cooling/heating cycles. The glass transition temperature, T _g, was taken as the inflection point in the DSC trace of the second heating cycle.

High-Resolution Mass Spectrometry

High-resolution mass spectra (HRMS) were obtained on a Q Exactive HF Orbitrap mass spectrometer (Thermo Scientific, Waltham, MA) at 240,000 resolution in positive electrospray ionization (ESI) mode. 50 nL samples were introduced using an Open Port Sampling Interface (OPSI) on the instrument using 75:25:0.1 (v/v/v) acetonitrile/water/formic acid as carrier solution at a 150 μL/min flow rate.

Tensile Measurements

Solutions for polymer solvent casting were prepared by dissolving the appropriate polymer in CHCl₃ (10 wt/v%) with 5 wt % added 6-ditert-butyl-4-methylphenol (BHT) and stirring overnight at room temperature to ensure complete dissolution. The solution was then cast into a Teflon dish, covered with a crystallization dish, and left to evaporate at room temperature overnight. The cast sample was dried under vacuum at room temperature for 24 h, thermally annealed under vacuum at 120 °C between metal plates, and allowed to cool slowly to RT. Dogbone samples for tensile measurements were prepared from these films by punching with a dogbone mold. The width and length of the dogbone samples were measured using a caliper and tensile strength was measured on an Instron machine (Instron 3343 Single Column Testing System). ASTM D1708 standard with a stretching rate of 1.0 mm/min was used for all measurements. Films cast for tensile measurements were also used for X-ray measurements.

Small and Wide-Angle X-ray Scattering

Small and wide-angle X-ray scattering (SAXS/WAXS) measurements were carried out on a Xenocs Xeuss 3.0 instrument equipped with D2+ MetalJet X-ray source (Ga Kα, λ = 1.3414 Å). The samples were aligned perpendicular to the direction of the X-ray beam (transmission mode) and the scattered beam was recorded on a Dectris Eiger 2R 4 M hybrid photon counting detector with a pixel dimension of 75 × 75 μm². The collected 2-dimensional (2D) SAXS/WAXS images were circularly averaged and expressed as intensity versus q, where q = (4π sin θ)/λ after subtraction of background scattering.

Results and Discussion

CDT is the thermodynamic product of PB CDP and can be produced in nearly quantitative conversion using appropriate catalysis and reaction conditions. In this study, we synthesized CDT by treating PB with the commercially available Ru-based olefin metathesis catalyst RuCl₂(H₂IMes)­[CH-(o-iPrO-C₆H₄)], HGII, at 0.17 M in CH₂Cl₂ at 35 °C (Figure A). The CDP reaction reached equilibrium within 16 h as judged by ¹H nuclear magnetic resonance (NMR) spectroscopy, after which the reaction mixture was quenched with ethyl vinyl ether (EVE) and concentrated. CDT was then distilled directly from the crude residue in 74% yield (Figure B). Characterization of the isolated CDT by ¹H NMR spectroscopy (Figures C and S4–S5) and high-resolution mass spectrometry (HRMS) confirmed the purity and identity of the desired butadiene cyclotrimer CDT.

1.

Simple synthesis of CDT via cyclodepolymerization of commercial PB. (A) Schematic of PB cyclodepolymerization. (B) Photograph of distillation apparatus used to recover CDT from the crude depolymerization mixture. (C) ¹H NMR spectrum of CDT obtained from cyclodepolymerization.

With monomer CDT in hand, we evaluated its copolymerization kinetic behavior. Monomers CDT and NI were dissolved in CH₂Cl₂ at a 1:1 molar ratio and treated with catalyst GIII at a 400:1 overall monomer/catalyst molar ratio (Scheme ). Aliquots of the polymerization mixture were removed at specified time points, quenched with EVE, and concentrated for subsequent analysis. The resulting copolymer samples were characterized by ¹H NMR spectroscopy to calculate monomer conversions and size-exclusion chromatography (SEC) to monitor time-dependent changes in copolymer MW distribution.

1. ROMP Copolymerization of NI and CDT .

Consistent with a gradient copolymerization, NI was consumed rapidly in the early stages of the reaction in pseudo-first-order fashion and was fully converted in <20 min (k _obs = 0.337 min^–1, Figures A, S1, and S2). During this period, only minimal CDT conversion was observed (ca. 7%). After NI was consumed, the rate of CDT conversion became dominant, albeit at a slower rate than NI (k _obs = 0.00529 min^–1, Figure S2). SEC analysis of these kinetic samples revealed a systematic reduction of polymer retention times with increasing polymerization time (Figure B). Copolymer MW distributions were monomodal and symmetrical except for the 20 min sample, which exhibited a notable high MW shoulder. We attribute this observation to slow reinitiation of the P­(NI) chains by CDT. Number-average MW, M_n , values increased linearly with increasing monomer conversion, indicating well-controlled polymerization behavior (Figure C). Dispersity, Đ, remained low during the NI consumption period and then gradually increased in response to the incorporation of CDT due to the onset of secondary metathesis reactions between backbone butadiene units (Figure D).

2.

Kinetic analysis of NI/CDT copolymerization. (A) Plot of conversion vs time for both monomers. (B) Overlaid SEC traces of the same time points shown in (A). (C) Plot of M_n as a function of total conversion, p _AB. The solid line represents a linear fit of these data. (D) Plot of Đ vs total conversion.

Reactivity ratios were calculated from the data in Figure A using a nonterminal kinetic model developed by Lynd and co-workers

$$p_{\mathrm{AB}}=1-n_{\mathrm{A}}(1-p_{\mathrm{A}})-(1-n_{\mathrm{A}}){(1-p_{\mathrm{A}})}^{r_{\mathrm{B}}}$$ 1

$$p_{\mathrm{AB}}=1-n_{\mathrm{A}}{(1-p_{\mathrm{B}})}^{r_{\mathrm{A}}}-(1-n_{\mathrm{A}})(1-p_{\mathrm{B}})$$ 2

where p _AB represents the total monomer conversion as described by p _AB = n _A p _A + n _B p _B, n _A and n _B are the number fractions of monomers A (NI) and B (CDT), p _A and p _B are the conversions of monomers A and B, and r _A and r _B are the reactivity ratios for these monomers. Reactivity ratios of r _NI = 36.0 and r _CDT = 0.0310 indicated a sharp compositional drift for this copolymerization (Figure S3). The difference in these reactivity ratios represents the second largest reported for ROMP behind the Kilbinger norbornene/oxanorbornene comonomer system. Taken together, these kinetic data clearly indicate the formation of block-like copolymers during NI/CDT copolymerization.

To evaluate the morphology and thermomechanical properties of these block-like copolymers, the NI/CDT copolymerization was conducted at a larger scale. Following polymerization at room temperature for 8 h to achieve >95% CDT conversion, the reaction was quenched with EVE and the synthesized copolymer P­(NI-grad-CDT) isolated via precipitation from CH₃OH. As shown in Table , P­(NI-grad-CDT) possessed a molar percentage of butadiene units approximately equal to the monomer feed ratio, a number-average molecular weight, M_n , of 75.5 kg mol^–1, and a dispersity, Đ, of 1.86. A small amount of low molecular weight P­(CDT) was likely produced as a byproduct of cross-metathesis during polymerization, evident as a second population of chains in the SEC trace at ca. 20 min (Figure S11). We note that the MW for this copolymer was much higher than has been reported for related ROMP gradient copolymer systems. Additional (co)­polymerizations were conducted to enable property comparisons including: (1) homopolymerization of NI to make poly­(NI); (2) homopolymerization of CDT to make P­(CDT); (3) traditional stepwise block copolymer synthesis to make P­(NI-b-CDT); and (4) ring-opening cross-metathesis polymerization of NI in the presence of PB to make P­(NI -co- CDT) using a method we developed previously. A summary of our characterization of these additional samples is also provided in Table , and additional characterization data and synthetic details can be found in the Supporting Information.

1. Characterization of Polymer Samples.

sample	butadiene source	mol % butadiene	M n,theo (kg mol–1)	Mn (kg mol–1)	M w (kg mol–1)	Đ
P(NI-grad-CDT)	CDT	48	83.1	75.5	140.5	1.86
P(NI-b-CDT)	CDT	42	83.1	77.8	104.2	1.34
P(NI -co- CDT)	PB	40	-	22.9	30.7	1.34
P(CDT)	CDT	100	64.9	87.3	180.6	2.07
P(NI)	-	-	50.7	50.3	54.9	1.09

^aMeasured using ¹H NMR spectroscopy.

^bCalculated based on the conversion of each monomer and the overall M/GIII ratio.

^cMeasured using SEC in THF calibrated with narrow PMMA standards.

^dP­(NI) homopolymer.

^eNot determined.

Small-angle X-ray scattering (SAXS) revealed clear morphological differences between the NI/CDT copolymers prepared by different methods. The random copolymer P­(NI -co- CDT) exhibited only a weak shoulder in the low-q region, suggesting limited short-range correlations rather than a well-defined periodic structure. P­(NI-b-CDT) exhibited a lamellar organization, with well-defined peaks at q ₁ = 0.017 Å^–1, 2q ₁, and 3q ₁ as shown in Figure A, inset. The presence of the second and third order peaks confirmed a periodic and well-ordered structure with a lamellar spacing of ∼35 nm. P­(NI-grad-CDT) displayed a pronounced primary scattering peak at q = 0.02 Å^–1, indicative of microphase separation and the formation of ordered nanoscale domains with characteristic sizes of ∼30 nm. Only very weak secondary scattering was observed for this sample (Figure A, inset), suggesting a lamellar-like organization similar to the diblock copolymer but with a relatively low long-range order. Furthermore, analysis of the high-q region in wide-angle X-ray scattering (WAXS) showed that all polymer samples were amorphous, as evidenced by the presence of an amorphous halo (Figure A).

3.

Comparison of polymer morphology, thermal, and mechanical properties. (A) Overlaid SAXS/WAXS spectra for P­(NI-b-CDT), P­(NI-grad-CDT), and P­(NI-co-CDT). Inset: qⁿ I­(q) vs q scaling plot, where n = 3 for P­(NI-grad-CDT) and n = 4 for P­(NI-b-CDT). Arrows mark the q-values corresponding to the peak maxima. (B) Stacked DSC thermograms for the various polymers. (C) Representative tensile stress/strain curves for P­(NI), P­(NI-b-CDT), and P­(NI-grad-CDT). The legend in (C) refers to all three panels.

Homopolymer P­(NI) showed a glass transition temperature of 157 °C as determined by differential scanning calorimetry (DSC, Figure B). P­(CDT) exhibited a much lower T _g at −88.7 °C and melting transition at 33.0 °C (Figure S16), likely due to the high proportion of trans-configured olefins comprising its polymer backbone. Block copolymer P­(NI-b-CDT), prepared by sequential monomer addition, had two T _g’s at −92.5 °C of 155 °C corresponding to its P­(CDT) and P­(NI) blocks, respectively. The T _g of P­(NI -co- CDT) was 114 °C, much lower than P­(NI) and closer to the theoretical value for a statistical copolymer calculated using the Fox equation (ca. 75 °C). Finally, the thermal behavior of P­(NI-grad-CDT) was more similar to the block copolymer P­(NI-b-CDT) than the comparatively more random P­(NI -co- CDT), with two broad T _g’s −84.7 and 143 °C that were assigned to the CDT- and NI-rich blocks of this block-like copolymer, respectively.

Thin films of P­(NI), P­(NI-b-CDT), and P­(NI-grad-CDT) were prepared via solvent casting from CHCl₃ followed by exhaustive drying to compare their mechanical properties. We note that we also attempted to prepare films from P­(CDT) and P­(NI -co- CDT); however, freestanding films with sufficient mechanical robustness for testing could not be obtained. P­(NI) exhibited a tensile stress at break, σ_b, of 47.3 MPa and a strain at break, ε_b, of 10.4% (Figure C and Table ). By contrast, the microstructure of P­(NI-b-CDT) and the presence of rubbery PB blocks significantly toughened this material compared with P­(NI). P­(NI-b-CDT) showed σ_b = 30.3 MPa, lower than its analogous homopolymer, but a much higher ε _b = 40.8%. Intriguingly, P­(NI-grad-CDT) possessed properties intermediate of the homo- and block copolymer analogues (σ_b = 41.8 MPa and ε_b = 17.8%). These results reflect the contribution of copolymer nanostructure to mechanical properties, particularly toughness, enabling precise targeting of material behavior by changing the synthetic approach. Overall, the observed tensile properties of unreinforced P­(NI-grad-CDT) were competitive with commodity thermoplastic materials such as poly­(ethylene terephthalate) (PET), polystyrene, or acrylonitrile butadiene styrene (ABS), providing a potentially high value target for converting PB waste into new, high-performance materials.

2. Summary of Thermomechanical Characterization of Synthesized (Co)­polymers.

sample	T g (°C)	T m/H m (°C/J g–1)	εbreak (MPa)	σbreak (%)	E (MPa)	toughness (MJ m–3)
P(NI-grad-CDT)	–84.7, 148	-	41.8 ± 2.1	17.8 ± 0.7	965 ± 17.8	5.5 ± 0.6
P(NI-b-CDT)	–92.5, 155	-	30.3 ± 1.0	40.8 ± 3.2	1167 ± 110	11.4 ± 1.3
P(NI-co-CDT)	114	-	-	-	-	-
P(CDT)	–88.7	33.0/53.7	-	-	-	-
P(NI)	157	-	47.3 ± 3.0	10.4 ± 4.0	1775 ± 155	4.2 ± 2.2

^aSample T _g as measured by DSC.

^bA heating rate of 5 °C min^–1 was used to observe the thermal transition associated with the CDT block(s).

^cTensile properties obtained from tensile testing experiments on a minimum of three independent samples.

^dNo melting transition observed.

^eSamples could not be prepared for tensile testing.

Conclusions

In summary, we have demonstrated that CDT, obtained directly from PB via a simple CDP procedure, undergoes facile copolymerization with NI to produce a block-like copolymer. The difference in reactivity ratios between these two monomers is among the largest reported for ROMP copolymerization systems. The block-like copolymer P­(NI-grad-CDT) exhibited bulk nanophase separation as confirmed by SAXS derived from the immiscibility of the P­(CDT) and P­(NI) ‘blocks’, imbuing the material with enhanced elasticity and toughness compared with the homopolymer analogue P­(NI). This work demonstrates how waste plastic can be repurposed using simple transformations to create high-value thermoplastic materials and opens new opportunities more broadly for the synthesis of copolymers by ROMP.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acspolymersau.5c00157.

• Methods, kinetic data, and polymer characterization data (PDF)

CRediT: Jeffrey C Foster conceptualization, investigation, methodology, project administration, writing - original draft; Isaiah Dishner investigation, writing - review & editing; Jackie Zheng investigation; Vera Bocharova investigation, writing - original draft; Vilmos Kertesz investigation; Tomonori Saito conceptualization, funding acquisition.

This manuscript has been authored by UT-Battelle LLC, under contract DE-AC05–00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

The authors declare no competing financial interest.