Exploring β-Myrcene Incorporation in Propene Copolymerization Using Half-Titanocene Catalysts

1. Introduction

In recent times, there has been a notable shift in focus towards synthesis of polymers derived from renewable resources from the viewpoint of circular economy, because the conventional petroleum-based polymer industry finds itself under immense pressure to meet escalating demands for energy and resources [1,2]. For this reason, synthesis of the new polyolefins, especially copolymers derived from renewable resources has been a long-term interest [3,4,5,6,7]. In this context, there is a growing interest to use of naturally abundant (mono)terpene such as myrcene, 7-methyl-3-methylene-1,6-octadiene (MY), for the synthesis of polymeric materials, including elastomers [8]. Indeed, this conjugated terpene, coming from citrus fruits and various plants, is a colorless oil that is found in nature as its β-isomer and, due to its natural availability and reactive conjugated 1,3-diene framework, may correspond to an alternative to isoprene and butadiene [9,10,11,12,13,14,15]. The copolymerization of olefins with conjugated dienes is inherently challenging due to mismatched monomer reactivities and the risk of catalyst deactivation. However, the catalytic technology based on the polymerization by transition metal catalysts, that is the core technology for industrial production of polyolefins, made possible to synthesize a wide range of new polymers with naturally rich terpene-based conjugated dienes and various olefins [15]. Recently, in their seminal work on ethene (E)/MY copolymerization, Nomura and coworkers reported that phenoxide-modified half titanocene catalysts were able to afford copolymers with rather high molecular weights, M_n = 6.60−8.50 × 10⁴ kg mol^-1, and narrow Đ (1.4−2.0), via subsequent cyclization after ethene insertion [16]. E/MY copolymers exhibit promising elastic properties, and the elongation at break increased upon increasing the MY contents, with a decrease in the tensile strength and toughness. Recently, Capacchione synthesized E/MY copolymers using an [OSSO]-type titanium complex activated by methylaluminoxane (MAO) as cocatalyst with remarkable trans-1,4 selectivity (92 mol%) and a well-defined “multiblock” architecture [17].

On the other hand, Hou has been the first one to use a scandium complex combined with [Ph₃C][B(C₆F₅)₄] affording propene(P)/MY copolymers with a MY content up to 34 mol%, narrow Đ (1.8−2.2) and molecular weight M_n up to 10 kg mol^-1 [18]. Despite the promising ability of MY incorporation in this catalysis, the catalyst systems exhibited low catalytic activities especially compared with those in the E/MY copolymerization, and the activity decrease upon increase in MY content.

To date, the copolymerization of MY with P has thus been only marginally addressed in the literature, and the contribution of Ti-based catalysts has yet to be systematically investigated. Thus, in this work we present a systematic study on the copolymerization of MY with P with good yield using two different families of half titanocene catalysts, the phenoxide-, Cp’TiCl₂(O-2,6-ⁱPr₂-4-C₆H₃) and ketimide-modified one, Cp’TiCl₂(N=C^tBu₂). The copolymerization yields high molecular weight random copolymers, despite the low amount of comonomer content. Thermal analysis of the resulting copolymers has been assessed as well.

2. Results

P/MY copolymers were synthesized using two distinct families of half titanocene catalysts shown in Scheme 1: the phenoxide-, Cp’TiCl₂(O-2,6-ⁱPr₂-4-C₆H₃) with Cp’ = Me₅Cp (1) or (Me₃Si)Cp (2) and ketimide-modified, Cp’TiCl₂(N=C^tBu₂) with Cp’= Me₅Cp (3) or Cp (4). All precursors were activated with dried MAO. The copolymerizations were conducted under optimized conditions at 4 bar of propene pressure, exploring various temperatures and comonomer feed ratios.

The phenoxide-modified Cp’TiCl₂(O-2,6-ⁱPr₂-4-C₆H₃) half-titanocenes have been chosen in this study due to its previously reported efficiency in E/MY copolymerization [16]. On the other hand, ketimide-modified catalysts were chosen for their proven ability to copolymerize propene with sterically demanding cyclic olefins, including norbornene, cyclopentene, cyclohexene, and their derivatives [19,20,21]. In particular, CpTiCl₂(N=C^tBu₂) catalyst 4 is able to efficiently catalyze the copolymerization of propene with cyclic olefins with good yield and norbornene incorporation [21].

All synthesized copolymers were analyzed by ¹H spectroscopy to analyze the copolymer composition and MY insertion mode, SEC (PS calibration) to determine molecular weight and Ð, and DSC to assess the thermal properties. The copolymerization results are summarized in Table 1. The results of the homopolymerization of propene with the different catalysts are reported as blank experiments.

Recently, some of the authors reported the propene homopolymerization by ketimide-based half-titanocene catalysts 3 and 4 under various temperatures and pressure [22]. These catalytic systems exhibited remarkable activity for the synthesis of atactic polymers with ultra-high molar masses, affording materials with high ductility and mechanical properties. However, no results were reported for the polymerization of propene with catalysts 1 and 2. So, first of all, the phenoxide-catalysts were evaluated in the propene homopolymerization at 4 bar and at different temperatures. While catalyst 1 exhibits high activity, catalyst 2 proved to be significantly less productive. Notably, the activity observed for the phenoxide based complexes were lower than those reported for ketimide catalysts (see for instance entry 3 vs entry 13, and entry 5 vs entry 17). Increasing the temperature from 25 to 60 °C resulted in a decrease in catalytic activity, possibly due to partial catalyst deactivation or enhanced chain-transfer processes at higher temperature, as suggested by the reduction in the molar mass. A similar trend has been observed with ketimide catalysts, although the presence of the permethylated ligand appeared to enhance the catalyst stability at higher temperatures [22].

The methyl pentad distribution of the polypropenes, listed in Table S1 and shown in Figures S1–S4 reported in the Supplementary Materials, indicates that all samples are slightly syndiotactoid, [rrrr] content ranging from 7.94 to 9.60, with the Bernoullian index around 1.0 suggesting a weak syndiospecific chain end control [23]. For catalyst 1, a markedly higher increase in regioerrors can be observed going from 40 to 60 °C. Furthermore, catalyst 2 affords 2,1 threo and erythro regioirregulaties of approximately 20% at 40 °C [24].

Catalysts 1 and 3 are also efficient for the copolymerization of propene and myrcene, achieving polymers in good yield. The behavior of catalyst 4 in the copolymerization of myrcene with propene differs with that observed in propene homopolymerization and propene/norbornene copolymerization; indeed, in the presence of myrcene, no copolymerization occurs. These results are consistent with those observed in E/MY copolymerization, where this catalyst exhibits low activity and negligible comonomer incorporation [16]. Catalyst 2, which exhibits low activity in propene homopolymerization, yields only negligible amount of copolymer. All these findings suggest that catalytic activity in the copolymerisation was affected by the cyclopentadienyl fragment employed. Specifically, the Cp* analogues are those that exhibited higher activities catalysts and afford high molar mass copolymers with unimodal molecular weight distributions. At both 40 and 60 °C catalyst 3 outperformed catalyst 1, although both catalysts showed a decrease in activity as the comonomer concentration in the feed increased (117.3 kg(mol_Ti·h)^-1, entry 14, vs 79.3 kg(mol_Ti·h)^-1 , entry 4, and 385.3 kg(mol_Ti·h)^-1, entry 18, vs 92.4 kg(mol_Ti·h)^-1 , entry 6). However, for catalyst 3, the decrease in activity with increasing MY content in the feed is more pronounced when compared to that of the polypropene.

Copolymers by both catalysts possess high molar mass as well as unimodal molecular weight distribution (Ð), with catalyst 3 affording the higher ones. Molar masses decreased as the MY content in the feed increased, however, no significant changes were observed when the temperature was raised from 40 to 60 °C.

Polymer composition was determined by ¹H spectroscopy according to equations S1-S4. NMR spectra were recorded in 1,1,2,2-tetrachloroethane-d₂ to ensure that all the diagnostic resonances were clearly resolved and free from the deuterated solvent peak. Additional ¹H spectra are also shown in the Supplementary Materials (Figures S5 and S6). Figure 1 shows the ¹H spectrum of entry 9 that is the copolymer at higher MY content.

Microstructural analysis shows that, regardless of the catalyst employed, incorporation of myrcene into the polymer chain remains limited, even upon variation of the reaction conditions, with MY contents not exceeding 3.10 mol%. This behavior is consistent with previous reports on diene/olefin copolymerizations, where steric hindrance and unfavorable insertion kinetics of bulky conjugated dienes restrict their incorporation. Notably, this trend is also evident for catalyst 1, which, despite its ability to incorporate up to 15.7 mol% of myrcene in E/MY copolymerization, exhibits significant lower comonomer incorporation under the present conditions [16]. Catalyst 3 promotes P/MY copolymerization with higher stereoselective control than catalyst 1, yielding up to 88 mol% of 1,4-units and 12 mol% of 3,4-units in the polymer backbone.

It is evident that, despite the low comonomer content in the polymer chain, increasing MY concentrations in the feed leads to a decrease in both activity and molar mass. This low incorporation of the terpene, along with the decrease in activity and molar mass, may be connected to the tendency of these half- titanocenes to promote cyclo-isomerization [25] Indeed, it is well known that ring-containing copolymers can be obtained with half-titanocene catalysts as well as zirconocene ones in the copolymerization of ethene or propene with butadiene and conjugated diene [26,27,28]. The presence of 1,2- cyclopentane rings has been firstly reported by Galimberti and co-workers in the copolymerization of ethene with 1,3-butadiene with metallocene catalysts such as bis(η⁵-cyclopentadienyl)zirconium dichloride or rac-ethylenebis[(4,5,6,7-tetrahydro)-indenyl]zirconium dichloride [26]. Besides, a highly sterically hindered zirconocene namely rac-methylenbis(3-tert-butyl-indenyl) zirconium dichloride has proven to be able to generate cyclopropane and cyclopentane rings in the copolymerization of propene with butadiene [26]. More recently, both cyclopentane and cyclohexane units have been detected in E/isoprene copolymers with the half-titanocene catalyst 1 used in this paper [29]. Furthermore, the microstructural analysis of E/MY copolymers from catalyst 1 reveals the presence of cyclopentane units, formed by 2,1- insertion of myrcene followed by cyclization after E insertion [16]. According to literature, the presence of these structures was linked to the absence of resonances at ca. 107 and 150 ppm, which are ascribed to methylidene carbon atoms. ¹³C-NMR spectra of copolymers from both catalysts 1 and 3 seem not to show these resonances. Further detailed microstructural analysis will be devoted to better elucidate this hypothesis.

All copolymers exhibited a single glass transition temperature (T_g), confirming their fully amorphous nature and homogeneous composition. The measured T_g values decreased with increasing MY content (Figure S7) and remained intermediate between those of the respective homopolymers (from −76.5 to −56 °C as a function of microstructure for polymyrcene [11,30,31] and 0 °C for atactic polypropene). The most significant T_g reduction was observed for the copolymer from catalyst 1 with the highest MY incorporation (3 mol%), i.e., entry 9, which exhibited a T_g of −17 °C. Notably, the incorporation of 1 mol% MY using catalyst 1 (entry 8) resulted in T_g of –11 °C. In contrast, entries 19 and 20 from catalyst 3 exhibited similar T_g values despite nearly doubling the MY content. This behavior is likely attributable to differences in the copolymer microstructure and molecular mass; specifically, entry 8 possesses a high 3,4-myrcene content (≈ 40%) and M_w of 90 kg mol^-1, whereas entries 19 and 20 contain significantly lower 3,4-myrcene level (≈ 20%) and M_w of about 200 kg mol^-1.

The thermal stability and decomposition profiles of selected samples synthetized with catalyst 3, namely PP homopolymer (entry 13) and the P/MY copolymers (entry 14 and entry 16), were investigated via thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG), as illustrated in Figure 2. The thermal results obtained from thermograms, i.e., temperatures at mass loss of 5, 10 and 50% (T_5%, T_10% and T_50%, respectively) and the temperature at the maximum degradation rate (T_max) are reported in Table S2.

The PP homopolymer exhibited the highest thermal stability among the samples, characterized by a well-defined single-step degradation process with an onset temperature of 422 °C. Its DTG curve shows a sharp, narrow peak centered at 471 °C, indicating a rapid and uniform mass loss. The incorporation of MY units into the polymer backbone resulted in a systematic decrease in thermal stability. As the MY content increased from 0.33 mol% (entry 14) to 2.52 mol% (entry 16), both the T_5% and T_max values shifted progressively toward lower temperatures. Specifically, entry 14 showed a T_5% reduction to approximately 400 °C and a T_max similar to that obtained for the PP homopolymer, whereas entry 16 (2.52 mol% MY) exhibited the lowest thermal resistance, with T_5% and T_max at 380 and 451 °C, respectively. These results suggest that the presence of MY units facilitates the initiation of thermal decomposition, as a consequence of the higher susceptibility of the double bonds in the terpene-derived units compared to the saturated propene backbone. Furthermore, the slight broadening of the DTG peak in the copolymers compared to the homopolymer reflects a more complex degradation process resulting from the random distribution of MY units along the chain.

3. Materials and Methods

All operation and manipulations were carried out under a dry nitrogen atmosphere using standard Schlenk-line and glove-box techniques. Before use, all glassware was dried in an oven at 130 °C and kept under a nitrogen atmosphere.

Propene and nitrogen were purified by passage through BTS-catalysts and molecular sieves. Toluene (Merck Life Science Srl, Milano, Italy) was dried over calcium chloride, then refluxed over metallic sodium for 48 h and distilled before use. β-Myrcene (Merck Life Science Srl) was purified by stirring overnight over CaH₂, then distilled under reduced pressure and finally stored under dry nitrogen and kept at −20 °C. Methylaluminoxane (MAO, 10 wt% solution in toluene, Merck Life Science Srl) was dried before use (80 °C, 3 h, 0.1 mmHg) to remove solvent and unreacted trimethylaluminum. C₂D₂Cl₄ was purchased from Merck Life Science Srl and used as received. Catalysts were synthesized as previously described [20,32].

3.1. Typical Reaction Procedure

A typical reaction procedure for the copolymerizations of P with MY by a half-titanocene catalyst was carried out as follows: a 250-mL Büchi (Uster, Switzerland) stainless-steel laboratory autoclave equipped with a mechanical stirrer and with an external thermostatic bath associated to a temperature control unit for temperature regulation is evacuated for 120 minutes at 80 °C and filled with nitrogen. After cooling down to room temperature, the reactor was filled with toluene, MY and with the solution of MAO in toluene previously prepared (total volume = 90 mL). After thermal equilibration at 40 °C, the reactor system was saturated with propene to the desired pressure. The reaction was initiated by injecting a 10 mL-toluene solution of the catalyst into the autoclave. The propene pressure was kept constant at 4 bar during the polymerization reaction. The polymerization was quenched by addition of 2 mL of ethanol and hydrochloric acid. The solution was then poured into ethanol to which a small amount of HCl is added, and the resultant polymer was collected by filtration and washed thoroughly. The polymer was then redissolved in toluene to remove any unreacted monomer, reprecipitated in ethanol and then dried in vacuo to constant weight for several hours.

3.2. Characterization

NMR spectra were recorded on a Bruker (Billerica, MA, USA) Avance 400 MHz spectrometer (400 MHz, ¹H; 100.58 MHz, ¹³C) operating at 100.58 MHz (¹³C) in the PFT mode working at 103 °C. The applied conditions were as follows: 10 mm probe, 90° pulse angle 13.5 μs power, 64K data points; acquisition time 4.52 s; relaxation delay 16 s; and 3K–4K transient. Proton broad-band decoupling was achieved with a 1D sequence using bi-waltz-16-32-power gate decoupling. The terpolymer sample (ca. 80 mg) was dissolved in tetrachloroethane-d₂ in a 10 mm NMR tube.

The weight-average molecular weight (M_w) and molecular weight distribution (M_w/M_n, Đ) of each polymer were determined at 145 °C using a size exclusion chromatography Waters (Milford, MA, USA) GPCV2000 high system. o-dichlorobenzene (containing 0.05 wt/v% 2,6-di-tert-butyl-p-cresol) was used as the eluent. The molecular weight was estimated based on a calibration curve using standard polystyrene samples.

Thermal properties of the samples were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC analysis was conducted using a PerkinElmer (Waltham, MA, USA) DSC 8000 instrument. The polymer samples were heated from −60 °C to 180 °C and then cooled (20 °C min⁻¹) under a nitrogen atmosphere; these heating/cooling cycles were repeated twice. The glass transition temperature (T_g) was determined from the second heating scan. TGA was performed using a PerkinElmer TGA7 analyzer to evaluate the thermal stability and decomposition profile of the polymer samples. Approximately 5 mg of the sample was placed in a platinum pan and heated from 50 to 700 °C at a constant heating rate of 10 °C min⁻¹. The measurements were conducted under a continuous nitrogen flow (25 mL min^-1).

4. Conclusions

Propene copolymerization with myrcene has been investigated in the presence of half titanocene catalysts. Catalysts 1 and 3, the permethylated ones, are suitable for the synthesis of propene/β-myrcene copolymers in terms of catalytic activity but show limited efficiency in MY incorporation, which does not exceed 3 mol% regardless the comonomer content in the feed or the polymerization conditions. The resulting copolymers are amorphous materials with T_g values that decrease as the comonomer content increases and remain intermediate between those of the corresponding homopolymers. Thermal analysis further indicates that the presence of comonomer units promotes the degradation, nonetheless the copolymers exhibit high thermal stability consistent with the PP homopolymer reference. Overall, these findings highlight that the efficient synthesis of propene/β-myrcene copolymers remain a significant challenge to overcome; further development of molecular catalysts is necessary to overcome the steric and kinetic barriers imposed by bulky bio-based monomer.