Catalytic process for co 2 to value-added functional polycarbonate

Abstract

The present invention relates to the catalytic process for utilization of carbon dioxide (CO2) to synthesize valuable chemicals such as polycarbonates. The present invention provides novel catalysts Mn (II) complex and a process to prepare the same. The present invention also provides a process to prepare polycarbonate using the novel catalytic Mn (II) complex. This process leverages the affordability and abundance of CO2 and transforms it into poly(ether)carbonate diol (PCD), a crucial precursor for high-performance polyurethanes.

Description

[0001] CATALYTIC PROCESS FOR CO2 TO VALUE-ADDED FUNCTIONAL POLYCARBONATE RELATED PATENT APPLICATION: This application claims the priority to and benefit of Indian Patent Application No. 202441051717 filed on July 05, 2024; the disclosures of which are incorporated herein by reference. FIELD OF THE INVENTION: The present invention relates to a catalytic process for the co-polymerization of epoxides and carbon dioxide (CO2) to produce polycarbonate diols. Specifically, the present invention involves the development of advanced Mn (II)-complex catalysts that facilitate this co-polymerization under ambient conditions. BACKGROUND OF THE INVENTION: Carbon dioxide (CO2) is a ubiquitous greenhouse gas with significant potential as a renewable carbon source for synthesizing valuable chemicals, thereby contributing to environmental sustainability. Affordability, harmlessness, and plentiful supply of CO2make it an ideal candidate. The urgent need to mitigate CO2emissions has spurred research into its transformation into useful products, leveraging its abundance and benign nature. The transformation of CO2into valuable chemicals is not just a scientific endeavor but also a strategic move to combat the escalating global warming crisis, which is exacerbated by increasing CO2 levels in the atmosphere. This transformation primarily occurs through either reductive or nonreductive pathways. Reductive processes, while effective, often require high energy inputs and potent reductants, limiting their sustainability. In contrast, nonreductive approaches, such as the direct utilization of CO2in polymer synthesis, offer a more sustainable alternative by minimizing energy demands and maximizing carbon utilization efficiency. Conversely, the nonreductive approach is emerging as a more sustainable alternative, predominantly because it demands lower energy inputs The synthesis of polycarbonate diols (PCDs) represents a pivotal application of CO2in polymer chemistry. PCDs are valued for their high molecular weight and mechanical properties, making them indispensable in polyurethane synthesis and biomedical materials. Traditionally, PCDs are synthesized by reacting CO2with diols in the presence of dehydrating agents or metal catalysts, processes that are plagued by inefficiencies, harsh conditions, and significant waste generation. Hence, there is a pressing demand for greener synthesis routes that eliminate these drawbacks while enhancing overall process sustainability. Recent advancements in catalysis have pioneered more efficient methods for PCD synthesis under milder conditions, circumventing the need for dehydrating agents. For example, Gu et al. (2019), ACS Sustainable Chemistry & Engineering, 7(6): 6304-631, discloses a CeO2and 2-furonitrile catalyst system for direct polycarbonate synthesis from CO2 and diols, achieving notable advancements in catalyst efficiency and sustainability. Publication Chapman et al. (2015), ACS Catalysis, 5(3): 1581-1588, discloses the use of Zn and Mg homogeneous catalysts to improve CO2 utilization efficiency in polycarbonate polyol production, highlighting significant strides in catalyst development for sustainable polymer synthesis. Moreover, the direct utilization of atmospheric CO2for PCD synthesis, as investigated by Gu et al. (2021), Green Chemistry, 23 (16), 5786-5796, represents a critical advancement in streamlining the carbon capture process and enhancing economic viability. This approach not only reduces environmental impact but also underscores the potential of CO2as a valuable feedstock for industrial applications. Furthermore, various advancements have been made in the production of polycarbonate-based materials, leveraging innovative catalysts to enhance efficiency and selectivity. Notable among these developments are processes employed by leading companies in the field: i). Novomer Process: utilizes a double metal cyanide (DMC) catalyst for creating alternating polycarbonate-based materials. This process has been instrumental in producing high-performance polymers with significant industrial applications. ii). Covestro AG: has developed a method to produce polycarbonate polyol using the Econic catalyst, a bimetallic macrocyclic homogeneous catalyst. This technology enables the efficient synthesis of polycarbonate polyols with improved properties and consistency. iii). Saudi Aramco: It employs a β-ketamide-derived zinc-based soluble catalyst for producing polycarbonate diols. This approach allows for the creation of polycarbonate diols with a range of molecular weights, catering to diverse application needs. The existing prior arts provide polycarbonate diol using several catalysts. However, there is a need in art to optimize catalytic systems for a broader substrate scope, improving catalyst stability, and enhancing overall process efficiency. Therefore, addressing these challenges is crucial for advancing CO2utilization in polymer synthesis toward more sustainable chemical production methods. OBJECTS OF THE INVENTION: The primary object of the present invention is to develop an efficient catalyst for the conversion of carbon dioxide (CO2) into high-value carbon compounds. Another object of the present invention is to provide a novel process for preparation of Mn(II) catalyst for the production of high value carbon compound such as Poly(ether)carbonate diol (PCD). Yet another object of the present invention is to optimize reaction conditions to maximize yield and quality of the produced polycarbonates while minimizing energy consumption and waste generation. Yet another object of the present invention is to characterize the Mn-(II) catalysts synthesized using the novel process. A further object of the present invention is to synthesize polycarbonates with diverse functional groups using catalytic Mn-(II) complexes suitable for various industrial applications, including packaging, coatings, and medical devices. SUMMARY OF THE INVENTION: A process for the preparation of Mn-(II) complex catalyst comprising the steps of: a) reacting pyridine derivative with heterocyclic compounds in presence of base and acetonitrile; b) refluxing the crude Mn-(II) complex at elevated temperatures under inert gas; c) cooling the mixture at room temperature followed by evaporation of solvent to extract heterocyclic Ligand (L); d) reacting heterocyclic ligand (L) with Manganese-(II) chloride tetrahydrate in presence of acetonitrile; and e) stirring, filtering and drying the mixture to obtain precipitates of Mn-(II) complex (MnL). The process as above, wherein the pyridine derivative of step (a) is selected from the group comprising 2-(chloromethyl) pyridine hydrochloride, 2-(2,6- bis(bromomethyl)pyridine, 2-(bromomethyl) pyridine, 2-(2,6- bis(chloromethyl)pyridine, 2-(2-chloroethyl) pyridine hydrochloride, or 4- (2-chloroethyl) pyridine hydrochloride. The process as above, wherein the pyridine derivatives are -(chloromethyl) pyridine hydrochloride or 2-(2,6-bis(bromomethyl)pyridine. The process as above, wherein the base in step (a) is selected from the group comprising potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and sodium hydroxide. The process as above, wherein the base is potassium hydroxide or potassium carbonate. The process as above, wherein the heterocyclic compounds in step (a) are selected from group comprising 3-methyl pyrazole, 3,5-dimethyl pyrazole, morpholine, 3 ethyl-1H pyrazole or 3-ethyl-5 methyl 1H pyrazole. The process as above, wherein the temperature in step b) is between 50 to 90°C. A Mn (II) complex catalyst of formula MnL1

[0002] for copolymerization of epoxide with carbon dioxide. A Mn-(II) complex catalyst of formula MnL2 for copolymerization of epoxide with carbon dioxide. A Mn-(II) complex catalyst of formula MnL3: for copolymerization of epoxide with carbon dioxide. A process for the synthesis of polycarbonate diols using the Mn-(II) complex catalyst, comprising the steps of: (i) dissolving the Mn-(II) complex in a suitable solvent; (ii) mixing the dissolved Mn-(II) complex with epoxides and CO2; (iii) performing copolymerization under ambient temperature and pressure; and (iv) terminating the polymerization reaction by the addition of an alcohol or diol. The process as above, wherein the Mn-(II) complex catalyst is selected from a group consisting of MnL1, MnL2, and MnL3. The process as above, wherein the suitable solvent in step (i) is selected from a group comprising methanol, ethanol, acetonitrile, and chloroform. The process as above, wherein the epoxide is selected from ethylene oxide (EO) and propylene oxide (PO). The process as above, wherein the alcohol is selected from the group comprising methanol, ethanol, propanol, butanol, or a combination thereof. The process as above, wherein the diols are selected from the group comprising ethylene glycol, propylene glycol, methane diol, butane diol. BRIEF DESCRIPTION OF THE DRAWINGS: FIG. 1: Copolymerization of epoxides with CO2in presence of Mn (II) complex catalyst providing high PCD (desired product) and low cyclic carbonate. FIG.2: Molecular structure of Mn-(II) complexes FIG.3: Non-Stirred Reactor Assembly FIG.4: IR spectra of polymer FIG.5: MALDTI-TOF ((Dithranol as a matrix) of Polymer FIG.6: NMR of Polymer (a):1H NMR of Polymer (b):13C NMR of Polymer FIG.7: NMR spectrum of L1 (a):1H (top) NMR spectrum of L1 (b):13C (bottom) NMR spectrum of L1 FIG.8: Mass analysis of L1 FIG.9: NMR spectrum of L2 (a):1H (top) NMR spectrum of L2 (b):13C (bottom) NMR spectrum of L2 FIG.10: Mass analysis of L2 FIG.11: MnL1 complex analysis (a): FT-IR of MnL1complex (left) (b): UV-Vis of MnL1 complex (right) (c): Single crystal structure of MnL1 complex FIG.12: MnL2complex analysis (a): FT-IR of MnL2complex (left) (b): UV-Vis of MnL2 complex (right) (c): Single crystal structure of MnL2complex FIG.13: MnL3complex analysis (a): FT-IR spectrum of MnL3 (Left) (b): FT-IR spectrum of MnL3 (Right) (c): Single crystal structure of MnL2complex FIG.14: Absorption spectrum of MnL3FIG.15: EPR spectrum of MnL3 FIG.16: Hysteresis loops of an array of Mn-PyNNN DETAILED DESCRIPTION OF THE INVENTION: Carbon technology stands at the forefront of sustainable innovation, harnessing CO2as a renewable resource to achieve carbon neutrality. To significantly cut down on CO2 emissions linked to fossil fuel usage, carbon recycling technology is anticipated to play a pivotal role. Among no-hydrogenative route applications, the transformation of CO2into polymers is particularly promising, enabling the production of high-value polymeric products. Poly(ether)carbonate diol (PCD), having a terminal hydroxyl group, exemplifies a high-value carbon compound pivotal in synthesizing high-performance polyurethanes. Renowned for its adhesive qualities, PCD is primarily employed as a precursor in polyurethane (PU) foam production. These foams are integral to various industries, serving as materials for synthetic leather in automotive seating, outdoor wall paints, and durable floor coatings. With an increasing global demand, the significance of PCD-based polyurethanes is on the rise. Traditionally, producing PCD involves a two-step process with considerable environmental repercussions. The present invention pioneered a single-step synthesis of PCD from CO2, marking a paradigm shift in the field. This method offers substantial economic and ecological advantages. Polycarbonate diols are commonly used in the synthesis of polyurethane resins, acrylic resins, and polyester resins. The polyurethane produced from polycarbonate diols or polycarbonate- based polyurethane pre-polymers surpasses that derived from polyether diols or polyester diols in terms of durability. The present invention mainly involves the development of efficient, scalable catalysts for the production of PCD with excellent selectivity as well as tuneable molecular weights and with enhanced CO2incorporation, particularly in the case of co-polymerization of epoxides (ethylene oxide (EO) / propylene oxide (PO)) and CO2. Polycarbonate diol will be produced at ambient temperature and low pressure of CO2. The present strategy allows these polymerizations to proceed with minimal additional energy input and greater control. In this copolymerization of epoxide with CO2 to produce polycarbonate, electron-rich epoxide and electron-deficient CO2 should be activated simultaneously. Notably, cyclic carbonate and homo- polymer formation are depressed under the present catalytic conditions. Scheme 1. Copolymerization of epoxides with CO2 In this copolymerization of epoxide with CO2 to produce polycarbonate, electron- rich epoxide and electron-deficient CO2 should be activated simultaneously. Accordingly, in one aspect, the present invention provides a process of preparation of Mn-(II) complexes as a novel catalyst for the copolymerization of carbon dioxide and epoxide, wherein the novel catalysts operate at ambient temperature and low CO2pressure with good polymer selectivity. The synthesis of homogenous Mn-(II) complexes catalyst involves the steps of: a) synthesis of heterocyclic ligand (L); and b) synthesis of heterocyclic Mn (II) complex. Step a) – synthesis of heterocyclic ligand (L): The synthesis of heterocyclic ligand (L) of step (a) involves reacting pyridine derivatives with heterocyclic compounds in presence of a suitable base and acetonitrile to arrive at the ligand (L). The suitable base is selected from the group comprising potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, sodium hydroxide. Wherein the preferred base is potassium hydroxide or potassium carbonate. The pyridine derivatives are selected from the group comprising 2-(chloromethyl) pyridine hydrochloride, 2-(2,6-bis(bromomethyl)pyridine, 2-(bromomethyl) pyridine, 2-(2,6-bis(chloromethyl)pyridine, 2-(2-chloroethyl) pyridine hydrochloride, 4-(2-chloroethyl) pyridine hydrochloride. The preferred pyridine derivative is 2-(chloromethyl) pyridine hydrochloride, 2-(2,6- bis(bromomethyl)pyridine. The heterocyclic compounds are selected from group comprising 3-methyl pyrazole, 3,5-dimethyl pyrazole, morpholine, 3 ethyl-1H pyrazole, 3-ethyl-5 methyl 1H pyrazole. The reaction was refluxed at 50 to 90oC between 10 to 14 hours under nitrogen gas. The completion of the reaction was analysed on TLC. The reaction mixture was cooled at room temperature. The solvent was evaporated under vacuum. The resulting product was extracted in a solvent and water mixture, wherein the organic layer was dried. The resulting ligand (L1, L2, L3) is used in the next step. Step b) - Synthesis of heterocyclic Mn-(II) complex: The synthesis of heterocyclic Mn-(II) complex involves reacting ligand (L) of step (a) with Manganese-(II) chloride tetrahydrate in presence of acetonitrile to provide heterocyclic Mn-(II) complex. The said process involves reacting heterocyclic ligand (L) with Manganese-(II) chloride tetrahydrate in presence of acetonitrile; and stirring, filtering, and drying the mixture to obtain precipitates of Mn (II) complex (MnL). Thus, the complexes stability and durability helped to perform all the catalytic copolymerization reactions under ordinary atmospheric conditions. Moreover, all the ligands and complexes (Mn) can be synthesized in a multi-gram scale. The synthesized complexes were effectively used for epoxide (propylene oxide) copolymerization with CO2,and the results are summarized in Table 1. This transformation is equally important for developing catalysts and optimizing reaction parameters (pressure, temperature, solvent (if necessary), mode of addition of starting materials, mol% of catalyst, initiator and its amount, and time). In order to achieve excellent selectivity towards polycarbonate diol with excellent CO2 incorporation, the effect of all these parameters will be examined carefully. Critical optimization is required to depress cyclic carbonate formation and achieve excellent selectivity (more than 95%) towards polycarbonate diol. The % of cyclic- and polycarbonate is determined by FT-IR spectroscopy. The yield and selectivity of the polymeric product are calculated based on the following equations. ^^^^^^^^ℎ^^ ^^^^ ^^^^^^^^^^^^^^ % Yield of product = 100 ^^ ^^^^^^^^ℎ^^ ^^^^ ^^^^^^^^^^^^^^+^^^^^^^^ℎ^^ ^^^^ ^^^^2+^^^^^^^^ℎ^^ ^^^^ ^^^^^^^^^^^^^^% of CO incorporat^^^^^^^^ℎ^^ ^^^^ ^^^^2 ^^^^^^^^^^^^^^^^ 2ed = 100 ^^^^^^^^^^ℎ^^ ^^^^ ^^^^2 ^^^^^^^^^^^^^^ Molecular structure of Mn (II) complexes: The ligand has a pyrazole-pyridine unit with an angle of about 120 Å, which makes it easier for epoxide and CO2 to interact with the Mn-center. Also, the pyrazole ligand is a good π-acceptor, which stabilizes low-valent manganese species and improves the kinetic stability of the Mn-(II)- complex. The co-polymerization of CO2 with epoxide is usually initiated by the addition of alcohol / diol. The alcohol is selected from group comprising methanol, ethanol, propanol, butanol or combination thereof. The diols are selected from the group comprising ethylene glycol, propylene glycol, methane diol, and butane diol. Indeed, the protonation of the polymer chain can either occur due to a hydroxyl starter in the reaction mixture or intentionally to generate low molecular weight polymers or to incorporate a chain-transfer agent. With this background research, the inventors intended to use diol as a starter / initiator in the copolymerization reaction. Thus, ethylene glycol / propylene glycol and PEG-450 were used as an initiator. All of them provided PCD with variable molecular weights. The optimization results are given below based on the activity of the Mn (II) complex and other as-synthesized Mn complexes. The optimal reaction condition is shown in Table 1. Table 1. Copolymerization results of the present invention. Initially, t-butyl ammonium halide as an additive (co-catalyst) was used to initiate the ring-opening of an epoxide and Mn-2 complex to activate CO2. After careful and systematic screening, it was observed that homo bi-metallic Mn-(II) is an optimal catalyst for the production of polycarbonate from propylene oxide and CO2. Among the synthesized scalable, bench-stable Mn-(II), Mn-complex (2) was found to be an efficient catalyst for polycarbonate production under mild reaction conditions (low temperature and pressure). Indeed, complex 2 synergistically activates the electron-rich propylene oxide and the electron-deficient carbon dioxide molecule. The non-stirred reactor assembly of the present invention is shown in the FIG.3. The IR spectra of the polymer are graphically represented in FIG. 4. The synthesized polymer was analyzed using FT-IR, NMR, and MALDI-TOF. Further, in the Mass analysis, Dithranol was used as a matrix. FIG.5 represents the MALDI- TOF (Dithranol as a matrix) of Polymer. Furthermore, FIG. 6 (a) and FIG.6 (b) graphically represent the1H and13C NMR of Polymer, respectively. In the mass spectrum, the following pattern was observed: ~ 58 difference = PO; ~ 16 difference = Terminal –OH ~ 44 difference = CO2; ~ 102 difference = PO + CO2 From FT-IR analysis, the stretching frequency of carbonate in the PCD appears to have a lower frequency region than in the cyclic carbonate. This helped to quantify the polymer vs cyclic carbonate. Similarly, the signal at 154 ppm at the13C NMR confirms the presence of carbonyl carbon in PCD. The signals around 65.2 to 68.0 correspond to poly-ether linkage in PCD. Mechanism for the formation of PCD via catalytic copolymerization of CO2 with epoxide: The copolymerization of epoxides and CO2is divided into three elementary steps: initiation, propagation, and termination. Termination also occurs via a chain transfer of the propagating chain through protic substrates or the addition of chain-transfer agents (CTA). Generally, the mechanism can proceed via three different pathways, with LnM-X being the homogeneous catalyst. In the monometallic pathway, the ring-opening of the precoordinated epoxide takes place through an intramolecular attack of the nucleophile X. Next, it forms a metal-carbonate chain-end after the insertion of CO2.

[0003] Scheme 2. Mechanism for the formation of PCD via catalytic copolymerization of CO2with epoxide. The alternating insertion of an epoxide and CO2 leads to the formation of the polycarbonate. Two possible side reactions are observed. If epoxides are inserted consecutively, polyether linkages result, and if the growing carbonate chain is prone to backbiting, a cyclic carbonate byproduct is formed. The polymerization reaction is usually terminated by the addition of alcohol / diol. This protonation of the growing polymer chain can either occur due to a hydroxyl starter in the reaction mixture or intentionally to generate low molecular weight polymers or to incorporate a chain-transfer agent. The resulting PCD is essential for producing polyurethane foams used in automotive seating, synthetic leather, outdoor paints, and floor coatings. The present invention addresses global warming by reducing CO2levels and offers a sustainable alternative with lower energy requirements compared to reductive conversion methods. In another aspect, the present invention characterized the scalable, air-stable Mn (II)-complexes synthesized using the present process. All the synthesized soluble, homogeneous complexes (Single site, Bimetallic, and polymetallic (2D-Cluster) were structurally characterized, and their catalytic activity towards CO2 copolymerization with propylene oxide (PO) was examined. The present invention has achieved excellent selectivity of the polyPC (97%) with ~35-43% CO2 incorporation. The present invention’s strategy enables polymerization with minimal energy and enhanced control, potentially revolutionizing the production of engineering polyurethane foams. The synthesized molecular Mn complexes are remarkably stable in both solution and solid states and can be easily managed in a regular atmospheric environment. This stability and robustness enable the execution of catalytic copolymerization reactions under standard atmospheric conditions. Additionally, the synthesis of all ligands and metal complexes is feasible on a multi- gram scale. Certain specific aspects and embodiment of the present invention will be explained in detail with reference to the following examples, which are provided only for purposes of illustration and should not be construed as limiting the scope of the invention in any manner. EXAMPLES: In copolymerization of epoxide with CO2to produce polycarbonate, both electron- rich epoxide and electron-deficient CO2should be activated simultaneously. After a careful literature survey and research experience, the inventors of the present invention have zeroed in on the best possible catalytic systems, and the following metal complexes have been synthesized and completely characterized.

[0004] Mn-complexes used for copolymerization of CO2with epoxide. Example 1. Synthesis and characterization of Ligand for catalyst synthesis: Example 1.1 Synthesis of L1: 2-(Chloromethyl) pyridine hydrochloride (0.6 g, 0.0036 mmol) was dissolved in 12 mL of acetonitrile, and 3-methyl pyrazole (0.326 g, 0.0039 mmol), potassium hydroxide (0.22 g, 0.0039 mmol) was refluxed at 90oC for 12hrs under nitrogen gas. The completion of the reaction was analyzed by TLC. The reaction mixture was cooled to room temperature, and the solvent was removed by evaporation under vacuum. The resulting product was extracted with CH2Cl2: H2O, and the organic layer was dried over Na2SO4. Orange oil. Yield (0.565 g, 91%). IR (KBr): ν = 3419.79, 2947.23, 2522.89, 1641.42, 1452.40, 1111.00, 1029.99, 599.86.1H NMR (400 MHz, CDCl3): δ 8.55 (d, 1H), 7.63 (t, 1H), 7.18 (t, 1H), 6.80 (d, 1H), 5.88 (s, 1H), 5.35 (s, 2H), 2.26 (s, 3H).13C NMR (100 MHz, CDCl3): δ 156.90, 149.18, 139.03, 137.17, 130.87, 122.68, 121.66, 121.12, 105.83, 104.22, 57.08, 13.54. GC-MS (CHCl3) m / z calcd for C10H11N3: 173.10 and found: 173.10. The1H (top) &13C (bottom) NMR spectrum of L1 is shown in the FIG. 7 (a) and FIG.7 (b) respectively, and the mass analysis is presented in FIG.8. Synthesis of L1 Example 1.2. Synthesis of L2: 2-(Chloromethyl) pyridine hydrochloride (0.3 g, 0.0018 mmol) was dissolved in 12 mL of acetonitrile and 3,5-dimethyl pyrazole (0.190 g, 0.0019 mmol), potassium hydroxide (0.11 g, 0.0019 mmol) were refluxed at 90oC for 12 hrs under nitrogen gas. The completion of the reaction was analyzed by TLC. The reaction mixture was cooled to room temperature, and the solvent was removed by evaporation under vacuum. The resulting product was extracted with CH2Cl2:H2O, and the organic layer was dried over Na2SO4. Orange oil. Yield (0.285 g, 85%). IR (KBr): ν = 3390.86, 1662.64, 1429.25, 1294.24, 1037.70, 763.81.1H NMR (400 MHz, CDCl3): δ 8.54 (d, 1H),7.61 (t, 1H), 7.16 (t, 1H), 6.80 (d, 1H), 5.88 (s, 1H), 5.35 (s, 2H), 2.25 (s, 3H), 2.17 (s, 3H).13C NMR (100 MHz, CDCl3): δ 157.31, 149.19, 148.06, 139.73, 137.11, 122.39, 120.96, 105.90, 104.11, 54.30, 13.51, 11.04. GC-MS (CHCl3) m / z calcd for C11H13N3: 187.13 and found: 187.16. The1H (top) &13C (bottom) NMR spectrum of L2 is shown in the FIG.9 (a) and FIG.9 (b) reactively and the Mass analysis of L2 represented in FIG.10. Synthesis of L2 Example 1.3. Synthesis L3: A solution of 2,6-bis(bromomethyl)pyridine (0.3 g, 1.13 mmol) in acetonitrile (30 mL) was added drop-wise to solution of morpholine (0.197 g, 2.26 mmol) and K2CO3(0.468 g, 3.39 mmol) in CH3CN (15 mL), the resulting reaction mixture was allowed to stir for 14 h at 80 °C, then cooled to room temperature. The solvent was evaporated under reduced pressure, and the reaction mixture was extracted using chloroform. The organic layer was collected and dried over anhyd.Na2SO4then evaporated in a vacuum under the reduced pressure afforded L3. Yield (0.282 g, 90%). IR (KBr): ν = 2800 (m), 1575 (m), 1454 (m), 1298 (m), 1111 (s), 906 (m).1H NMR (500 MHz, CHCl3-d) δ = 7.65 - 7.52 (m, 1H), 7.31 (d, J = 7.6 Hz, 2H), 3.84 - 3.69 (m, 8H), 3.66 (s, 4H), 2.51 (s, 8H).13C NMR (126 MHz, CHLOROFORM-d) δ = 157.7, 136.7, 121.4, 77.3, 76.7, 66.9, 64.8, 53.7. HRMS (EI): m / z Calcd for C15H24O2N3: 278.1869; Found: 278.1863. Synthesis of L3 Example 2. Synthesis of well-defined Mn (II) complexes: Example 2.1. Synthesis of MnL1 complexes: A Ligand L1 (0.1 g, 0.00057 mmol) was dissolved in 6 mL of acetonitrile and MnCl2.4H2O. (0.112 g, 0.00057mmol) was added. The reaction mixture was stirred for 3 hrs at room temperature. The resulting precipitate was filtered and dried. The precipitate was crystallized by ether diffusion method at room temperature for 2 days. The colourless crystals of MnL1 were formed. Yield (0.129 g, 89 %). IR (KBr): ν = 3442.94, 1635.64, 1521.84, 1433.11, 1317.38, 1107.14, 1014.56, 948.98, 761.88. Synthesis of MnL1 Example 2.2. Synthesis of MnL2: A Ligand L2 (0.1 g, 0.00051 mmol) was dissolved in 6mL of acetonitrile and MnCl2.4H2O. (0.1g, 0.00051mmol) was added. The reaction mixture was stirred for 3 hours at room temperature. The resulting precipitate was filtered and dried. The precipitate was crystallized by ether diffusion method at room temperature for 2 days. The Colourless crystal of MnL2 was formed. Yield (0.119 g, 82%). IR (KBr): ν = 3444.87, 1602.85, 1438.90, 1313.52, 1151.50, 1049.28, 769.60, 680.87, 603.72. Synthesis of MnL2 Example 2.3. Synthesis of MnL3: To a stirred solution of ligand PyNNN (0.05 g, 0.18 mmol) in CH3CN (8 mL) was added MnCl2.4H2O (0.035 g, 0.18 mmol), then the reaction mixture was stirred at room temperature for 3h and the reaction mixture was filtered. Diffusion of diethyl ether into the filtrate, after 3 days, colorless crystals of 1a afforded, which were filtered off and washed with diethyl ether and dried in air. Yield (0.067 g, 92 %). IR (KBr): ν in cm-1= 2968 (s), 2851 (s), 1608 (s), 1452 (s), 1290 (s), 1112 (2), 1001 (s), 868 (s), 814 Notably, all the synthesized well-defined molecular complexes MnL-1, MnL-2 and MnL-3 as represented in FIG.2 are very stable in the air (both under solution and solid-state) and easy to handle under an ordinary atmosphere. Example 3. The characterization of MN-complexes of MnL1, MnL2, and MnL3: Example 3.1. Characterization of MnL1 complex: The FT-IR of the MnL1 complex is represented in FIG. 11 (a), and the graphical representation of the UV-Vis of the MnL1 complex is shown in FIG.11 (b). Further, the single crystal structure of the MnL1 complex is represented in FIG.11 (c). Example 3.2. Characterization of MnL2 complex The FT-IR of the MnL2 complex is represented in FIG. 12 (a), and the graphical representation of the UV-Vis of the MnL2 complex is shown in FIG. 12 (b). Further, the single crystal structure of the MnL2 complex is represented in FIG.12 (c). Example 3.3. Characterization of MnL3 complex: The FT-IR of the MnL3 complex is represented in FIG. 13 (a) and FIG. 13 (b). Further, the single crystal structure of the MnL3 complex is represented in FIG. 13(c). FT-IR Spectra: FT-IR spectra of ligand PyNNN exhibit characteristic ν(C=N) stretching vibration band at 1583 cm-1. The complex MnPyNNN shows the C=N stretching vibration band at 1608 cm-1. The IR stretching band shift confirms the coordination of the PyNNN ligand with the Mn2+center. Example 4. Absorption spectra of MnL3: The UV-Vis absorption spectra for the ligand (PyNNN) and complex MnL3 (i.e. MnPyNNN) were recorded in CH3CN. The free ligand displayed the absorption band at 203 nm and 264 nm could be attributed to π-π* and n-π* transitions. The complex MnL3 (Mn-PyNNN) exhibits the absorption band at 267 nm, which is slightly red-shifted from the ligand absorption band. The absorption spectrum of MnL3is graphically presented in FIG.14. Example 5. EPR spectra analysis of MnL3: The EPR spectrum of the complex Mn-PyNNN was recorded in the solid state at 298 K. The EPR spectrum of Mn-PyNNN exhibits two g values at g1 = 4.52 and g2 = 2.02 with no hyperfine splitting observed. The EPR spectra analysis of MnL3 is graphically presented in FIG.15. Example 6. VSM Spectra Analysis: A vibrating Sample Magnetometer (VSM) is used to analyze the magnetic properties of the Mn-PyNNN complex at room temperature in the applied field range from -17500 to 17500 Oe. A small intrinsic coercivity indicates the existence of magnetic behavior of the complex. Also, room temperature magnetic hysteresis loop measurement suggests that the complex Mn-PyNNN is soft ferromagnetic in nature. The hysteresis loops of an array of Mn-PyNNN is graphically presented in FIG.16. Example 7. Structural analysis of MnL3: Synthesis of a bench stable (air and moisture stable; both in solid and solution state) L3 (Py-NNN) based Mn (II) complex, MnL3 (i.e Mn-PyNNN) achieved by the reaction of Py-NNN with MnCl2.4H2O in acetonitrile afforded neutral, colorless solid in a 92% yield. The crystals of MnL3 are suitable for single-crystal X-ray crystallography and were obtained by vapor diffusion of diethyl ether into the MeOH solution of the complex. The complex Mn-PyNNN crystallized in the monoclinic space group P21 / n. The crystallographic data, selected bond distances, and angles are given in Tables 2 and 3. The crystal structure reveals that the Mn(II) center has five coordinated, distorted trigonal bipyramidal geometry with the three nitrogen donor atoms of Py-NNN ligand (L) and two chloride ions. The equatorial plane is defined by the two chloride anions (Cl(1) and Cl(2)) and a nitrogen atom (N(1)) of the Py-NNN ligand. Two nitrogen atoms (side arms, N(2) and N(3)) of Py-NNN are located in the axial positions. The bond lengths of Mn(1)–Cl(1) and Mn(1)–Cl(2), Mn(1)–N(1), Mn(1)–N(2) and Mn(1)–N(3) are found to be 2.3499(6) Å, 2.3472(6) Å, 2.1651(12) Å, 2.4648(13) Å and 2.4358(13) Å, respectively. The bond angles between N(1)- Mn(1)-Cl(1), N(1)-Mn(1)-Cl(2) and N(3)-Mn(1)-N(2) are 114.72(4), 131.56(4) and 144.13(4), respectively. According to Addison et al., the distortion parameter τ provides a measure of the degree of square pyramidal (SP) versus trigonal bipyramidal (TBP) geometry adopted by the five-coordinated metal (II) complexes (For an ideal SP complex, τ = 0; while τ = 1 for an ideal TBP). The experimental τ value for the Mn1 atom in complex Mn-PyNNN is calculated as 0.49. Thus, the coordination geometry of manganese in complex Mn-PyNNN is almost described as distorted TBP. The Single crystal X-ray structure of MnL3is represented in FIG.13 (c). Table 2. Crystallographic parameter of Mn-PyNNN complex (MnL3) Table 3. Selected Bond length [Å] and angle [°] for Mn-PyNNN Example 8. Chemical structure of additives (co-catalyst) used in the polymerization reaction: Initially, t-butyl ammonium halide as an additive (co-catalyst) was used to initiate the ring-opening of an epoxide and Mn-II complex to activate CO2. After careful and systematic screening, it was observed that homo bi-metallic Mn-(II) is an effective catalyst for the production of polycarbonate from propylene oxide and CO2. Among the synthesized scalable, bench-stable Mn (II)complexes, Mn- complex II was found to be an efficient catalyst for polycarbonate production under mild reaction conditions (low temperature and pressure). Indeed, complex II activates the electron-rich propylene oxide and the electron-deficient carbon dioxide molecule synergistically. Chemical structure of additives (Co-catalyst) used in the polymerization reaction. Several reaction parameters were studied to achieve the optimal yield and selectivity of polycarbonate production. The optimized reaction conditions revealed that the effect of the co-catalyst (t-butyl ammonium halide) is negligible under the present catalytic conditions. Example 9. Experimental procedure: Mn Complex II (5 mg), and starter (5 g of diol) were dissolved in 25 mL propylene oxide and injected via an inlet port into a Parr autoclave (100 mL). The reactor was subsequently charged to 100 psi with CO2 and stirred at 50oC (at 980 rpm speed). After 6 h, all the volatiles were removed under reduced pressure, and the obtained crude polymeric product was analysed by1H NMR and IR spectroscopy. Thus, the present invention introduces a novel catalytic process utilizing scalable and air-stable Mn (II)-complexes. It achieves high selectivity and enables adjustable molecular weights, marking a substantial advancement in polymer synthesis technology. The technical features, advantages, and applications of the present invention are highlighted as follows: Technical Features: 1. Scalable and Air-Stable Catalyst: The invention utilizes scalable and air- stable Mn-(II) complexes, ensuring practical and robust application in industrial settings. 2. Mild and Benign Conditions: Catalytic copolymerization of CO2with epoxides occurs under mild conditions (temperature ≈ 50°C, pressure ≈ 15 atm), reducing energy consumption and operational costs. 3. High Selectivity: Achieves a high selectivity of approximately 97% for polycarbonate diol (PCD) formation, minimizing unwanted by-products. 4. High CO2Incorporation: Incorporates up to ~35-43 wt% of CO2into the polymeric product, contributing to CO2 sequestration and utilization. 5. Tunable Molecular Weight: Offers tunable molecular weights of the resultant PCD, ranging from approximately 1000 to 4000, catering to various industrial needs. 6. Depressed By-Product Formation: The catalytic process significantly depresses the formation of cyclic carbonates and homopolymers, enhancing the efficiency and purity of the desired product. 7. Innovative Catalysts: Includes single-site, bimetallic, and polymetallic (2D- cluster) Mn-(II) complexes, structurally characterized and optimized for high catalytic activity. Advantages: 1. Environmental Impact: Utilizes CO2 as a raw material, thus contributing to the reduction of greenhouse gases and mitigating global warming. 2. Sustainability: The nonreductive pathway of CO2conversion into valuable chemicals demands lower energy inputs compared to traditional reductive methods, promoting sustainable industrial practices. 3. High-Performance Polyurethanes: The PCD produced is a high-value precursor for synthesizing high-performance polyurethanes, which are essential in various industries, including automotive, construction, and coatings. 4. Industrial Relevance: Addresses the increasing global demand for PCD- based polyurethanes, which are critical for applications such as synthetic leather, automotive seating, and durable floor coatings. 5. Robust and Stable Catalysts: The synthesized Mn complexes are stable in both solution and solid states, allowing for easy handling and operation under regular atmospheric conditions. 6. Efficiency: The process achieves excellent polymerization control with minimal energy input, enhancing overall production efficiency and scalability. Applications and Commercialization: 1. Polyurethane Foams: PCD, with its terminal hydroxyl group, is pivotal in the production of polyurethane foams, which are used in automotive seating, synthetic leather, outdoor wall paints, and floor coatings. 2. Adhesive Qualities: PCD's renowned adhesive qualities make it a valuable component in various industrial applications, particularly in manufacturing high-performance materials. 3. Market Demand: With the rising global demand for durable and high- performance polyurethane products, the invention's efficient and scalable production method positions it favorably for commercial success. 4. Validation: The technology has been validated in the laboratory, achieving significant milestones in the synthesis and application of scalable, air-stable Mn (II) complexes. The catalysts demonstrate excellent selectivity (97%) and substantial CO2 incorporation (~35-43%), with depressed by-product formation, showcasing their practical applicability in industrial polymer synthesis. 5. Improved Technological Advancement: Compared to existing methods, such as the Novomer process utilizing a double metal cyanide (DMC) catalyst for creating alternating polycarbonate-based materials, and methods by Covestro AG and Saudi Aramco using Econic and β-ketamide-derived zinc-based catalysts respectively for producing polycarbonate diols, this invention represents a significant leap forward. The developed scalable and efficient catalyst for producing Poly(ether)carbonate diol (PCD) ensures exceptional selectivity and offers adjustable molecular weights up to 5000. It enhances CO2incorporation to approximately 43%, particularly beneficial for ethylene oxide (EO) co-polymerization with CO2. Notably, this innovation effectively depresses cyclic carbonate and homopolymer formation, marking a substantial improvement in catalytic efficiency and product purity.

Claims

We claim:

1. A process for preparation of Mn-(II) complex catalystcomprising the steps of: a). reacting pyridine derivative with heterocyclic compounds in the presence of base and acetonitrile; b). refluxing the crude Mn-(II) complex at elevated temperatures under inert gas; c). cooling the mixture at room temperature followed by evaporation of solvent to extract heterocyclic Ligand (L);d). reacting heterocyclic ligand (L) with Manganese-(II) chloride tetrahydrate in the presence of acetonitrile; and e). stirring, filtering, and drying the mixture to obtain precipitates of Mn (II) complex (MnL).

2. The process as claimed in claim 1, wherein the pyridine derivative of step (a) is selected from the group comprising 2-(chloromethyl) pyridine hydrochloride, 2-(2,6-bis(bromomethyl)pyridine, 2-(bromomethyl)pyridine, 2-(2,6-bis(chloromethyl)pyridine, 2-(2-chloroethyl) pyridine hydrochloride, or 4-(2-chloroethyl) pyridine hydrochloride.

3. The process, as claimed in claim 2, wherein the pyridine derivatives are - (chloromethyl) pyridine hydrochloride or 2-(2,6-bis(bromomethyl)pyridine.

4. The process, as claimed in claim 1, wherein the base in step (a) is selected from the group comprising potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and sodium hydroxide.

5. The process, as claimed in claim 4, wherein the base is potassium hydroxide or potassium carbonate.

6. The process as claimed in claim 1, wherein the heterocyclic compounds in step (a) are selected from the group comprising 3-methyl pyrazole, 3,5- dimethyl pyrazole, morpholine, 3 ethyl-1H pyrazole or 3-ethyl-5 methyl 1H pyrazole.

7. The process, as claimed in claim 1, wherein the temperature in step b) is between 50 to 90°C.

8. A Mn-(II) complex catalyst of formula MnL1for copolymerization of epoxide with carbon dioxide.

9. A Mn-(II) complex catalyst of formula MnL2for copolymerization of epoxide with carbon dioxide.

10. A Mn-(II) complex catalyst of formula MnL3:for copolymerization of epoxide with carbon dioxide.

11. A process for the synthesis of polycarbonate diols using the Mn (II) complex catalyst, comprising the steps of: (i) dissolving the Mn (II) complex in a suitable solvent; (ii) mixing the dissolved Mn (II) complex with epoxides and CO2; (iii) performing copolymerization under ambient temperature and pressure; and (iv) terminating the polymerization reaction by the addition of an alcohol or diol.

12. The process is as claimed in claim 11, wherein the Mn (II) complex catalyst is selected from a group consisting of MnL1, MnL2, and MnL3 as claimed in preceding claims.

13. The process as claimed in claim 11, wherein the suitable solvent in step (i) is selected from a group comprising methanol, ethanol, acetonitrile, and chloroform.

14. The process as claimed in claim 11, wherein the epoxide is selected from ethylene oxide (EO) and propylene oxide (PO).

15. The process as claimed in claim 11, wherein the alcohol is selected from the group comprising methanol, ethanol, propanol, butanol, or a combination thereof.

16. The process, as claimed in claim 11, wherein the diols are selected from the group comprising ethylene glycol, propylene glycol, methane diol, and butane diol.

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