Ring-opening Polymerization Of Propylene Oxide Research Articles

Microstructural Insights into Ethylene Glycol‐Mediated Polyether Polyols Using Double Metal Cyanide Catalyst

AbstractPolyether polyols are well established category of polymers, which are synthesized via ring opening polymerization (ROP) industrially using Double Metal Cyanide (DMC) catalyst with glycols as initiators. Establishing the role of glycol in initiation and its involvement in the polymer chain during the ROP of propylene oxide (PO) via DMC catalysis is of utmost importance. In this work, the experimental method is established to understand the role of ethylene glycol (EG) and its participation during initiation and chain propagation. Furthermore, the microstructure analysis of synthesized polyether polyols reveals that ethylene glycol initiates from both sides and participated in random copolymerization besides its role as an initiator in the polymeric backbone during the ROP of PO. Also, in‐depth studies of the polymeric microstructural distribution are carried out using 1H‐1H COSY, 1H‐13C HMQC, and 1H‐13C HMBC with the aid of multi‐pulse 2D correlation techniques along with carbon‐13 NMR chemical shifts. In‐depth understanding of the microstructure of the polyether polyols is established with the aid of the obtained polymeric regio‐irregular structures. The reactivity ratios of PO are found to be 0.89 and 0.57 by modified Kelen–Tudos and Fineman–Ross method, respectively, indicating higher reactivity of PO monomer in comparison to EG.

A Binary Silicon-Centered Organoboron Catalyst with Superior Performance to That of Its Bifunctional Analogue.

This work reported that a silicon-centered alkyl borane/ammonium salt binary (two-component) catalyst exhibits much higher activity than its bifunctional analogue (one-component) for the ring-opening polymerization of propylene oxide, showing 7.3 times the activity of its bifunctional analogue at a low catalyst loading of 0.01 mol %, and even 15.3 times the activity at an extremely low loading of 0.002 mol %. By using 19 F NMR spectroscopy, control experiments, and theoretical calculation we discovered that the central silicon atom displays appropriate electron density and a larger intramolecular cavity, which is useful to co-activate the monomer and to deliver propagating chains, thus leading to a better intramolecular synergic effect than its bifunctional analogue. A unique two-pathway initiation mode was proposed to explain the unusual high activity of the binary catalytic system. This study breaks the traditional impression of the binary Lewis acid/nucleophilic catalyst with poor activity because of the increase in entropy.

Controlled Oxyanionic Polymerization of Propylene Oxide: Unlocking the Molecular-Weight Limitation by a Soft Nucleophilic Catalysis.

The oxyanionic ring-opening polymerization of propylene oxide (PO) from an exogenous alcohol activated with benign (complexed) metal-alkali carboxylates is described. The equimolar mixture of potassium acetate (KOAc) and 18-crown-6 ether (18C6) is demonstrated to be the complex of choice for preparing poly(propylene oxide) (PPO) in a controlled manner. In the presence of 18C6/KOAc, hydrogen-bonded alcohols act as soft nucleophiles promoting the PO SN 2 process at room temperature and in solvent-free conditions while drastically limiting the occurrence of parasitic hydrogen abstraction generally observed during the anionic ROP of PO. The resulting PPO displays predictable and unprecedented molar masses (up to 20kg mol-1 ) with low dispersities (ĐM < 1.1), rendering the 18C6/KOAc complex the most performing activator for the oxyanionic polymerization of PO reported to date. Preliminary studies on the preparation of block and statistical copolyethers are also reported.

Synthesis and optimization of polypropylene glycol-glycidyl azide polymer-polypropylene glycol as a novel triblock copolymer binder

The novel energetic triblock copolymer of polypropylene glycol-glycidyl azide polymer-polypropylene glycol (PPG-GAP-PPG) ( $$ \bar{M}_{\text{n}} $$ = 1639 g·mol−1) was synthesized and optimized by cationic ring-opening polymerization of propylene oxide using low-molecular-weight glycidyl azide polymer (GAP) ( $$ \bar{M}_{\text{n}} $$ =1127 g·mol−1) as the initiator and boron trifluoride etherate (BF3OEt2) as the catalyst. This research reveals a new method for polymerization of PPG at room temperature and atmospheric pressure. The synthesized GAP and triblock copolymer were characterized by fourier-transform infrared (FT-IR), gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (1H and 13C NMR). Optimization of temperature and amount of catalyst for the synthesis of PPG-GAP-PPG were also performed. The effect of the temperature and the amount of catalyst on the percentage yield of the reaction and the molecular weight of the triblock copolymer of PPG-GAP-PPG was studied. The optimized temperature and catalyst amount for the synthesis of triblock copolymer of PPG-GAP-PPG were 10–15°C and 1% w/w respectively. The thermal stability of the triblock copolymer PPG-GAP-PPG was studied by differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques. The DSC result showed that the glass transition temperature (Tg) of the triblock copolymer (Tg = −63°C) is lower than the neat low molecular weight GAP (Tg = –45°C). Also, the differential thermogravimetry (DTG) result indicated that this triblock copolymer is more stable than the GAP. Synthesis and optimization of the novel energetic triblock copolymer of polypropylene glycol-glycidyl azide polymer-polypropylene glycol (PPG-GAP-PPG) was conducted. The Tg of the triblock copolymer (Tg = −63°C) was lower than the GAP (Tg = –45°C). Also, the DTG result indicated that PPG-GAP-PPG was more stable than the GAP.

N-Heterocyclic carbene/Lewis acid-mediated ring-opening polymerization of propylene oxide. Part 2: Toward dihydroxytelechelic polyethers using triethylborane

Propylene oxide (PO) is polymerized by metal-free ring-opening polymerization (ROP) at 25 °C using N-heterocyclic carbenes (NHCs) and triethylborane (Et3B) as a bicomponent catalytic system. Poly(propylene oxide)s with predictable molar mass up to 60 000 g·mol−1 and low dispersity (Ð < 1.10) were obtained without the occurrence of undesirable transfer reaction to the monomer. In presence of an alcohol as the initiator, the ROP of PO follows an anionic mechanism assisted by monomer activation improving the efficiency of NHCs for the polymerization of substituted epoxides. Et3B is involved both in the formation of a complexed active center and in the activation of PO. Interestingly, dihydroxytelechelic PPOs can be readily synthesized not only using 1,4-benzenedimethanol but also water, both serving as difunctional initiators. Block copolyethers are also prepared by PPO chain extension experiments. All (co)polyethers have been thoroughly characterized by 1H NMR spectroscopy, SEC and MALDI-TOF mass spectrometry as means to elucidate the reaction mechanisms involved in this chemistry.

N-Heterocyclic carbene/Lewis acid-mediated ring-opening polymerization of propylene oxide. Part 1: Triisobutylaluminum as an efficient controlling agent

Ring-opening polymerization (ROP) of propylene oxide (PO) is achieved at 25 °C either in bulk or in solution, using N-heterocyclic carbenes (NHCs) and triisobutylaluminum (i-Bu3Al) as a bicomponent catalytic system. Transfer to monomer was not observed and poly(propylene oxide)s with predictable molar masses up to 60 000 g·mol−1 and low dispersities were obtained. In presence/absence of an alcohol as the initiator, the polymerization of PO follows anionic or zwitterionic ROP mechanisms, respectively. The addition of the Lewis acid strongly improves the efficiency of NHCs for the polymerization of substituted epoxides. It is established that i-Bu3Al is involved both in the formation of an initiating/propagating complex of moderate basicity/nucleophilicity and in the coordination of PO, enabling the activation of the monomer towards the complexed nucleophilic active species. Block copolyethers are also prepared by PPO chain extension experiments. All (co)polyethers were thoroughly characterized by 1H NMR spectroscopy, SEC and MALDI-TOF mass spectrometry as means to prove the control and benefit of this NHC approach for epoxides ROP.

Adherence behavior of 1-poly(propylene oxide)-2-(1-pyrenyl)benzimidazoles on graphene oxide surface

1-Poly(propylene oxide)-2-(1-pyrenyl)benzimidazole (PyrBI-PPO(n), n denotes the degree of polymerization (DP) of poly(propylene oxide) (PPO)) was synthesized by the ring-opening polymerization of propylene oxide (PO) with NH-deprotonated 2-(1-pyrenyl)benzimidazole (PyrBI). The DP of the PPO chain depended on the feed ratio of PO/(NH-deprotonated PyrBI). The solubility of PyrBI-PPO(n) in water depended on the PPO chain length. The reaction of water-soluble PyrBI-PPO(n) (n = 26, 46, and 125) with graphene oxide (GO) in water at various feed ratios of GO/PyrBI-PPO(n) caused the adherence of the pyrenyl group on the GO surface and produced water-soluble composites, namely, PyrBI-PPO(n)-GO. The adherence behavior of PyrBI-PPO(n) on the GO surface was investigated by UV–vis and photoluminescence spectroscopy analyses.

Effect of α-, β-, γ-, and δ-dicarbonyl complexing agents on the double metal cyanide-catalyzed ring-opening polymerization of propylene oxide

Organic complexing agent (CA) plays a crucial role in generating active sites of double metal cyanide (DMC) catalyst in the ring-opening polymerization (ROP) of epoxides. Considering dicarbonyl compounds are versatile and present a great variety of coordination modes besides the usual bidentate behavior of monoanions, the usefulness of the dicarbonyl compounds as CAs for Zn(II)-Co(III) DMC catalysts is investigated. The structural properties of the catalysts were defined by X-ray powder diffraction patterns, elemental analysis, and thermogravimetric analysis. The interaction of the dicarbonyl ligand on the catalyst surface is evaluated by integrating infrared, UV–vis absorption and X-ray photoelectron spectroscopies. The turnover frequencies (calculated by mol-PO per mol-Zn per minute) of the DMC catalysts bearing β-, γ-, and δ-diesters are significantly higher than those of conventional catalysts specifically bearing tert-butyl alcohol as a CA. The resultant polyols exhibit very low levels of unsaturation and narrow polydispersity indexes. The molecular weight and functionality of the resultant polyols can be tuned by modifying the polymerization parameters. Systematic studies on the effect of the new CAs on the catalytic behavior offer great potential for the synthesis of high quality polyols.

Synthesis, thermal stability and kinetic decomposition of triblock copolymer polypropylene glycol–poly glycidyl nitrate–polypropylene glycol (PPG–PGN–PPG)

An energetic triblock copolymer PPG–PGN–PPG (Mn = 1886 g mol−1) was synthesized for the first time by cationic ring-opening polymerization of propylene oxide with low molecular weight poly glycidyl nitrate (PGN) (Mn = 1061 g mol−1) as a macroinitiator, in the presence of boron trifluoride etherate (BF3·OEt2) as the catalyst. The product obtained in high yield was characterized by FTIR, gel permeation chromatography and 1H and 13C NMR spectroscopy. The thermal properties of the triblock copolymer were characterized by differential scanning calorimeter (DSC). The result approved that the glass transition temperature of the triblock copolymer (Tg = − 58 °C) is lower than PGN (Tg = − 35 °C); also, it was more stable than that of PGN. The effect of heating rates (10, 20, 30 and 40 °C min−1) on the decomposition of the copolymer was evaluated. The decomposition temperature of this compound increased as the heating rate increased. The kinetic parameters such as activation energy and frequency factor for the thermal decomposition of the triblock copolymer were obtained from the DSC and DTG data by non-isothermal methods proposed by the ASTM E696, Flynn–Wall–Ozawa (FWO) and Kissinger methods. The values by the FWO method are in good agreement with ASTM and Kissinger methods.

Mechanistic insights on Zn(II)−Co(III) double metal cyanide-catalyzed ring-opening polymerization of epoxides

Electron donating complexing agents (CAs) are one of the key components to achieve active Zn(II)–Co(III) double metal cyanide (DMC) catalysts for the ring-opening polymerization (ROP) of propylene oxide (PO). The effect of the type of CAs on the ROP of PO is mechanistically studied by exploiting a series of dicarbonyl compounds, α-, β-, and γ-diketones and β-ketoesters, as CAs for DMC catalysts. The keto and/or enol modes of complexation of CAs on the catalyst surface is defined by combining structure-sensitive characterization tools such as infrared, absorption and X-ray photoelectron spectroscopies, finding keto complexation exhibits higher activity than enol complexation. Polymerizations of 1,2-epoxyhexane, glycidyl isopropyl ether, epichlorohydrin, and 1,2-epoxytetradecane are also conducted to get further insights on the mechanistic pathways of DMC-catalyzed ROP of epoxides. The interaction of oxygen atoms from both CAs and PO with the dormant sites during the activation of catalyst and the consequent formation of active sites by the removal of the coordinated CAs has been investigated using 1H NMR analysis combined with DFT calculations. Once the active sites are initiated in cationic pathway and started to propagate, the DMC catalysts are fragmented into small pieces, allowing the unexpected high activity of the DMC catalysts. The propagation proceeds in either cationic or coordinative route depending on the presence of polyol initiator. The further understanding of the mechanistic pathways using a series of new dicarbonyl CAs expands the variety and scope of DMC-catalyzed ROP of epoxides.

Phosphoniums as catalysts for metal-free polymerization: Synthesis of well-defined poly(propylene oxide)

The anionic ring-opening polymerization of propylene oxide (PO) was initiated with glycerol and catalyzed by three new synthetic phosphonium salts, tetrakis (pyrrolidino) phosphonium (Py4P1+), tetrakis (piperidino) phosphonium (Pi4P1+), tetrakis (morpholino) phosphonium (Mo4P1+), and the known tetrakis [cyclohexyl (methyl) amino] phosphonium (Cy4P1+) and tetrakis [tris (dimethylamino) phosphonoamino] phosphazene (P5+). The effects of substituents on the polymerization behavior, especially the molecular weight and its distribution, degree of unsaturation, and the sequential structures of poly (propylene oxide) (PPO) were investigated. The structures of these catalysts and PPOs were characterized by FT-IR, 1H and 13C NMR, and GPC. The results indicate that Cy4P1+, Py4P1+, and Pi4P1+ have lower optimum reaction temperatures at 90, 70, and 70 °C, respectively, and are better than traditional catalysts KOH and double metal cyanide. PPO samples with high molecular weight, narrow polydispersity, and high functionality were accessible when catalyzed with Cy4P1+, Pi4P1+, and P5+ at the optimum temperature. Notably, Pi4P1+ formed unimodal distribution PPO with 9000 g/mol, 2.93 of functionality, and 0.008 mmol/g degree of unsaturation. Majority segments of PPO from five catalysts adopted the stereoregular head-to-tail structure, exhibiting excellent regularity.

Grignard-based anionic ring-opening polymerization of propylene oxide activated by triisobutylaluminum

Better known as alkylating agents, Grignard reagents are investigated as deprotonating agents of an alcohol to generate magnesium alkoxides for the initiation and propagation of the anionic ring-opening polymerization of propylene oxide. By using an excess of triisobutylaluminum, this magnesium–aluminum system enables a full conversion polymerization in a few hours yielding controlled poly(propylene oxide) up to 10000g/mol with relatively low dispersity. Characterizations by NMR and MALDI-ToF-mass spectrometry allowed the determination of the chain-ends and therefore the associated initiation mechanisms. This study revealed that propylene oxide can be polymerized in presence of a magnesium-based counter-ion. Concomitant initiations by alkoxide, halide and hydride are discussed.

The synthesis and the bulk rheological properties of the highly-branched block polyethers

Several kinds of highly-branched block polyethers were synthesized via anion ring-opening polymerization of propylene oxide (PO) and ethylene oxide (EO), using phenol-amine resin (PA) as the initiator. The rheological properties determined by rotational rheometer all followed the regular rules of polymer systems: under a certain conditions, the bulk polyethers were pseudoplastic and non-Newtonian fluid, and with the increasing of the shear rate and temperature, the apparent viscosity of the block copolymers were reduced. In addition, modulus determination showed that such polyether molecules presented preferable viscosity compared to the elasticity, meanwhile, storage modulus, loss modulus and compound viscosity all decreased with the increasing of temperature. Storage modulus and loss modulus increased along with the scanning frequency increasing. But compared with the same kind of linear polymers, the significant difference was the low melt viscosity, which attributed to the special three-dimension space structure hindering the entanglement of chains. Furthermore, the rheological properties among the several block polyethers showed differences obviously. In other words, the number of block and the content of EO all have a significant effect on the rheological properties, specifically, the modulus will increase with the increasing of the block number and the EO content.

Cosolvent effect on the aggregation of amphipathic highly branched diblock polyether in aqueous solution

An amphipathic highly branched diblock polyether, PA60-2-40, was synthesized via anion ring-opening polymerization of propylene oxide (PO) and ethylene oxide (EO), using phenol-amine resin (PA) as the initiator. The molecular structure was confirmed by FT-IR and 1H NMR. The surface tension and fluorescence spectroscopy measurements provide information about the effect of cosolvents on the critical micelle concentration (CMC) and the standard Gibbs energy (ΔGomic). The cosolvents have a significant effect on the CMC of PA60-2-40; the CMC first decreases and then increases with increasing content of n-pentanol, but with an increase of ethylene glycol concentration, there is durative decrease of the CMC. The micellization becomes more spontaneous in the presence of cosolvents with more hydroxyl groups, but the CMC does not monotonically increase or decrease with the increasing carbon chain length of aliphatic monohydric alcohols. The values of surface activity parameters (Гmax, Amin and ∆Gomic) calculated all coincide with the results of the experiment. Additionally, TEM and DLS demonstrate that the size of the micelles can be changed by the addition of alcohols, but the aggregation morphology will not change.

Ring-opening polymerization of propylene oxide by double metal cyanide catalysts prepared by reacting CoCl2 with various metal cyanide salts

Polymerization of propylene oxide (PO) has been carried out by using a series of double metal cyanide (DMC) catalysts prepared by reacting dicationic CoCl2 with dianionic K2Ni(CN)4, K4Fe(CN)6, trianionic K3Fe(CN)6, or K3Co(CN)6 in the presence of tert-butyl alcohol as the complexing agent and poly(tetramethylene ether) glycol as the co-complexing agent. The resulting DMC catalysts are characterized by elemental analysis, X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray diffractometry. Screening the catalytic activity in PO polymerization shows that it is dependent on the type of catalyst, relative ratio of cationic cobalt and anionic metal salts, and polymerization conditions. Thus, the DMC catalyst prepared by reacting CoCl2 with K2Ni(CN)4 yielded the highest activity. The polyols produced by DMC catalysts had a very low level of unsaturation. The molecular weight and particularly the polydispersity of the resulting polyols are sensitive to the type of anionic metal salts.

Surface properties and aggregation behaviors of amphiphilic highly-branched block polyethers in aqueous solution

A series of new highly-branched block polyethers, PA-PO-EO, were synthesized via anion ring-opening polymerization of propylene oxide (PO) and ethylene oxide (EO), using phenol-amine resin (PA) as the initiator. The molecular structures were confirmed by end group analysis, FT-IR and 1H NMR. The surface properties and self-assembly behaviors in aqueous solution were investigated by surface tension, fluorescence spectroscopy, UV–vis spectra and TEM. The results indicated that the CMC of polyethers aqueous solution presented a wide range, and the scope of CMC corresponded to the content of EO. Moreover, the proportion of PA had a great effect on the properties of the whole polymer molecules, and the number of blocks was a critical factor to the process of self-assembly. A corresponding critical size of the spherical micelles formed by different samples was presented from TEM photographs. Accordingly, the possible self-assembly mechanisms were put forward to explain the formation of all kinds of aggregates. In a word, the PA and PPO blocks aggregated to form a relatively hydrophobic core, which was stabilized by a hydrophilic PEO corona.

Ring-Opening Polymerization of Epoxides Catalyzed by Uranyl Complexes: An Experimental and Theoretical Study of the Reaction Mechanism

A comprehensive computational study on the ring-opening polymerization of propylene oxide catalyzed by uranyl chloride [UO(2)Cl(2)(THF)(3)] and the uranyl aryloxide [UO(2)(OAr)(2)(THF)(2)] (Ar = 2,6-(t)Bu(2)C(6)H(3)) is reported. The initiation and propagation steps have been probed and significant differences between the two catalysts discovered. The initiation step involving uranyl chloride is an intermolecular process because the orientation of the lone pair on the initiating chloride nucleophile is optimally oriented toward the empty σ*-antibonding orbital of the epoxide, which lowers the activation barrier by 22 kcal mol(-1). Thus, initiation is orbitally controlled. Propagation occurs through a dimeric species, and low-temperature fluorescence spectroscopy has been used to probe this experimentally. In contrast the initiation step for the uranyl aryloxide catalyzed mechanism is intramolecular because of the steric constraints imposed by the bulky substituents on the aryl ring and the fact that the lone pair on the nucleophile is able to approach the propylene oxide coordinated to the same uranium center. Thus, initiation is principally sterically controlled. Propagation is, however, intermolecular, and this can be traced to steric effects. Experimental evidence in the form of fluorescence spectroscopy and diffusion NMR has been used to explore the propagation process in solution.

Combination of phosphazene base and triisobutylaluminum for the rapid synthesis of polyhydroxy telechelic poly(propylene oxide)

Anionic ring-opening polymerization of propylene oxide, initiated with protonated phosphazene alkoxides and based on monomer activation with a Lewis acid, is investigated. Alcohols bearing one or several protected hydroxyl groups, namely 1,2-isopropylideneglycerol, 1,2:3,4-di-O-isopropylidene-D-galactopyranose and poly(propylene glycol), are firstly deprotonated with a phosphazene base. Their combination with triisobutylaluminum enables quantitative and controlled synthesis of poly(propylene oxide) in a very short time (hours), at room temperature and in hydrocarbons. The chain-ends determination showed a main population initiated by the protonated phosphazene alkoxide. A population coming from the residual transfer to monomer is also observed but in very low intensity. Molar masses of 80 000 g mol−1 with relatively narrow dispersity could be prepared. After deprotection of protected hydroxyl functions, tri- and penta-hydroxy telechelic poly(propylene oxide) were obtained. Dihydroxy telechelic polymers of high molar masses are also rapidly and directly synthesized from poly(propylene glycol).

Preparation of a Certified Reference Material of Trifunctional Polypropylene Oxide

Due to the lack of trifunctional polypropylene oxide (TPPO) certified reference material (CRM) with precise molecular weight on the market, it is therefore very important to prepare such CRM to calibrate the molecular weight testing equipments, and assure the quality in polyurethanes manufacture. In this paper, the certification of TPPO CRM was introduced based on the size-exclusion chromatography with a multi-angle laser light scattering (SEC-MALLS) method and end-group analysis method. First, TPPO was synthesized via controlled ring-opening polymerization of propylene oxide in the condition of both high temperature and pressure using double metal cyanide (DMC) complexes as catalyst, and TPPO oligomer as coinitiator. Then, the molecular weight homogeneity of TPPO CRM was evaluated by using the F-distribution model, and the storage stability of the molecular weight of TPPO CRM was assessed. All potential uncertainty factors for the certification of TPPO CRM were evaluated using cause-effect diagram. The results showed that the chemical structure of TPPO CRM was an anticipatory structure, which was confirmed by IR. The molecular weight homogeneity of TPPO CRM was qualified. The storage stability period of the final product was one year with respect to its molecular weight, and the certification results of TPPO CRM obtained by SEC-MALLS and end-group analysis were 4996±162 and 5025±146, respectively.

Calcium chloride doped zinc-cobalt metal-cyanide complex: Unexpected highly activity towards ring-opening polymerization of propylene oxide

Highly active calcium chloride (CaCl 2) doped Zn-Co III double metal-cyanide (Ca-DMC) catalysts were firstly reported. Ca-DMCs presented a very higher polymer yield (>54 kg polymer/g catalyst) at relative low temperature (80–115 °C) toward ring-opening polymerization (ROP) of propylene oxide (PO) than did DMC catalysts without modification.