Microwave‐Assisted Polymer Synthesis: State‐of‐the‐Art and Future Perspectives

Summary: Monomodal microwaves have overcome the safety uncertainties associated with the precedent domestic microwave ovens. After fast acceptance in inorganic and organic syntheses, polymer chemists have also recently discovered this new kind of microwave reactor. An almost exponential increase of the number of publications in this field reflects the steadily growing interest in the use of microwave irradiation for polymerizations. This review introduces the microwave systems and their applications in polymer syntheses, covering step‐growth and ring‐opening, as well as radical polymerization processes, in order to summarize the hitherto realized polymerizations. Special attention is paid to the differences between microwave‐assisted and conventional heating as well as the “microwave effects”.

Results of search on number of publications on microwave‐assisted polymerizations, sorted by year.     

Microwave‐Assisted Single‐Step Synthesis of Poly(alkylene hydrogen phosphonate)s by Transesterification of Dimethyl Hydrogen Phosphonate with Poly(ethylene glycol)

Summary: Poly(alkylene hydrogen phosphonate)s with a number‐average molecular weight of about 3 000 Da were obtained by a transesterification of dimethyl hydrogen phosphonate with poly(ethylene glycol) (PEG 400) under microwave irradiation with a very short reaction time (55 min) relative to that of classical thermal heating (9 h). The structure of the resulting polymer was confirmed by ¹H, ³¹P, and ¹³C NMR spectroscopy. The molecular weight was determined by ¹H, ³¹P{H} NMR spectroscopy, MALDI‐TOF, and GPC.

The transesterification of dimethyl hydrogen phosphonate with poly(ethylene glycol).     

Microwave‐Assisted Synthesis of Crosslinked Poly(vinyl alcohol) Nanocomposites Comprising Single‐Walled Carbon Nanotubes,Multi‐Walled Carbon Nanotubes,and Buckminsterfullerene

We report a facile method to accomplish the crosslinking reaction of PVA with SWNTs, MWNTs, and C‐60 using MW irradiation. Nanocomposites of PVA crosslinked with SWNT, MWNT and C‐60 were prepared expeditiously by reacting the respective carbon nanotubes with 3 wt.‐% PVA under MW irradiation, maintaining a temperature of 100 °C, representing a radical improvement over literature methods to prepare such crosslinked PVA composites. This general preparative procedure is versatile and provides a simple route to manufacture useful SWNT, MWNT and C‐60 nanocomposites.

     

Microwave‐Assisted Solid‐Phase Synthesis of Antifreeze Glycopeptides

Microwave‐assisted solid‐phase synthesis allows for the rapid and large‐scale preparation and structure–activity characterization of tandem repeating glycopeptides, namely monodispersed synthetic antifreeze glycopeptides (syAFGPs, H‐[Ala‐Thr(Galβ1,3GalNAcα1→)‐Ala]_n‐OH, n=2–6). By employing novel AFGP analogues, we have demonstrated that of the monodispersed syAFGP_n (n=2–6, degree of polymerization, DP=2–6, M_w=1257–3690 Da), syAFGP₅ (DP=5, M_w=3082 Da) and syAFGP₆ (DP=6, M_w=3690 Da) exhibit the ability to form typical hexagonal bipyramidal ice crystals and satisfactory thermal hysteresis activity. Structural characterization by NMR and CD spectroscopy revealed that syAFGP₆ forms a typical poly‐L ‐proline type II helix‐like structure in aqueous solution whereas enzymatic modification by sialic acid of the residues at the C‐3 positions of the nonreducing Gal residues disturbs this conformation and eliminates the antifreeze activity.     

新型手性聚（甲基）丙烯酸酯类聚合物的合成与表征

手性聚丙烯酸酯;新型手性聚（甲基）丙烯酸酯类聚合物的合成与表征     

Facile Microwave‐Assisted Solid‐Phase Synthesis of Highly Fluorescent Nitrogen–Sulfur‐Codoped Carbon Quantum Dots for Cellular Imaging Applications

Carbon quantum dots (CQDs) have recently attracted significant attention for both their fundamental science and technological applications as a new class of fluorescent zero‐dimensional nanomaterials with a size below 10 nm. However, the reported methods of synthesis were generally less suitable for the large‐scale production of the CQDs with high‐fluorescent quantum yield (QY). In the paper, a novel one‐pot microwave‐assisted drying synthesis approach was presented to prepare CQDs with high QY of 61.3 % for the first time. The production yield of CQDs was 35±3 % in weight. The as‐prepared CQDs were characterized by various techniques such as TEM, AFM, XRD, XPS, FTIR spectroscopy, UV/Vis absorption spectroscopy, and fluorescence spectroscopy. The results showed that the high QY of CQDs was largely attributed to the dual doping of nitrogen and sulphur into CQDs. Such CQDs were then used as live‐cell imaging reagents due to their high QY, good water dispersibility, fine biocompatibility, high photostability, and low cytotoxicity.     

Green Polymer Chemistry VIII: Synthesis of Halo‐ester‐Functionalized Poly(ethylene glycol)s via Enzymatic Catalysis

Halo‐ester‐functionalized poly(ethylene glycol)s (PEGs) are successfully prepared by the transesterification of alkyl halo‐esters with PEGs using Candida antarctica lipase B (CALB) as a biocatalyst under the solventless conditions. Transesterifications of chlorine, bromine, and iodine esters with tetraethylene glycol monobenzyl ether (BzTEG) are quantitative in less than 2.5 h. The transesterification of halo‐esters with PEGs are complete in 4 h. ¹H and ¹³C NMR spectroscopy with MALDI‐ToF and ESI mass spectrometry confirm the structure and purity of the products. This method provides a convenient and “green” process to effectively produce halo‐ester PEGs.

     

Microwave‐Assisted Aqueous Synthesis: A Rapid Approach to Poly‐functionalized Indoline‐Spiro Benzofurodiazepines

New cycloaddition has been established for the synthesis of indoline‐spiro benzofurodiazepine derivatives. Those reactions were conducted by reacting readily available and inexpensive starting materials, such as benzene‐1,2‐diamines, tetronic acid and indoline‐2,3‐diones, in aqueous solution under microwave irradiation. When mono‐substituted benzene‐1,2‐diamines as an amine component was employed, the reaction regioselectively resulted in the poly‐functionalized indoline‐spiro benzofurodiazepine with good yields. The present green synthesis shows attractive characteristics such as the use of water as reaction media, concise one‐pot conditions, short reaction periods, easy work‐up/purification and reduced waste production without the use of any strong acids or metal promoters.     

无溶剂条件下利用微波促进下的多组分反应对2,6-二芳基-4-苯乙烯基吡啶化合物进行非预期的绿色简易合成

在无溶剂条件下,通过微波促进下的3-芳基丙烯醛肟、1-芳基乙酮及醋酸铵的多组分反应,非预期地实现了2,6-二芳基-4-苯乙烯基吡啶化合物的绿色简易合成。该方法具有环境友好、反应时间短、产率高、成本低、操作简便以及适用范围广等优点。     

Single‐Step Microwave‐Mediated Synthesis of Oxazoles and Thiazoles from 3‐Oxetanone: A Synthetic and Computational Study

The direct microwave‐mediated condensation between 3‐oxetanone and primary amides and thioamides has delivered moderate to good yields of (hydroxymethyl)oxazoles and (hydroxymethyl)thiazoles. The reactions use a sustainable solvent and only require short reaction times. These are highly competitive methods for the construction of two classes of valuable heteroarenes, which bear a useful locus for further elaboration. Electronic structure calculations have shown that the order of events involves chalcogen atom attack at sp³ carbon and alkyl–oxygen cleavage. The critical role of acid catalysis was shown clearly, and the importance of acid strength was demonstrated. The calculated barriers were also fully consistent with the observed order of thioamide and amide reactivity. Spontaneous ring opening involves a modest degree of C? O cleavage, moderating the extent of strain relief. On the acid‐catalysed pathway, C? O cleavage is less extensive still, but proton transfer to the nucleofuge is well advanced with the carboxylic acid catalysts, and essentially complete with methanesulfonic acid.     

一种以水相中微波促进下的多组分反应合成具有重要生物意义的茚并[2,1-e]吡唑并[5,4-b]吡啶的绿色方法

在无催化剂的条件下,以醛、3-甲基-1-苯基-1H-吡唑-5-胺和1,3-茚二酮为原料,通过水相中微波促进下的多组分反应,成功地实现了具有重要生物意义的茚并[2,1-e]吡唑并[5,4-b]吡啶的绿色合成。这种方法具有环境友好、反应时间短、产率高、价廉、操作简单以及广泛的适用范围等显著优点。     

Optimization of Focused Microwave‐Assisted Extraction of DDT and Derivatives from Soil Samples

Optimization of focused microwave (FMW)‐assisted extraction of 4,4′‐DDE, 4,4′‐DDD, and 4,4′‐DDT from soil samples was carried out using central composite designs. All the extracts were analyzed with GC/MS and some of them also with GC/AED using columns of different polarities for each of the techniques. The extraction of the analytes was carried out in two ways: with acetic acid as microwave radiation absorbent solvent and n‐decane to concentrate the analytes and with reagent water and iso‐octane as solvents. In the first case, the influence of the extraction temperature, the extraction time, and the addition of sodium chloride were studied and the optimum conditions for the extraction of 1 g of soil with 5 mL of acetic acid and 2 mL of n‐decane were 1.30 mol L^–1 sodium chloride at 98°C for 9.3 min. In the second case, the temperature was kept constant (94°C) and the influence of the concentration of sodium chloride and the extraction time were studied. The optimum conditions were 5 mL of a 2.0 mol L^–1 sodium chloride together with 2 mL of iso‐octane for 15 min. The recoveries obtained by water‐FMW extraction were significantly lower than those obtained by the acetic acid‐FMW procedure. These last recoveries were in good agreement with those obtained by closed microwave assisted‐extraction with acetone‐n‐hexane as solvent.     

Scale‐up of Microwave‐Assisted Polymerizations in Batch Mode: The Cationic Ring‐Opening Polymerization of 2‐Ethyl‐2‐oxazoline

Summary: The use of microwave heating in polymer science is a rapidly growing field of research leading to faster and cleaner polymerization procedures. However, the majority of the investigations are performed at small scales (≈1 mL), which is far away from potential commercial applications of microwave‐assisted polymerizations. In addition, it has been shown in organic chemistry that microwave‐assisted reaction protocols can be directly scaled without the need for process optimization. In this contribution, we have investigated the direct scaling of microwave‐assisted polymerization procedures under pressure conditions using the cationic ring‐opening polymerization of 2‐ethyl‐2‐oxazoline as the model system. This polymerization was performed at scales ranging from 4.0 mmol (1 mL) to 1.0 mol (250 mL) in different microwave synthesizers covering both monomode and multimode devices.

Scale‐up of microwave‐assisted polymerizations in batch mode: the cationic ring‐opening polymerization of 2 ethyl‐2‐oxazoline.     

Synthesis of Poly(2‐oxazoline)‐Based Hydrogels with Tailor‐Made Swelling Degrees Capable of Stimuli‐Triggered Compound Release

A 32‐membered library of poly(2‐oxazoline)‐based hydrogels of the composition p EtOx _m‐p PhOx _n‐p PBO _q (m/n = 150/0, 100/50, 50/100, and 0/150; q = 1.5–30) was prepared from 2‐ethyl‐ ( EtOx ), 2‐phenyl‐2‐oxazoline ( PhOx ), and phenylene‐1,3‐bis‐(2‐oxazoline) ( PBO ). The polymerizations were performed from ground monomer mixtures at 140 °C in a single‐mode microwave reactor in reaction times as short as 1 h. Purified hydrogels, containing no residual monomers, were obtained in yields of 95% or higher. Acid‐mediated hydrolysis rates as well as swelling degrees of the hydrogels were adjustable over a broad range; swelling degrees in water/ethanol/dichloromethane ranged from 0 to 13.8/11.7/20.0. The hydrogels could incorporate organic molecules according to in situ or post‐synthetic routines. Post‐synthetic routines enabled for the preparation of hydrogels from which the incorporated compounds were only released through diffusion processes if the solvent was changed or through hydrogel degradation if the pH was lowered.     

Microwave‐Assisted Ring‐Opening Polymerization of ε‐Caprolactone in the Presence of Ionic Liquid

Summary: Microwave‐assisted ring‐opening polymerization of ε‐caprolactone in the presence of 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid using zinc oxide as a catalyst is investigated. By adding 30 wt.‐% ionic liquid, poly(ε‐caprolactone) with a weight‐average molar mass of 28 500 g · mol⁻¹ is obtained at 85 W for 30 min. The results indicate that the polymerization could be efficiently enhanced in the presence of ionic liquids under microwave irradiation because ionic liquids can effectively absorb microwave energy.

     

Branching by reactive end groups. II. Synthesis,branching, and melt rheology of (meth)acrylate/p‐t‐butylphenol‐coterminated bisphenol a polycarbonates

(Meth)acrylate/p‐t‐butylphenol (PTBP)‐coterminated bisphenol A polycarbonates (PCs) were prepared by interfacial processes and subsequently were reacted at high temperatures (≥200 °C) to form new branched polymers. Two interfacial methods were used to prepare the precursor linear PCs, one with (meth)acryloyl chloride [(M)AC] and the other with (meth)acrylic acid [(M)AA]. Both processes involve phosgenation in the presence of catalytic amounts of triethylamine. The process that used (M)AC formed disproportionately large amounts of bisphenol A di(meth)acrylate, whereas the process using (M)AA required about 50% more phosgene to achieve high (M)AA conversions than typical interfacial PC processes. The branching of the acrylate/PTBP PCs occurred with heating at temperatures greater than or equal to 250 °C. The molecular weight and degree of branching depended on the mole ratio of the thermally reactive and nonreactive coterminators, the total amount of coterminators, and the reaction conditions. The functionality of the branch points formed appeared to be dependent on the acrylate concentration. The branching of the methacrylate/PTBP PCs required the presence of a free‐radical initiator and temperatures up to about 200 °C. The methacrylate end group was less effective than the acrylate on a molar basis in increasing the branched polymer molecular weight and degree of branching. The melt rheology of the branched acrylate/PTBP PCs showed the expected increase in low shear viscosity and shear rate sensitivity with increasing weight‐average molecular weight and acrylate‐end‐group concentration. Small changes in the total terminator concentration and, therefore, the linear precursor polymer molecular weight produced large effects in the low shear rate melt viscosity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2340–2351, 2000     

Chemistry of 2‐oxazolines: A crossing of cationic ring‐opening polymerization and enzymatic ring‐opening polyaddition

Chemistry of 2‐oxazolines is involved in the polymer synthesis fields of cationic ring‐opening polymerization (CROP) and enzymatic ring‐opening polyaddition (EROPA), although both polymerizations look like a quite different class of reaction. The key for the polymerization to proceed is combination of the catalyst (initiator) and the design of monomers. This article describes recent developments in polymer synthesis via these two kinds of polymerizations to afford various functional polymers having completely different structures, poly(N‐acylethylenimine)s via CROP and 2‐amino‐2‐deoxy sugar unit‐containing oligo and polysaccharides via EROPA, respectively. From the viewpoint of reaction mode, an acid‐catalyzed ring‐opening polyaddition (ROPA) is considered to be a crossing where CROP and EROPA meet. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1251–1270, 2010