Ring-expansion Polymerization Research Articles

Smart and Efficient Synthesis of Cyclic Poly(N-isopropylacrylamide)s by Ring Expansion RAFT (RE-RAFT) Polymerization and Analysis of Their Unique Temperature-Responsive Properties.

Cyclic polymers have many interesting properties compared to their linear analogs, but there are very few examples of their synthesis. This is because most cyclic polymers have been synthesized by stepwise processes, including synthesizing homo- or hetero-telechelic end-functionalized precursor polymers and consecutive intramolecularly coupling of both ends of the polymers. This requires a complicated synthesis, and the product yields are very low because the target cyclic polymers are usually synthesized under highly dilute conditions, consequently, making it difficult to systematically analyze the properties of cyclic polymers. In the present research, we have synthesized cyclic polymers using a ring expansion polymerization method. Particularly, the ring expansion RAFT polymerization (RE-RAFT polymerization) that we have developed using a cyclic chain transfer agent is a smart method that can synthesize cyclic polymers very efficiently. In this paper, we successfully synthesized cyclic-poly(N-isopropylacrylamide), which is widely known as a thermo-responsive polymer, by RE-RAFT polymerization. Furthermore, we have compared the thermo-responsive properties of the cyclic-poly(N-isopropylacrylamide)s with those of their linear analogs.

Zwitterionic ring expansion polymerization of tert‐butyl glycidyl ether with B(C6F5)3 towards the generation of cyclic chains

AbstractThe synthesis of cyclic polymers via zwitterionic ring expansion polymerization is limited to a few number of monomer and catalyst pairs. Herein we report the synthesis of cyclic poly(tert‐butyl glycidyl ether) through the polymerization of tert‐butyl glycidyl ether (tBGE) with B(C6F5)3 in different reaction conditions that include different solvents, monomer to initiator ratio, monomer concentration and temperature. We found that bimodal molecular weight distribution is formed in almost all reaction conditions caused by cyclization of short chains. Subsequent chain elongation leads to the formation of cycles of higher molecular weight, particularly in cyclohexane and under bulk conditions. The formation of non‐cyclic byproducts is common in all the systems investigated. Low molecular weight cyclic chains (Mn = 0.7 kDa, Ð = 1.1) of high topological purity were successfully isolated by preparative gel permeation chromatography. By using a click scavenging protocol, the non‐cyclic byproducts were eliminated from the high molecular weight fraction (Mn = 3 kDa, Ð = 1.3) generating pure cyclic chains. Mechanistic investigation using density functional theory calculations was performed on the formation of zwitterionic intermediates and the transfer reaction to the monomer, which notably affects chain growth by the attack of the glycidyl oxygen of the monomer on the growing chain.

Cyclic polyglycolides via ring-expansion polymerization with cyclic tin catalysts

Glycolide was polymerized in bulk with two cyclic catalysts − 2,2-dibutyl-2-stanna-1,3-dithiolane (DSTL) and 2-stanna-1,3-dioxa-4,5,6,7-dibenzepane (SnBiph). The monomer/initiator ratio, temperature (140 – 180 °C) and time (1–––4 days) were varied. The MALDI TOF mass spectra exclusively displayed peaks of cyclic polyglycolide (PGA) and revealed an unusual “saw-tooth pattern” in the mass range below m/z 2 500 suggesting formation of extended ring crystallites. The DSC measurements indicated increasing crystallinity with higher temperature and longer time, and after annealing for 4 d at 160 °C a hitherto unknown and unexpected glass transition was found in the temperature range of 170–185 °C. Linear PGAs prepared by means of metal alkoxides under identical conditions did not show the afore-mentioned features of the cyclic PGAs, neither in the mass spectra nor in the DSC measurements. All PGAs were also characterized by SAXS measurements, which revealed relatively small L-values suggesting formation of thin crystallites in all cases with little influence of the reaction conditions.

Ring-ring equilibration (RRE) of cyclic poly(L-lactide)s by means of cyclic tin catalysts

With 2,2-dibutyl-2-stanna-1,3-dithiolane (DSTL) and 2-stanna-1,3-dioxa-4,5,6,7-dibenzoxepane (SnBiPh) as catalysts ring-expansion polymerizations (REP) were performed either in 2 M solution using three different solvents and two different temperatures or in bulk at 140 and 120 °C. A kinetically controlled rapid REP up to weight average molecular masses (Mẃs) above 300 000 was followed by a slower degradation of the molecular masses at 140 °C, but not at 120 °C Furthermore, a low molecular mass cyclic poly(L-lactide) (cPLA) with a Mn around 16 000 was prepared by polymerization in dilute solution and used as starting material for ring-ring equilibration at 140 °C in 2 M solutions. Again, a decrease of the molecular mass was detectable, suggesting that the equilibrium Mn is below 5 000. The degradation of the molecular masses via RRE was surprisingly more effective in solid cyclic PLA than in solution, and a specific transesterification mechanism involving loops on the surface of crystallites is proposed. This degradation favored the formation of extended-ring crystallites, which were detectable by a “saw-tooth pattern” in their MALDI mass spectra.

Snapshot of cyclooctyne ring-opening to a tethered alkylidene cyclic polymer catalyst.

Cyclooctyne reacts with the trianionic pincer ligand supported alkylidyne [ t BuOCO]WCC(CH3)3(THF)2 (1) to yield tungstacyclopropene (3) and tungstacyclopentadiene (4) complexes. The ratio of 3 and 4 in the reaction mixture depends on the stoichiometry of the reaction. The maximum concentration of 3 occurs with one equiv. of cyclooctyne and 4 is the exclusive product of the reaction above three equivalents. Both complexes 3 and 4 convert to the cyclooctyne ring-opened product 5 upon heating. While the conversion of 4 to 5 is accompanied by formation of polycyclooctyne as a white precipitate during the reaction, conversion of 3 to 5 is homogeneous. Exhibiting Ring Expansion Polymerization (REP), complexes 4 and 5 initiate the polymerization of phenylacetylene to generate cyclic poly(phenylacetylene) (c-PPA).

Cyclic and linear poly(L‐lactide)s by ring‐expansion polymerization with Bu2SnO, Oct2SnO, and Bu2SnS

AbstractTo elucidate the usefulness of commercial Bu2SnO as catalyst for syntheses of linear and/or cyclic polylactides l‐Lactide was polymerized in bulk with variation of the LA/Cat ratio, temperature and time. Based on a ring‐expansion polymerization (REP) mechanism cyclic polylactides (PLAs) with weight average molecular weights in the range of 200,000–300,000 are obtained at the highest temperature (180 °C). Polymerizations at 150 °C yielded crystalline, mainly cyclic polylactides which, after annealing, showed high melting temperatures (up to 197.5 °C) and high crystallinities (>80%). Polymerization at 120 °C confirmed the trend towards more linear chains with lower temperatures but yielded extended‐ring crystals showing a “saw‐tooth pattern” in the mass spectra. Oct2SnO gave similar results as Bu2SnO. Bu2SnS proved a sluggish polymerization catalyst, but a good transesterification catalyst in solid PLA.

Facile Synthesis of Linear and Cyclic Poly(diphenylacetylene)s by Molybdenum and Tungsten Catalysis.

Improved methods for the synthesis of linear and cyclic poly(diphenylacetylene)s by polymerization of the corresponding diphenylacetylenes using MoCl5 - and WCl4 -based catalytic systems have been developed. MoCl5 induces migratory insertion polymerization of diphenylacetylenes in the presence of arylation reagents such as Ph4 Sn and ArSnn Bu3 to produce cis-stereoregular linear poly(diphenylacetyelene)s with high molecular weights (number-average molar mass (Mn )=30,000-3,200,000) in good yields (up to 98 %). On the other hand, WCl4 induces ring expansion polymerization of diphenylacetylenes in the presence of Ph4 Sn or reducing reagents to produce cis-stereoregular cyclic poly(diphenylacetylene)s with high molecular weights (Mn =20,000-250,000) in moderate to good yields (up to 90 %). Both catalytic systems are applicable to the polymerization of various diphenylacetylenes having polar functional groups such as esters that are not efficiently polymerized by conventional methods using WCl6 -Ph4 Sn and TaCl5 -n Bu4 Sn systems.

Facile Synthesis of Linear and Cyclic Poly(diphenylacetylene)s by Molybdenum and Tungsten Catalysis

AbstractImproved methods for the synthesis of linear and cyclic poly(diphenylacetylene)s by polymerization of the corresponding diphenylacetylenes using MoCl5‐ and WCl4‐based catalytic systems have been developed. MoCl5 induces migratory insertion polymerization of diphenylacetylenes in the presence of arylation reagents such as Ph4Sn and ArSnnBu3 to produce cis‐stereoregular linear poly(diphenylacetyelene)s with high molecular weights (number‐average molar mass (Mn)=30,000–3,200,000) in good yields (up to 98 %). On the other hand, WCl4 induces ring expansion polymerization of diphenylacetylenes in the presence of Ph4Sn or reducing reagents to produce cis‐stereoregular cyclic poly(diphenylacetylene)s with high molecular weights (Mn=20,000–250,000) in moderate to good yields (up to 90 %). Both catalytic systems are applicable to the polymerization of various diphenylacetylenes having polar functional groups such as esters that are not efficiently polymerized by conventional methods using WCl6‐Ph4Sn and TaCl5‐nBu4Sn systems.

Controlled synthesis of cyclic helical polyisocyanides and bottlebrush polymers using a cyclic alkyne-Pd(II) catalyst.

Cyclic polymers have very unique structure and properties, and thus have drawn intense research attention. However, controlled synthesis of cyclic polymers with predictable molar mass and narrow distribution is still a challenging task. In this study, we developed a novel cyclic catalyst that initiates the ring-expansion polymerisation of isocyanides, producing a series of cyclic helical polymers with predictable molecular weight and low dispersity. Interestingly, the ring-expansion polymerization of the isocyanide macromonomers gives well-defined cyclic bottlebrush polymers. The cyclic topology was demonstrated using transmission electron microscopy.

About the Influence of (Non‐)Solvents on the Ring Expansion Polymerization of l‐Lactide and the Formation of Extended Ring Crystals

AbstractRing‐expansion polymerizations (REPs) catalyzed by two cyclic tin catalysts (2‐stanna‐1.3‐dioxa‐4,5,6,7‐dibenzazepine [SnBiph] and 2,2‐dibutyl‐2‐stanna‐1,3‐dithiolane [DSTL]) are performed at 140 °C in bulk. Small amounts (4 vol%) of chlorobenzene or other solvents are added to facilitate transesterification reactions (ring–ring equilibration) in the solid poly(l‐lactide)s. In the mass range up to m/z 13 000 crystalline PLAs displaying a so‐called saw‐tooth pattern in the MALDI‐TOF mass spectra are obtained indicating the formation of extended‐ring crystals. The characteristics of extended‐ring crystallites and folded‐ring crystallites are discussed. Furthermore, extremely high melting temperatures (Tm's up to 201.2 °C) and melting enthalpies (ΔHm's up to 106 J g−1)) are found confirming that ΔHmmax, the ΔHm of a perfect crystal, is around or above 115 J g−1 in contrast to literature data.

Ring-expansion polymerization (REP) of L-lactide with cyclic tin catalysts – About formation of extended ring crystals and optimization of Tm and ΔHm

L-Lactide was polymerized in bulk at 140 °C with three different cyclic tin catalysts and the time was varied from 1 d up to 14 d. The MALDI TOF spectra confirmed the formation of cyclic polylactides (PLAs) and displayed a characteristic change of peak intensity distribution with formation of a “saw tooth pattern”. This pattern confirms a previous hypothesis that cyclic PLAs tend to form crystallites with extended ring conformation and relatively smooth surface. This type of crystallites is formed under thermodynamic control by transesterification on the surface of the crystallites. In this way PLAs with extraordinarily high melting temperatures (Tm's up to 200.6 °C) and extraordinarily high melting enthalpy were obtained (ΔHm's up to 105 J g−1). These ΔHm values require a revision of the maximum ΔHm value calculated in the literature for ideal PLA crystals.

Cyclic Polycarbonates by N-Heterocyclic Carbene-Mediated Ring-Expansion Polymerization and Their Selective Depolymerization to Monomers

Cyclic Polycarbonates by N-Heterocyclic Carbene-Mediated Ring-Expansion Polymerization and Their Selective Depolymerization to Monomers

Nanobowls from Amphiphilic Core–Shell Cyclic Bottlebrush Polymers

Cyclic bottlebrush polymers are macrocycles that contain pendent side-chain polymers on nearly every repeat unit. We leveraged a method to generate cyclic bottlebrush polymers utilizing ring-expansion polymerization (REP) followed by subsequent grafting-from via atom-transfer radical polymerization (ATRP) to produce core–shell cyclic bottlebrush polymers and investigated their self-assembly behavior in water. The bottlebrush polymers were comprised of a cyclic backbone with side chains composed of a hydrophobic polystyrene block and a hydrophilic poly(acrylic acid) block. A two-step morphological transformation from spheres to porous spheres to nanobowls (spheres with a single large pore) was observed with solvent exchange from tetrahydrofuran to water. In contrast, under identical experimental conditions, an analogous linear bottlebrush polymer with a similar backbone and side chain degrees of polymerizations did not aggregate into nanobowls but rather exhibited a porous sphere morphology. These unusual morphologies and their propensities to transform can be attributed to changes in the internal viscosity of the aggregates during the solvent exchange process. Additionally, the rate of solvent exchange was found to influence the propensity for shape transformation to occur.

A Supramolecular Strategy for the Synthesis of Cyclic Oligomers and Polymers by Ring Expansion.

Ring-opening metathesis polymerization (ROMP) of strained macrocycles is a key method to prepare diverse polymers. However, lack of ring strain in most macrocycles is an impediment to polymerization. In this paper, the polymerization/oligomerization of unstrained macrocycles was achieved using a supramolecular approach, leading selectively to cyclic products. Diphenyl thiourea and other guest molecules were used as additives to the ROMP reaction of unstrained macrocycles. An intermediate host-guest complex leads to the stabilization of the open form of the macrocycle after treatment with Grubbs catalysts, thereby favoring polymerization by inhibiting the ring-closing reaction back to the monomer. This proof-of-concept enables ring-expansion polymerization of unstrained macrocycles leading to cyclic polymers with molecular weights up to 6700 Da.

Controlled Synthesis of Cyclic-Helical Polymers with Circularly Polarized Luminescence.

Cyclic polymers attract attention because of their endless structure and unique properties, which differ from the linear analogs. However, the synthesis of cyclic polymers is difficult and prohibits their functions and applications. In this study, we reported chiral cyclic PdII -catalysts that initiate a living ring-expansion polymerization of isocyanides, yielding a single-handed cyclic-helical poly(phenyl isocyanide), with predictable molecular weight (Mn ) and low dispersity (Mw /Mn ), in good yield. Using this method, cyclic bottlebrush polymers were prepared via the grafting-onto strategy. The cyclic topology was confirmed using various spectroscopic data and atomic force microscope observation. Moreover, the cyclic polymer brushes, comprising of a one-handed helical backbone, showed interesting photoluminescence and circularly-polarized luminescence.

Ring‐Expansion Polymerization of ε‐Caprolactone, Glycolide, and l‐lactide with a Spirocyclic Tin(IV) Catalyst Derived from or 2,2′‐Dihydroxy‐1,1′‐Binaphthyl – New Results and a Revision

AbstractIn contrast to other cyclic tin bisphenoxides, polymerizations of glycolide and l‐lactide with the spirocyclic tin(IV) bis‐1,1′‐bisnapthoxide yield linear chains having a 1,1′‐bisnapthol end group and no cycles. In the case of l‐lactide, LA/Cat ratio and temperature are varied and at 160 °C or below, all polylactides mainly consist of even‐numbered chains. A total predominance of even‐numbered chains is also found for homopolymerization of glycolide, or the copolymerization of glycolide and l‐lactide, when conducted <120 °C. Linear chains having a bisnaphthol end group are again the main reaction products of ring‐expansion polymerizations (REP) of ε‐caprolactone, but above 150 °C cycles are also formed.

Synthetic Strategy for Mechanically Interlocked Cyclic Polymers via the Ring-Expansion Polymerization of Macrocycles with a Bis(hindered amino)disulfide Linker

Synthetic Strategy for Mechanically Interlocked Cyclic Polymers via the Ring-Expansion Polymerization of Macrocycles with a Bis(hindered amino)disulfide Linker

Synthesis, Properties, and Applications of Bio-Based Cyclic Aliphatic Polyesters.

Cyclic polymers have long been reported in the literature, but their development has often been stunted by synthetic difficulties such as the presence of linear contaminants. Research into the synthesis of these polymers has made great progress in the past decade, and this review covers the synthesis, properties, and applications of cyclic polymers, with an emphasis on bio-based aliphatic polyesters. Synthetic routes to cyclic polymers synthesized from bioderived monomers, alongside mechanistic descriptions for both ring closure and ring expansion polymerization approaches, are reviewed. The review also highlights some of the unique physical properties of cyclic polymers together with potential applications. The findings illustrate the substantial recent developments made in the syntheses of cyclic polymers, as well as the progress which can be made in the commercialization of bio-based polymers through the versatility this topology provides.

Cyclic Poly(α-peptoid)s by Lithium bis(trimethylsilyl)amide (LiHMDS)-Mediated Ring-Expansion Polymerization: Simple Access to Bioactive Backbones.

Cyclic polymers display unique physicochemical and biological properties. However, their development is often limited by their challenging preparation. In this work, we present a simple route to cyclic poly(α-peptoids) from N-alkylated-N-carboxyanhydrides (NNCA) using LiHMDS promoted ring-expansion polymerization (REP) in DMF. This new method allows the unprecedented use of lysine-like monomers in REP to design bioactive macrocycles bearing pharmaceutical potential against Clostridioides difficile, a bacterium responsible for nosocomial infections.

Thioacyl-Transfer Ring-Expansion Polymerization of Thiiranes Based on a Cyclic Dithiocarbamate Initiator

We report the novel thioacyl-transfer ring-expansion polymerization (REP) of thiiranes, using 3H-benzothiazol-2-thione (BTT) as the cyclic dithiocarbamate initiator and tetrabutylammonium chloride (TBAC) as the catalyst. REP proceeded in a controlled manner to produce cyclic polysulfides with a narrow Mw/Mn of 1.1 and whose cyclic structures were confirmed by MALDI-TOF mass spectrometry. Time-course studies revealed unique sigmoidal profiles due to the stable five-membered cyclic structure of the BTT initiator. The GPC curves were analyzed by the Gaussian fitting method, which revealed that the cyclic polymers are accompanied by another fraction with nearly twice the molecular weight of the main product due to ring-crossover reactions between cyclic polysulfides during REP. The BTT-initiated cyclic polysulfides produced in this manner act as macroinitiators for the REP of thiiranes, highlighting the living characteristics of the BTT-initiated REP developed herein. Post-polymerization further enabled the preparation of cyclic block copolymers with different combinations of thiiranes.