Cyclic Poly Research Articles

Direct observation of the formation of a cyclic poly(alkyl sorbate) via chain‐growth polymerization by an N‐heterocyclic carbene initiator and ring‐closing without extreme dilution

Abstract1,3‐Di‐tert‐butylimidazol‐2‐ylidene (NHCtBu), which is one of the typical N‐heterocyclic carbene (NHC), was found to initiate the anionic chain‐growth polymerization of methyl sorbate (MS) in toluene at −20°C. The polymerization was accelerated by an aluminum Lewis acid, that is, methylaluminum bis(2,6‐di‐tert‐butyl‐4‐methylphenoxide) (MAD), to afford cyclic poly(MS)s. Subsequently, thiol‐ene click reactions of poly(MS)s with two different alkanethiols were performed using a photoradical initiator to obtain graft‐like polymers, which was for the microscopic observation of the chains directly by transmittance electron microscope (TEM). The contamination of linear polymers was hardly observed. This means that the polymers synthesized by our synthetic strategy were indeed cyclic polymers. We also compared the theoretical and experimental chain diameter and width for the graft‐like cyclic polymers, where these values were dependent on the molecular weight of the cyclic polymers regulated by the feed monomer/initiator ratio and length of the alkanethiols, respectively.

Cyclic Poly(4-methyl-1-pentene): Efficient Catalytic Synthesis of a Transparent Cyclic Polymer

Cyclic polymers possess properties that are significantly different from their linear analogs, such as higher densities, smaller hydrodynamic volumes, and higher glass transition temperatures. Poly...

Polymer Topology Determines the Formation of Protein Corona on Core-Shell Nanoparticles.

Linear and cyclic poly(2-ethyl-2-oxazoline) (PEOXA) adsorbates provide excellent colloidal stability to superparamagnetic iron oxide nanoparticles (FexOy NPs) within protein-rich media. However, dense shells of linear PEOXA brushes cannot prevent weak but significant attractive interactions with human serum albumin. In contrast, their cyclic PEOXA counterparts quantitatively hinder protein adsorption, as demonstrated by a combination of dynamic light scattering and isothermal titration calorimetry. The cyclic PEOXA brushes generate NP shells that are denser and more compact than their linear counterparts, entirely preventing the formation of a protein corona as well as aggregation, even when the lower critical solution temperature of PEOXA in a physiological buffer is reached.

Melt density, equilibrium melting temperature, and crystallization characteristics of highly pure cyclic poly(ε-Caprolactone)s

A series of cyclic and linear poly (ε-caprolactone)s (c-PCLs: c-PCLn, n = 30–200, average degree of polymerization; l-PCLs: l-PCLn, n = 30–200) in extremely high purity have been quantitatively investigated in terms of melt density, equilibrium melting temperature crystallization nature in connection to molecular topology effects. Synchrotron X-ray reflectivity analysis determined higher melt densities for c-PCLs than those of the linear counterparts. c-PCL was determined to exhibit 8.4 °C higher equilibrium melting temperature than that of the linear counterpart. Such higher equilibrium melting temperature may originate from a cooperative effort of thermodynamic factors (larger equilibrium heat of fusion and smaller equilibrium entropy) and morphological factors (thicker crystalline layer and higher fold surface energy). Comparing with l-PCL, c-PCL revealed apparently faster crystallization behavior at a chosen Tc but slower crystallization behavior at a same degree of supercooling because of the higher equilibrium melting temperature. Both c-PCL and linear counterpart were observed to follow heterogeneous nucleation and growth mechanism in crystallizations. Overall, this study achieved comprehensive insights into cyclic topology effects on the melt density, equilibrium temperature, and crystallization behavior of PCL.

Direct observation of cyclic poly(N-substituted maleimide)s with broad size distributions synthesized by anionic polymerization using an N-heterocyclic carbene and successive ring closure without high dilutions

The N-heterocyclic carbene 1,3-di-tert-butylimidazol-2-ylidene (NHCtBu) and its 1:1 adduct with methyl sorbate (MS) were found to initiate anionic polymerization of N-substituted maleimides (RMIs) in organic solvents at −20 °C to afford linear and cyclic poly(RMI)s, respectively. Quantitative monomer consumption was observed when a bulky aluminum Lewis acid, such as methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD), was used as an additive. In the presence of the NHCtBu:MS adduct as an initiator, monomer consumption was followed by ring closure without the need for highly diluted conditions to give cyclic poly(RMI)s containing α-terminal MS units, which exhibited an Mn of 3.4 × 103–6.2 × 103 and a broad polydispersity index (Mw/Mn = 2.43–2.65). Dilution was not required due to the presence of an α-terminal NHCtBu group, which acted as a counter cation for the propagating center during polymerization. The broad molecular weight distributions of the obtained polymers were ascribed to a chain transfer upon abstraction of the α-carbonyl proton of the RMIs, particularly to the neighboring NHCtBu cation unit. 1H NMR and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass analyses indicated that the proton transfer of the acidic proton was induced by the propagating anion. The topologies of the linear and cyclic poly(RMI)s were directly observed by transmission electron microscopy (TEM). Direct Observation of Cyclic Poly(N-substituted maleimide)s with Broad Size Distributions Synthesized by Anionic Polymerization using an N-Heterocyclic Carbene and Successive Ring Closure Without High Dilutions.

Functional Nanoassemblies of Cyclic Polymers Show Amplified Responsiveness and Enhanced Protein-Binding Ability.

The physicochemical properties of cyclic polymer adsorbates are significantly influenced by the steric and conformational constraints introduced during their cyclization. These translate into a marked difference in interfacial properties between cyclic polymers and their linear counterparts when they are grafted onto surfaces yielding nanoassemblies or polymer brushes. This difference is particularly clear in the case of cyclic polymer brushes that are designed to chemically interact with the surrounding environment, for instance, by associating with biological components present in the medium, or, alternatively, through a response to a chemical stimulus by a significant change in their properties. The intrinsic architecture characterizing cyclic poly(2-oxazoline)-based polyacid brushes leads to a broad variation in swelling and nanomechanical properties in response to pH change, in comparison with their linear analogues of identical composition and molecular weight. In addition, cyclic glycopolymer brushes derived from polyacids reveal an enhanced exposure of galactose units at the surface, due to their expanded topology, and thus display an increased lectin-binding ability with respect to their linear counterparts. This combination of amplified responsiveness and augmented protein-binding capacity renders cyclic brushes invaluable building blocks for the design of "smart" materials and functional biointerfaces.

Improvement in the transience and mechanical performance of flexible Poly(phthalaldehyde) substrates

Cyclic poly(phthalaldehyde) (PPHA) is a metastable polymer that depolymerizes to ortho-phthalaldehyde (o-PHA) at ambient temperatures. Both high molecular weights and a long shelf-life can be achieved. After an external stimulus trigger has been activated to break polymer backbone bonds, PPHA undergoes rapid depolymerization from the solid state. Flexible PPHA substrates can be made by mixing the polymer with traditional non-volatile plasticizers, however the solubility of plasticizers in PPHA is sometimes poor, and any solid products left behind after depolymerization can hinder the transience of the mixture. Hansen solubility parameters for PPHA were obtained and used to predict the solubility of plasticizers in PPHA. The freezing point of the degraded remnants is proportional to the total mole fraction of additives present in any given PPHA formulation. The colligative properties of o-PHA were obtained and used to estimate the freezing point depression of the transient remnants for multi-component mixtures. A single additive approach to plasticizing PPHA is presented in this study using diethyl adipate (DEA). Mechanical properties of PPHA films containing DEA were characterized. Glass transition temperature (Tg) values between 99.5 °C and 12.5 °C can be achieved using THF solvent to cast the PPHA-DEA film. A Tg of 187 °C was extrapolated for pure PPHA with no residual solvent. The flexibility of the DEA loaded PPHA films greatly improved at high DEA loadings. DEA also improved the transient depolymerization performance of DEA loaded PPHA resulting in complete liquification after depolymerization.

Morphology and growth rate of spherulite of cyclic poly(ε-caprolactone) having a triazole group at the closing point

To clarify the effect of triazole (TR) group at the closing point within the cyclic polymer chain on the crystal growth, we studied morphology and the growth rate of spherulite G of cyclic poly(ε-caprolactone) with a TR group at the closing point (C-TR-PCL) as a function of the degree of supercooling ΔT using polarizing optical microscopy. We prepared C-TR-PCLs synthesized by the ring-closing reaction from their linear precursors with Mw = 30000 and 50000, and cyclic PCL polymerized by ring expansion reaction (C-PCL) having no TR group with Mw = 26000, and linear PCL with/without a TR group at a chain end (L-TR-PCL-OH and L-PCL) with Mw = 26000 and 36000, where Mw was determined by GPC using linear polystyrene standards.We found that C-TR-PCL and L-TR-PCL-OH show banded spherulites at small ΔT and band-less normal spherulites with maltese cross at large ΔT. Whereas, C-PCL and L-PCL only showed band-less normal spherulites at any ΔT. This speculates that steric hindrance of TR group near the surface of crystals will become significant at small ΔT. G obeyed the equation, G = G0exp(−B/TΔT), for all the samples. B of C-PCL and L-PCL is constant irrespective of TΔT, however, that of C-TR-PCL and L-TR-PCL-OH showed two values depending on TΔT. At the region of lower B, i.e., small ΔT, the banded spherulites could be observed. The change of B corresponds to the morphological change of the spherulites. Since the size of the crystalline region closely related to lamellar thickness becomes large at small ΔT, TR groups excluded from the crystals become to assemble near the surface of the crystals. This is the reason why B becomes low at small ΔT. In C-TR-PCL with higher Mw and L-TR-PCL-OH, the decrease of B with decreasing ΔT becomes small compared to that of C-TR-PCL with lower Mw due to relatively small effect of TR group.

Ring-Opening Copolymerizaton of Cyclohexene Oxide and Succinic Anhydride by Zinc and Magnesium Schiff-Base Complexes Containing Alkoxy Side Arms

The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is a promising method for the synthesis of new polyesters with various polymer properties. Among previously reported metal catalysts for ROCOP, the Schiff-base complexes have gained significant attention because of their ease of synthesis and modification. In this work, zinc and magnesium complexes containing Schiff-base ligands with different alkoxy side arms [-(CH2)2O- and -(CH2)3O-] were synthesized and shown to have a cubane metal core by X-ray crystal structures. All complexes were studied in the ROCOP of cyclohexene oxide (CHO) and succinic anhydride (SA) in toluene at 110 °C. The zinc complex having a shorter side arm is the most active catalyst for copolymerization, giving poly(CHO-alt-SA) with narrow dispersity and negligible ether linkage. On the other hand, magnesium complexes were not active because of the formation of stable carboxylate species. The detailed analysis of polyester obtained from zinc complexes unexpectedly revealed three different types of polymer structures occurring at different polymerization times. Cyclic polymer was generated at the beginning by intramolecular transesterification of the alkoxy side arm, giving a low-molecular-weight polyester. At higher conversion, cyclization diminished, giving just a linear polyester but with minor competitive formation of higher-molecular-weight polyester having cyclohexanediol as an end group. On the basis of a thorough understanding of the polymerization mechanism, the desired cyclic poly(CHO-alt-SA) was successfully synthesized using a low monomer/catalyst ratio.

Diblock, Triblock and Cyclic Amphiphilic Copolymers with CO2 Switchability: Effects of Topology.

The combination of topology and CO2 switchability could provide new options for amphiphilic copolymers. Cyclic molecules supply novel topologies, and CO2 switching provides stimulus responsiveness. Cyclic poly(2–(diethylamino)ethyl methacrylate)–b–poly(ethylene oxide) and their corresponding block copolymers were prepared from poly(ethylene oxide) and 2–(diethylamino)ethyl methacrylate via atom transfer radical polymerization and Keck allylation with a Hoveyda–Grubbs catalyst. Changes in conductivity, surface activity, and hydrodynamic size were examined to illustrate the switchability of the produced amphiphilic copolymers upon contact with CO2 in the presence of water. The reversible emulsification and switchable viscosity behaviors of the copolymers were also demonstrated.

Direct Observation of a Cyclic Vinyl Polymer Prepared by Anionic Polymerization using N‐Heterocyclic Carbene and Subsequent Ring‐Closure without Highly Diluted Conditions

AbstractA 1:1 adduct of methyl sorbate (MS) and 1,3‐di‐tert‐butylimidazol‐2‐ylidene (NHCtBu) initiates anionic polymerization of a nonconjugated polar alkene, allyl methacrylate (AMA) in toluene at −20 °C. After the monomer is consumed quantitatively using a bulky aluminum Lewis acid, methylaluminum bis(2,6‐di‐tert‐butyl‐4‐methylphenoxide) (MAD), as an additive, successive ring‐closure occurs without highly dilute conditions to give a cyclic poly(AMA) containing α‐terminal MS unit, and an Mn of 8.8 × 103−58.5 × 103 with a narrow molecular dispersity index (Mw/Mn = 1.14–1.37). The lack of a need for dilution is due to the fact that an α‐terminal NHCtBu group is acting as the counter cation for the propagating center in the polymerization. From 1H NMR and matrix assisted laser desorption/ionization (MALDI‐TOF) mass spectra, combined with transmittance electron microscope (TEM) observation of a synthesized poly(AMA) with longer alkyl side chains prepared via a thiol‐ene click reaction, it is concluded that once the monomer is consumed, nucleophilic attack at the neighboring methine of the α‐terminal NHCtBu residue by the propagating anionic center causes ring‐closing to cyclic poly(AMA).

Ring–Ring Equilibration in Solid, Even‐Numbered Cyclic Poly(l‐lactide)s and their Stereocomplexes

AbstractEven‐numbered cyclic poly(d‐lactide) and poly(l‐lactide) are prepared by ring‐expansion polymerization. The cyclic pol(l‐lactide) is annealed either at 120 or at 160 °C for several days. The progress of transesterification in the solid state is monitored by the formation of odd‐numbered cycles via matrix‐assisted laser desorption/ionization‐time of flight mass spectrometry. The changes of the crystallinity are monitored by differential scanning calorimetry, wide‐ and small‐angle x‐ray scattering (WAXS and SAXS) measurements. Despite total even‐odd equilibration at 160 °C, the crystallinity of poly(l‐lactide) is not reduced. Furthermore, the crystallinity of the stereocomplexes of both cyclic polylactides do not decrease or vanish, as expected, when a blocky or random stereosequence is formed by transesterification. This conclusion is confirmed by 13C NMR spectroscopy. These measurements demonstrate that transesterification is a ring–ring equilibration involving the loops on the surfaces of the lamellar crystallites thereby improving crystallinity and 3D packing of crystallites without significant broadening of the molecular weight distribution.

Molecular weight dependence of the growth rate of spherulite of cyclic poly(ε-caprolactone) polymerized by ring expansion reaction

To elucidate the molecular weight (Mw) dependence of the growth rate of spherulite (G) of cyclic polymers polymerized by the ring expansion reaction, we studied the G of cyclic poly(ε-caprolactone) (C-PCL) growing from the melt as a function of the crystallization temperature (Tc) and degree of supercooling (ΔT) using polarizing optical microscopy. We prepared C-PCLs comprising an exclusively repeating unit, excluding other structural units with Mw = 17000, 33000, 41000, 74000 and 85000 and linear PCLs (L-PCL) with Mw = 16000, 35000 and 60000, where Mw was determined by GPC using linear polystyrene standards.G obeyed the equation, G = G0exp(−B/TΔT), for all the samples. The activation energies of secondary nucleation (B) of C-PCL were almost identical except for the C-PCL with the lowest Mw. Since B is proportional to the end surface free energy of the secondary nucleus, it implies that the regularity of the folding surface of C-PCL with high Mw is independent of Mw.Compared to C-PCL and L-PCL having almost the same Mw, G of L-PCL is higher than that of C-PCL at the same TΔT. This suggests that G of C-PCL and L-PCL is not solely controlled by the entanglement because the entanglement of C-PCL is lower than that of L-PCL. We found the following associations: G0∝Mw−1.2 for C-PCL and G0∝Mw−0.67 for L-PCL. The difference in the power of the G0 dependence on Mw reflects the difficulty of performing sliding diffusion in the secondary nucleus. This strongly supports the hypothesis that the absorption behavior of C-PCL chains in the secondary nucleus on the growth front is significantly different from that of L-PCL.

Limitations of ion mobility spectrometry-mass spectrometry for the relative quantification of architectural isomeric polymers: A case study.

In this work, the suitability of ion mobility spectrometry-mass spectrometry (IMS-MS) for the quantification of two isomers was investigated. A cyclic poly(L-lactide) was prepared through photodimerization of its linear homologue. Since IMS-MS can be used to differentiate cyclic polymer ions from their linear analogues because of their more compact three-dimensional conformation, the present work envisaged the use of IMS-MS for the quantification of residual linear polymers within the cyclic polymer sample. Using the standard addition method to plot calibration curves, the fraction of linear contaminants in the sample was determined. By doing so, unrealistically high values of contamination were measured. These results were explained by an ionization efficiency issue. This work underlines some intrinsic limitations when using IMS-MS in the context of the relative quantification of isomers having different ionization efficiencies. Nevertheless, the linear-to-cyclic ratio can be roughly estimated by this method.

Matrix-assisted laser desorption/ionization behavior of neat linear and cyclic poly(L-lactide)s and their blends.

RationaleNumerous new tin catalysts that enable the synthesis of cyclic polylactides with broad variation in their molecular mass were recently developed. The abundance of cyclics in matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectra is, however, frequently reported to greatly exceed that of linears. Thus, the MALDI ionization behavior of various end‐capped linear poly(L‐lactide)s and one cyclic poly(L‐lactide) was investigated and compared.MethodsNeat compounds and various blends of cyclic and linear species were prepared and studied under identical conditions with regard to sample preparation and instrumental condition, except for the laser power. For this purpose, two different MALDI‐TOF mass spectrometers were applied.ResultsOur results reveal that cyclics indeed show a slightly better ionization in MALDI, although their ionization as a neat compound seems to be less effective than that of linear polylactides. The ionization of most linear polylactides investigated does not depend on the end group structure. However, linear polylactides containing 12‐bromododecyl end groups reveal an unexpected saturation effect that is not caused by fragmentation of the polymer or the end group, or by electronic saturation of the detector digitizer. Furthermore, polylactides with a 2‐bromoethyl end group did not show such a saturation effect.ConclusionsAn overestimation of cyclic species in MALDI‐TOF mass spectra of poly(L‐lactide)s must be considered, but the commonly assumed peak suppression of linear polymers in mixtures of both structures can be excluded.

Triggered Transience of Plastic Materials by a Single Electron Transfer Mechanism.

Transient polymers rapidly and controllably depolymerize in response to a specific trigger, typically by a chain-end unzipping mechanism. Triggers, such as heat, light, and chemical stimuli, are generally dependent on the chemistry of the polymer backbone or end groups. Single electron transfer (SET), in contrast to other triggering mechanisms, is achievable by various means including chemical, electrochemical, and photochemical oxidation or reduction. Here, we identify SET and subsequent mesolytic cleavage as the major thermal triggering mechanism of cyclic poly(phthalaldehyde) (cPPA) depolymerization. Multimodal SET triggering is demonstrated by both chemical and photoredox-triggered depolymerization of cPPA. Redox-active small molecules (p-chloranil and 1,3,5-trimethoxybenzene) were used to tune the depolymerization onset temperature of cPPA over the range 105–135 °C. Extending this mechanism to photoredox catalysis, N-methylacridinium hexafluorophosphate (NMAPF6) was used to photochemically degrade cPPA in solution and thin films. Finally, we fabricated photodegradable cPPA monoliths with a storage modulus of 1.8 GPa and demonstrated complete depolymerization within 25 min of sunlight exposure. Sunlight-triggered depolymerization of cPPA is demonstrated and potentially useful for the manufacture of transient devices that vanish leaving little or no trace. Most importantly, this new mechanism is likely to inspire other SET-triggered transient polymers, whose development may address the ongoing crisis of plastic pollution.

Synthesis of cyclic poly(2-ethyl-2-oxazoline) with a degradable disulfide bond

Cyclic poly(2-ethyl-2-oxazoline) with a degradable disulfide bond was first synthesized by combining cationic ring opening polymerization and reversible thiol/disulfide exchange chemistry at a high concentration of the polymer solution.

High Tm poly(L-lactide)s via REP or ROPPOC of l-lactide

A new kind of high melting (HTm) pol(l-lactide) was discovered when cyclic poly(l-lactide)s were prepared by ring-expansion polymerization with cyclic tin catalysts at 130–160 °C in bulk.

High molar mass cyclic poly(l-lactide) obtained by means of neat tin(ii) 2-ethylhexanoate

l-Lactide was polymerized in bulk at 120, 140, 160 and 180 °C with neat tin(ii) 2-ethylhexanoate (SnOct2) as the catalyst and a cyclic topology was detected.

Synthesis of cyclic Poly(methyl methacrylate) via N-Heterocyclic carbene (NHC) initiated-anionic polymerization and subsequent ring-closing without need of highly dilute conditions

A cyclic polymethacrylate was prepared via the anionic polymerization of methyl methacrylate (MMA) initiated by a 1:1 adduct of methyl sorbate (MS) and a N-heterocyclic carbene (NHC) without the need for highly dilute conditions, because an α-terminal imidazolimium group is neighboring to the propagating anion during the polymerization. The polymerization proceeded quantitatively using methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) as the Lewis acid to give the poly (MMA) with the MS unit in the α-terminus, and an Mn of 7.2 × 103–29.1 × 103 with a molecular weight distribution (Mw/Mn = 1.16-1.56). From proton (1H) NMR and matrix assisted laser desorption/ionization (MALDI-TOF) mass spectra, coupled with size exclusion chromatography (SEC) of the poly (MMA) produced before and after ozonolysis of the internal carbon-carbon double bond in the MS unit, we found that ring-closing reaction was induced by nucleophilic substitution of the propagating anion to the neighboring carbon of the α-terminal carbenyl (imidazolimium) group to synthesize cyclic poly (MMA) following complete the monomer consumption.