Copolyesters of ε-caprolactone and l-lactide catalyzed by a tetrabutylammonium phthalimide-N-oxyl organocatalyst

Aliphatic polyesters are biocompatible materials that can be used in biomedical applications. We report here the use of tetrabutylammonium phthalimide-N-oxyl catalyst (TBAPINO), as a thermally stable organocatalyst for the ring-opening polymerization (ROP) of cyclic esters under mild conditions. In the solution ROP of ε-caprolactone (ε-CL), quantitative conversion and M_n of ∼20 000 g mol⁻¹ are achieved in a wide temperature range from −15 to 60 °C. Under bulk condition, the conversion of ε-CL reaches over 85% at 120 °C within 2 h. The living ROP character of l-lactide (l-LA) catalyzed over TBAPINO is proved by multiple additions of monomer in the bulk polymerization. The catalyst shows comparable selectivity towards the ring-opening polymerization of l-LA and ε-CL. Their copolymerization over TBAPINO is carried out in one-pot bulk condition in terms of the reaction time, monomer feed ratio, and sequence of addition. The colorless poly(ε-caprolactone-co-lactide) (PCLA) is obtained with considerable conversion of both monomers with the M_n over 22 000 g mol⁻¹.

By utilizing tetrabutylammonium phthalimide-N-oxyl organocatalyst, copolymer PCLA with M_n over 20 000 g mol⁻¹ was synthesized by sequential ring-opening polymerization of ε-caprolactone and l-lactide under bulk conditions.     

Bimetallic aluminum complexes bearing novel spiro-phenanthrene-monoketone/OH derivatives: synthesis,characterization and the ring-opening polymerization of ε-caprolactone

A series of spiro-phenanthrene-monoketone/OH derivatives (L1–L6) were synthesized and fully characterized with ¹H/¹³C NMR spectroscopy and elemental analyses. By treating ligands with AlMe₃, oxygen-bridged binuclear aluminum complexes (Al1–Al6) were isolated and characterized by ¹H/¹³C NMR spectroscopy. The molecular structures of ligands (L2, L4 and L5) and complex Al1 were determined by single crystal X-ray diffraction. In the presence of benzyl alcohol (BnOH), these aluminum complexes demonstrated high efficiency towards the ring-opening polymerization of ε-caprolactone (ε-CL), resulting in PCL in a linear manner with the BnO-end group. In addition, complexes Al1 and Al5 exhibited good catalytic activities even without BnOH. Moreover, complexes Al3 and Al6 with the bulkier substituent of ⁱPr at the ortho-position of the arylamines demonstrated better catalytic activities than the analogs. Moreover, substituents on the backbone also affected catalytic behaviors.

Bimetallic aluminum complexes bearing novel spiro-phenanthrene-monoketone/OH derivatives were synthesized, and displayed good activity toward the ring-opening polymerization of ε-caprolactone.     

Glycol-functionalized ionic liquids for high-temperature enzymatic ring-opening polymerization

Enzymatic ring-opening polymerization (ROP) is a benign method for preparing polyesters, such as polylactides and other polylactones. These reactions are typically carried out at relatively high temperatures (60–130 °C), however, there is a deficiency of enzyme-compatible solvents for such thermally-demanding biocatalytic processes. In this study, we have prepared a series of short-chained glycol-grafted ionic liquids (ILs) based on a phosphonium, imidazolium, pyridinium, ammonium, or piperidinium cationic headgroup. Most of these glycol-grafted ILs exhibit relatively low dynamic viscosities (33–123 mPa s at 30 °C), coupled with excellent short-term thermal stabilities with decomposition temperatures (T_dcp) in the 318–403 °C range. Significantly, the long-term thermal stability under conditions matching those for enzymatic ROP synthesis (130 °C for 7 days) is excellent for several of these task-specific ILs. Using Novozym 435-catalyzed ROP, these ILs are demonstrated to be viable solvents for the enzymatic production of reasonable yields (30–48%) of high molecular mass (M_w ∼20 kDa) poly(l-lactide) and poly(ε-caprolactone) compared to solventless conditions (12–14 kDa).

New glycol-functionalized ionic liquids exhibit high thermal stability and are lipase-compatible, leading to a high molecular weight of polyester in the enzymatic ring-opening polymerization reaction.     

Ternary composites by an in situ hydrolytic polymerization process

Polyamide 6/modified silica composite materials have been prepared by a coupled polymerization procedure. For this purpose, the three-component-system we presented in a previous publication, consisting of ε-aminocaproic acid (ε-ACA), ε-caprolactam (ε-CL), and 1,1′,1′′,1′′′-silanetetrayltetrakis-(azepan-2-one) (Si(ε-CL)₄), has been combined with other silicon monomers with one or two methyl groups (MeSi(ε-CL)₃ and Me₂Si(ε-CL)₂). The simultaneous polymerization of ε-CL and silicon monomers leads to the in situ formation of silica/polysiloxane particles and the surrounding polyamide 6 matrix in one step. Moreover, 3-aminopropyltriethoxysilane has been added to the three-component-system to achieve covalent bonding between organic and inorganic phases and to inhibit agglomeration of the silica particles. Chemical structures and morphologies of the composites have been investigated by solid-state NMR and FTIR spectroscopy as well as electron microscopy and SEC measurements. Structural effects on thermal properties have been studied by DSC and TGA measurements.

Polyamide 6/silica/polysiloxane composites have been prepared in a one-step process using lactam-substituted silicon monomers.     

Binary catalytic system for homo- and block copolymerization of ε-caprolactone with δ-valerolactone

The combined interaction of 2,3,6,7-tetrahydro-5H-thiazolo[3,2-a] pyrimidine (ITU) as the organocatalytic nucleophile with YCl₃ as Lewis acid cocatalyst, generating ITU/YCl₃, was employed for homo- and copolymerization of ε-caprolactone (CL) with δ-valerolactone (VL). Poly(caprolactone) (PCL) and poly(caprolactone)–poly(ethylene glycol)–poly(caprolactone) (PCL–PEG–PCL) triblock copolymer and poly(valerolactone)–poly(caprolactone)–poly(ethylene glycol)–poly(caprolactone)–poly(valerolactone) (PVL–PCL–PEG–PCL–PVL) pentablock copolymer were successfully prepared by ring-opening polymerization (ROP) of CL employing ITU/YCl₃ as catalyst in the presence of benzyl alcohol (BnOH) or poly(ethylene glycol) (PEG) as initiator, respectively. The reaction was systematically optimized, and the architecture, molecular weight and thermal properties of the polymers were characterized by NMR, FTIR, SEC and DSC analyses. Finally, a plausible polymerization mechanism was proposed.

The combined interaction of 2,3,6,7-tetrahydro-5H-thiazolo[3,2-a] pyrimidine (ITU) as the organocatalytic nucleophile with YCl₃ as Lewis acid cocatalyst, generating ITU/YCl₃, was employed for homo- and copolymerization of CL with VL.     

Preparation of functionalized poly(1-butene) from 1,2-polybutadiene via sequential thiol-ene click reaction and ring-opening polymerization

Poly(1-butene) is a kind of unique poly(α-olefin) material that displays exceptional creep resistance and environmental stress cracking resistance, and therefore currently finds wide application in the fields of packaging, films, pipes, etc. However, very few current researchers are paying attention to functional poly(1-butene) despite its great research significance, perhaps due to the general paucity of catalytic systems for synthesizing this material. Therefore, in the present study, we set out to develop an alternative method to prepare polar poly(1-butene)s. Specifically, 1,2-enriched poly(1,3-butadiene), used as a starting material, was partially hydrogenated to afford a quasi-poly(1-butene) polymer containing C C double bonds. These double bonds were further modified by subjecting the quasi-poly(1-butene) polymer to a thiol-ene reaction in the presence of thiol compounds, and a series of polar poly(1-butene)s that bore significantly improved surface properties were obtained. By using hydroxyl-containing thiol compounds, ring-opening polymerization (ROP) of ε-caprolactone was further implemented. Here, polar poly(ε-caprolactone) was incorporated as side chains, and we were able to control the chain length by adjusting the feeding ratio. The water contact angles of the resultant polymers, i.e., those containing the poly(ε-caprolactone) side chains, were as low as 59.4°, indicating a greater hydrophilicity resulting from the incorporation of these side chains.

An alternative method for preparing functionalized poly(1-butene) from 1,2-polybutadiene via sequential thiol-ene click reaction and ring-opening polymerization is reported.     

Ring opening polymerization of d,l-lactide and ε-caprolactone catalysed by (pyrazol-1-yl)copper(ii) carboxylate complexes

1,2-Bis{(3,5-dimethylpyrazol-1-yl)methyl}benzene (L) reacts with [Cu(OAc)₂] and C₆H₅COOH, 4-OH-C₆H₄COOH, 2-Cl-C₆H₄COOH and (3,5-NO₂)₂-C₆H₃COOH to afford the copper complexes [Cu₂(C₆H₅COO)₄(L)₂] (1), [Cu₂(4-OH-C₆H₄COO)₄(L)₂] (2), [Cu₂(2-Cl-C₆H₄COO)₄(L)₂]_n (3) and [Cu{(3,5-NO₂)₂-C₆H₃COO}₂L]_n (4) which are characterised by IR, mass spectrometry, elemental analyses, and X-ray crystallography. The structural data revealed two geometries that are adopted by the complexes: (i) paddle wheel in 1, 2·7H₂O, 3 and (ii) regular chains in 3 and 4. Magnetic studies show strong antiferromagnetic couplings in the paddle wheel complexes and a weak antiferromagnetic coupling in the monometallic chain one. Catalysis studies performed with these complexes (1–4) showed that they initiate ring opening polymerization (ROP) of ε-caprolactone (ε-CL) under solvent-free conditions and d,l-lactide in toluene at elevated temperatures. Polycaprolactone (PCL) and poly(d,l-lactide) (PLA) obtained from the polymerization reactions are of low molecular weights (858 for PCL and 602 Da for PLA for initiator 1) and polydispersity indices (typically 2.16 for PCL and 1.64 for PLA with 1 as the initiator). End group analysis of the polymers, determined by MALDI-ToF MS, indicates that the polymers have benzoate, hydroxyl, methoxy and cyclic end groups.

We report the synthesis, structure and complete characterization of four new pyrazolyl carboxylate-based copper(ii) complexes that catalyze the ring opening polymerization of ε-caprolactone under solvent-free conditions and of d,l-lactide in toluene.     

Bismuth subsalicylate,a low-toxicity catalyst for the ring-opening polymerization (ROP) of l-lactide (l-LA) with aliphatic diol initiators: synthesis,characterization, and mechanism of initiation

The ring-opening polymerization (ROP) of l-lactide (l-LA) was induced by the catalytic action of bismuth subsalicylate (BiSS) using linear aliphatic diols [HO(CH₂)_nOH, where n = 2, 3, 4, 5, 6, and 8] as initiators and chain transfer agents. The theoretical and experimental degree of polymerization (DP) in all samples of α,ω-hydroxy telechelic poly(l-lactide) (HOPLLAOH) had a good agreement in all samples, an effect attributed to the interaction of BiSS with HO(CH₂)_nOH inducing a transfer reaction. HOPLLAOH was synthesized and characterized by a range of analytical techniques, confirming the insertion of methylene groups from the initiator into the main chain of the polyester. The glass-transition temperature (T_g) of HOPLLAOH was found to be proportional to the number of methylene groups present in the diol. Various parameters regarding the ROP of l-LA were studied, such as temperature, time of reaction, amount of catalyst, and the nature of the diols. A kinetic study of the reaction allowed the determination of the rate constants (k) and activation energy (E_a). A mechanism of initiation is proposed based on a computational study using density functional theory (DFT), evidencing the role of the alkyl diol as an initiator, producing an alkoxide (Bi–OROH). This species then acts as a nucleophile, attacking the carbonyl group, inducing its insertion, and ultimately completing the ring-opening of l-LA.

Bismuth subsalicylate (BiSS) acted as a catalyst in the ring-opening polymerization of l-lactide (l-LA) in the presence of alkyl diols as initiators.     

Photo-responsive polymeric micelles and prodrugs: synthesis and characterization

Bio-recognizable and photocleavable amphiphilic glycopolymers and prodrugs containing photodegradable linkers (i.e. 5-hydroxy-2-nitrobenzyl alcohol) as junction points between bio-recognizable hydrophilic glucose (or maltose) and hydrophobic poly(α-azo-ε-caprolactone)-grafted alkyne or drug chains were synthesized by combining ring-opening polymerization, nucleophilic substitution, and “click” post-functionalization with alkynyl-pyrene and 2-nitrobenzyl-functionalized indomethacin (IMC). The block-grafted glycocopolymers could self-assemble into spherical photoresponsive micelles with hydrodynamic sizes of <200 nm. Fluorescence emission measurements indicated the release of Nile red, a hydrophobic dye, encapsulated by the Glyco-ONB-P(αN₃CL-g-alkyne)_n micelles, in response to irradiation caused by micelle disruption. Light-triggered bursts were observed for IMC-loaded or -conjugated micelles during the first 5 h. Following light irradiation, the drug release rate of IMC-conjugated micelles was faster than that of IMC-loaded micelles. Selective lectin binding experiments confirmed that glycosylated Glyco-ONB-P(αN₃CL-g-alkyne)_n could be used in bio-recognition applications. The nano-prodrug with and without UV irradiation was associated with negligible levels of toxicity at concentrations of less than 30 μg mL⁻¹. The confocal microscopy and flow cytometry results indicated that the uptake of doxorubicin (DOX)-loaded micelles with UV irradiation by HeLa cells was faster than without UV irradiation. The DOX-loaded Gluco-ONB-P(αN₃CL-g-PONBIMC)₁₀ micelles effectively inhibited HeLa cells'' proliferation with a half-maximal inhibitory concentration of 8.8 μg mL⁻¹.

Bio-recognizable and photocleavable amphiphilic glycopolymers and prodrugs containing photodegradable linkers as junction points between hydrophilic glycose and hydrophobic poly(α-azo-ε-caprolactone)-grafted alkyne or drug chains were synthesized.     

Tailor-made block copolymers of l-, d- and rac-lactides and ε-caprolactone via one-pot sequential ring opening polymerization by pyridylamidozinc(ii) catalysts

Three-coordinated Zn(ii) complexes bearing sterically encumbered bidentate monoanionic [N,N⁻] pyridylamido ligands efficiently catalyze the ring opening polymerization of lactide (LA) and ε-caprolactone (CL). Owing to the polymerization controlled nature and high rate, precise stereodiblock poly(LLA-b-DLA) with different block lengths can be easily produced by one-pot sequential monomer addition at room temperature in short reaction times. NMR, SEC and DSC analyses confirm the production of highly isotactic diblock copolymers which crystallize in the high melting stereocomplex phase. Stereo-triblock and tetrablock copolymers of l-LA, d-LA and rac-LA have been synthesized similarly. Finally, a diblock poly(CL-b-LA) has been easily obtained by sequential addition of ε-caprolactone and lactide under mild conditions.

New 3-coordinated Zn ROP catalysts afford lactide stereo-block copolymers with variable block lengths and steric structures and diblock ε-caprolactone-lactide copolymers at room temperature and in short reaction times.     

Synthesis and characterization of 4-arm star-shaped amphiphilic block copolymers consisting of poly(ethylene oxide) and poly(ε-caprolactone)

Star-shaped polymers exhibit lower hydrodynamic volume, glass transition temperature, critical micelles concentration (CMC), and higher viscosity and drug-loading capacity compared to their linear counterparts. In the present study, amphiphilic biodegradable 4-arm star-shaped block copolymers, based on poly(ethylene oxide) (PEO) as a hydrophilic part and poly(ε-caprolactone) (PCL) as a hydrophobic segment, are synthesized by ring-opening polymerization of ε-caprolactone employing pentaerythritol as an initiator and stannous octoate as a catalyst, followed by the coupling reaction with carboxyl-functionalized monomethoxy poly(ethylene oxide) (MeO-PEO-COOH). The structures of intermediates were deduced through ¹H-NMR and FT-IR spectroscopy. Average chemical composition of the star block copolymer is determined by proton NMR. Information related to molar mass distribution of targeted products and their precursors is obtained by size exclusion chromatography (SEC). However, due to its inherent poor resolution SEC could not reveal whether all the parent homopolymers are coupled to each other or remained unattached to the other segment. In order to comprehensively characterize the synthesized star-block copolymers, liquid chromatography at critical conditions of both blocks is employed. The study allowed for separation of homopolymer precursors from targeted star-block copolymers. The study exposed heterogeneity of star block copolymers that was not possible by conventional techniques.

LCCC on RP columns proves to be efficient for separation of precursors from targeted star block copolymers in a single run.     

High pressure as a novel tool for the cationic ROP of γ-butyrolactone

In this study, we report the acid-catalyzed and high pressure assisted ring-opening polymerization (ROP) of γ-butyrolactone (GBL). The use of a dually-catalyzed approach combining an external physical factor and internal catalyst (trifluoromethanesulfonic acid (TfOH) or p-toluenesulfonic acid (PTSA)) enforced ROP of GBL, which is considered as hardly polymerizable monomer still remaining a challenge for the modern polymer chemistry. The experiments performed at various thermodynamic conditions (T = 278–323 K and p = 700–1500 MPa) clearly showed that the high pressure supported polymerization process led to obtaining well-defined macromolecules of better parameters (M_n = 2200–9700 g mol⁻¹; Đ = 1.05–1.46) than those previously reported. Furthermore, the parabolic-like dependence of both the molecular weight (M_W) and the yield of obtained polymers on variation in temperature and pressure at either isobaric or isothermal conditions was also noticed, allowing the determination of optimal conditions for the polymerization process. However, most importantly, this strategy allowed to significantly reduce the reaction time (just 3 h at room temperature) and increase the yield of obtained polymers (up to 0.62 g_PGBL/g_GBL). Moreover, despite using a strongly acidic catalyst, synthesized polymers remained non-toxic and biocompatible, as proven by the cytotoxicity test we performed in further analysis. Additional investigation (including MALDI-TOF measurements) showed that the catalyst selection affected not only M_W and yield but also the linear/cyclic form content in obtained macromolecules. These findings show the way to tune the properties of PGBL and obtain polymer suitable for application in the biomedical industry.

Well-defined poly(γ-butyrolactone) was synthesized with great efficiency via high pressure assisted cationic ROP of hardly polimerizable γ-butyrolactone.     

Studies on asymmetric total synthesis of (−)-β-hydrastine via a chiral epoxide ring-opening cascade cyclization strategy

Herein, facile and enantioselective approaches to synthesize the core phthalide tetrahydroisoquinoline scaffold of (−)-β-hydrastine via both a CF₃COOH-catalyzed (86% ee) and KHMDS-catalyzed (78% ee) epoxide ring-opening/transesterification cascade cyclization from chiral epoxide under very mild conditions are described. The key elements include a highly enantioselective epoxidation using the Shi ketone catalyst and an intramolecular CF₃COOH-catalyzed cascade cyclization in one pot, and a late-stage C-3′ epimerization under MeOK/MeOH conditions as the key steps to achieve the first total synthesis of (−)-β-hydrastine (up to 81% ee).

Herein, both CF₃COOH-catalyzed (86% ee) and KHMDS-catalyzed (78% ee) chiral epoxide ring-opening cascade cyclization to facile and enantioselective synthesis of the core phthalide tetrahydroisoquinoline scaffold of (−)-β-hydrastine are described.     

Controlled organocatalyzed d,l-lactide ring-opening polymerizations: synthesis of low molecular weight oligomers

A systematic approach to the synthesis of organocatalyzed oligo(d,l-lactide) demonstrates that choice of initiator, catalytic ratio, and reaction time yields well-controlled oligomers. Ring-opening polymerization of d,l-lactide with the initiator α-methyl propargyl alcohol, a secondary alcohol, used in excess of 4-dimethylaminopyridine catalyst mitigates cyclicization, transesterification, and catalyst-initiated side reactions. This approach enables the design of uniform lactide oligomers for controlled release applications, such as delivery systems for drugs, prodrugs, and molecular sensors.

A systematic approach to the synthesis of organocatalyzed oligo(d,l-lactide) demonstrates that choice of initiator, catalytic ratio, and reaction time yields well-controlled oligomers.

Poly(lactide) (PLA) is a versatile polymer with properties suitable for a range of controlled release applications. Molecular weight and copolymer composition control degradation and drug release, with degradation yielding biocompatible lactic acid products. PLA and copolymers of lactide and glycolide (PLGA) have been used pre-clinically and clinically for release of drugs,^1–3 prodrugs,^4–6 and molecular sensors.^7–9 Highly reproducible PLA chemistries are particularly important to ensure control over the release of drugs with narrow therapeutic windows and to enhance efficacy by reducing dependence on patient compliance.^10–13 Stannous octoate is commonly employed as an organometallic catalyst of lactide ring-opening polymerization (ROP).^14,15 However, tin catalysts, which are challenging to fully remove during purification, can result in toxicity.¹⁶ Alternatives such as strongly basic amine organocatalysts are favored, particularly 4-dimethylaminopyridine (DMAP), which was pioneered by Nederberg et al.¹⁷ DMAP-mediated ROP is used for one-pot PLA polymerizations and conjugations,¹⁸ diblock or triblock copolymerizations with lactide as a first or second block,^19–21 grafting lactide to cellulose polymer fibers,²² and synthesizing star-shaped/cross-linked PLA networks.²³ PLA has also been exploited to tether and release drugs from linear polymers and hydrogel depots.²⁴ Drug tethers are typically low molecular weight oligo(lactide) composed of fewer than seven lactide repeat units to avoid crystallinity and associated challenges with solubility and control over degradation rates.²⁵ Self-catalysis, or direct polycondensation, of lactic acid at increased temperature and reduced pressure for an extended time yields low-molecular weight (800–3200 Da) oligo(lactide), in contrast to organocatalysts that are commonly employed for high molecular weight PLA synthesis. However, extensive setup and environmental control decrease the accessibility of these reactions.^26,27 Furthermore, with neither an initiator nor a catalyst used in polycondensation, mixtures of α-hydroxy and ω-carboxy PLA are formed, limiting the versatility of post-polymerization drug functionalization.²⁸ Alternatively, click chemistry²⁹ may be exploited to control subsequent modification of oligo(lactide) by employing a ‘clickable’ alcohol to initiate oligo(lactide) synthesis.³⁰Here, we explored bulk ROP of d,l-lactide by propargyl alcohol initiator and DMAP catalyst to synthesize low molecular weight oligo(lactide) linkers. To enable subsequent click reactions and to mitigate crystallinity, propargyl alcohol (PA) and d,l-lactide (L) were used.²⁷ Although not studied here, similar approaches have shown products do not epimerize, and we expect oligomers to be atactic and amorphous.¹⁷ To investigate the molecular weight and polydispersity of oligo(lactide), an initial polymerization was designed similar to that of Nederberg et al.¹⁷ and Coulembier and Dubois¹⁸ using a PA : L : DMAP ratio of 1 : 20 : 4 (Scheme 1; see Table S1, ESI^†). The neat polymerization was stirred at 130 °C under a nitrogen environment. After 5, 10, 15, 30, and 60 minutes, reaction vials were opened to atmosphere and cooled before dissolution in dichloromethane (DCM) and precipitation in hexanes. ¹H-NMR spectroscopy identified successful synthesis of PA-functional oligo(d,l-lactide) (PA-ODLA) with ∼99% conversion of d,l-lactide after only 5 minutes of polymerization (Fig. 1a; see Fig. S3, ESI^†). Integration of peaks C, E, and F indicated linkers were ∼19 lactide units, or an average M_n of 2825 Da. However, matrix assisted laser desorption ionization time of flight mass spectrometry suggested M_n was 752 Da. As M_n determined by NMR was based on average end-group analysis and assumed PA-initiated oligo(lactide), and M_n determined by MALDI analysis represented all species present, these data indicated that not all oligo(lactide) chains were initiated by PA. Rather, distinct series of peaks periodically separated by 144 Da, the M_n of lactide, were formed during polymerization (α, β, γ, δ, ε, and ζ; Fig. 1b; see Table S2, ESI^†). Peaks 72 Da less than these peaks were also identified (α′–ζ′). Additional reactions were conducted to identify the formed products: one with the initiator α-methyl propargyl alcohol (αMPA; see Fig. S4, ESI^†), and another with only d,l-lactide and DMAP but no initiator (see Fig. S9, ESI^†).Open in a separate windowScheme 1Ring-opening polymerization of lactide by alcohol initiator. X = H, propargyl alcohol; CH₃, α-methyl propargyl alcohol.Open in a separate windowFig. 1Five minute polymerizations of 1 : 20 : 4 propargyl alcohol : lactide : 4-dimethylaminopyridine form varied products. (a) ¹H-NMR, with hydrogen peaks assignments shown. (b) MALDI-TOF, where colors represent peak sets that repeat every 72 Da and the inset shows a full 144 Da peak set.The desired product, PA-ODLA “α” (Fig. 1a), was formed via DMAP base activation of PA, which initiated ROP (see Fig. S5, ESI^†). Base activation of the alcohol was confirmed similar to previous reports^20,31 using ¹H-NMR of PA mixed with DMAP in CDCl₃ to identify ppm shifts for hydroxyl groups (see Fig. S6, ESI^†). Peak α′, 72 Da less than α, was the result of transesterification of PA-ODLA (see Fig. S7, ESI^†), which is a common side-reaction during ROP of lactide.^18,20 Two undesired products, peaks ζ and ζ′, were cyclic PA-ODLA with and without transesterification, respectively. Cyclicization increases over time during ROP of lactide^32–34 and is undesired because hydroxyl end groups are not available for subsequent conjugation. These peaks were due to radical-mediated dehydration with hydroxyl end group participation,³⁵ as the addition of hydroquinone (HQ), a radical scavenger, eliminated peaks ζ and ζ′ (see Fig. S8, ESI^†). However, a peak appeared that corresponded to HQ-DMAP-ODLA and lacked alkyne functionalities for subsequent click reactions.Another undesired product, DMAP-ODLA, or peak γ, formed via DMAP nucleophilic attack of lactide (see Fig. S9, ESI^†). Peak γ′, separated by 72 Da from peak γ, resulted from transesterification of DMAP-ODLA. By conducting reactions of only DMAP and lactide, only PA and lactide, and only αMPA and lactide, it was confirmed that DMAP can both initiate and catalyze lactide ROP, but PA and αMPA cannot (see Fig. S10, ESI^†). DMAP-ODLA formed due to PA : DMAP ratios less than 1, as previously described,¹⁸ and is undesired because ‘clickable’ propargyl end groups are not present. Finally, peaks β, δ, and ε are likely ion fragments, as these peaks were only present when using MALDI in linear, but not reflector, mode (see Fig. S11, ESI^†).To increase the amount of oligo(lactide) with alkyne and hydroxyl functionalities, side reactions were systematically addressed. It was noted that transesterification increased with time of polymerization and was greater for PA than for αMPA, as less nucleophilic secondary alcohols are unable to participate in transesterification reactions.³⁰ Thus, reactions were conducted at 130 °C for 5 minutes using αMPA as the initiator to optimize desired product (αMPA-ODLA, peak α).Ratios of αMPA : L : DMAP were investigated to mitigate undesired DMAP-initiated and cyclic ODLA (Fig. 2a; see Fig. S12, ESI^†). Increasing αMPA : L at a constant αMPA : DMAP increased the intensity of α relative to both ζ and γ (Fig. 2b; see Fig. S13, ESI^†). Similar behavior was identified when holding L : DMAP constant and increasing αMPA : L (Fig. 2c; see Fig. S14, ESI^†). These results suggest that αMPA : L controls cyclicization, as the distribution of ζ is similar among reactions in Fig. 2a, and the ratio of ζ : α decreases between ratios of 1 : 10 and 1 : 2 in Fig. 2b. αMPA : DMAP controls DMAP-ODLA generation, as the distribution of γ is similar within Fig. 2b and smallest at a ratio of 2 : 1. Finally, lower ratios of L : DMAP appear to increase α, with 2 : 1 yielding the greatest amount of desired product α. Altogether, higher αMPA : L, higher αMPA : DMAP, and lower L : DMAP ratios yield higher levels of α relative to γ and ζ.Characterization of 5 minute αMPA polymerizations. X = % conversion, M_n = molecular weight, PDI = polydispersity

pH-responsive polymeric vesicles from branched copolymers

A new type of branched copolymer, poly(l-lactide)₂-b-poly(l-glutamic acid) (PLLA₂–PLGA), based on polypeptide PLGA is synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG–NCA) with amino-terminated PLLA₂–NH₂ and subsequent deprotection. The branched copolymer is characterized by ¹H NMR, FTIR and GPC measurements. The self-assembly of the copolymers in aqueous media has been systematically discussed. A pyrene probe has been used to demonstrate the aggregated formation of PLLA₂–PLGA in solution by measuring the critical micelle concentration (cmc). The morphology and size of the micelles have further been studied by transmission electron microscopy (TEM), dynamic light scattering (DLS) and field emission scanning electron microscopy (ESEM). We demonstrated that the Rh of the vesicle is depending on solution pH and salt concentration. The vesicles show good stability with remained shapes and sizes during the lyophilizing process. These vesicles have great potential in the application of drug delivery.

A new type of branched copolymer, poly(l-lactide)₂-b-poly(l-glutamic acid), based on polypeptide PLGA is synthesized by the ring-opening polymerization of N-carboxyanhydride of γ-benzyl-l-glutamate with amino-terminated PLLA₂–NH₂ and subsequent deprotection.     

Mesoporous anodic α-Fe2O3 interferometer for organic vapor sensing application

In this work, we reported the utilization of mesoporous α-Fe₂O₃ films as optical sensors for detecting organic vapors. The mesoporous α-Fe₂O₃ thin films, which exhibited obvious Fabry–Perot interference fringes in the reflectance spectrum, were successfully fabricated through electrochemical anodization of Fe foils. Through monitoring the optical thickness of the interference fringes, three typical organic species with different vapor pressures and polarities (hexane, acetone and isopropanol) were applied as probes to evaluate the sensitivity of the α-Fe₂O₃ based interferometric sensor. The experiment results showed that the as-synthesized mesoporous α-Fe₂O₃ interferometer displayed high reversibility and stability for the three organic vapors, and were especially sensitive to isopropanol, with a detection limit of about 65 ppmv. Moreover, the photocatalytic properties of α-Fe₂O₃ under visible light are beneficial for degradation of dodecane vapor residues in the nano-pores and refreshment of the sensor, demonstrating good self-cleaning properties of the α-Fe₂O₃-based interferometric sensor.

Mesoporous α-Fe₂O₃ interferometers with well-resolved optical fringes can display high sensitivity to organic vapors.     

New strategy for synthesis of hydroxyl-terminated poly(3-hexylthiophene) and its block copolymer using an η3-π-allyl hydroxyl-functionalized Ni(i) initiator

A novel hydroxyl-terminated π-allyl nickel initiator [η³-Ni(CH₂CHCHCH₂OCOCH₃)][BPh^F₄] is synthesized for the first time. ¹H-NMR analysis confirms that the Ni initiator possesses a typical π-allyl structure with a protected hydroxyl group. The Ni initiator is applied to synthesize hydroxyl-terminated poly(3-hexylthiophene). It successfully initiates the homopolymerization of thiophene, and yields narrow molecular weight distribution, hydroxyl-terminated poly(3-hexylthiophene). Moreover, it is found that [η³-Ni(CH₂CHCHCH₂OCOCH₃)][BPh^F₄] has high initiating activity in the polymerization of styrene and butadiene. A hydroxyl-terminated polybutadiene-b-poly(3-hexylthiophene) block copolymer is synthesized by using [η³-Ni(CH₂CHCHCH₂OCOCH₃)][BPh^F₄] to improve the workability and brittleness of poly(3-hexylthiophene).

A novel hydroxyl-terminated π-allyl nickel initiator is synthesized for the first time. Hydroxyl-terminated PB-b-P3HT block copolymer and hydroxyl-terminated P3HT are synthesized by using the initiator to improve the workability of P3HT.     

Theoretical prediction of some layered Pa2O5 phases: structure and properties

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure. Although the layered structure has been observed with the isostructural transition Nb and Ta metal pentoxides experimentally, the detailed structure and properties of the layered Pa₂O₅ are not clear and understandable. Our theoretical prediction explored some possible stable structures of the Pa₂O₅ stoichiometry according to the existing M₂O₅ structures (where M is an actinide Np or transition Nb, Ta, and V metal) and replacing the M ions with protactinium ions. The structural, mechanical, thermodynamic and electronic properties including lattice parameters, bulk moduli, elastic constants, entropy and band gaps were predicted for all the simulated structures. Pa₂O₅ in the β-V₂O₅ structure was found to be a competitive structure in terms of stability, whereas Pa₂O₅ in the ζ-Nb₂O₅ structure was found to be the most stable overall. This is consistent with Sellers''s experimental observations. In particular, Pa₂O₅ in the ζ-Nb₂O₅ structure is predicted to be charge-transfer insulators. Furthermore, we predict that ζ-Nb₂O₅-structured Pa₂O₅ is the most thermodynamically stable under ambient conditions and pressure.

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure.     

A green route to prepare metal-free phthalocyanine crystals with controllable structures by a simple solvothermal method

Exploring the environmentally friendly and low-cost synthesis strategies of phthalocyanine (Pc) crystals in just one step is an absolute challenge. The solvothermal synthesis of phthalocyanine crystals shows the advantages of high-quality crystalline products, facile reaction and purification, and low cost. Nevertheless, only a few metal phthalocyanine crystals have been successfully synthesized via solvothermal reactions. In this study, we found that the crystalline β metal-free phthalocyanine needles could be directly prepared via the tetrapolymerization of phthalodinitrile catalyzed by DBU in solvothermal reactions. Similar to the preparation of β-phthalocyanine crystals, the α metal-free phthalocyanine crystals with the specific multiply-laminated structures can be obtained through solvothermal reactions assisted by DBN. SEM characterization showed that the individual β metal-free phthalocyanine has a well-defined quadrangular shape with smooth faces. However, the α metal-free phthalocyanine exhibits a distinctive undulating surface morphology. Both phthalocyanines showed satisfactory thermal stability (from room temperature to about 300 °C), excellent resistance to acid/alkali solution, and fast photoelectric response properties (order of magnitude of response time, 10⁻⁶ s) as tested by TG-DSC and TPV, respectively. It is noted that ethanol was used as the reaction medium and the resulting phthalocyanine crystals can be facilely purified using hot ethanol to dissolve the impurities adsorbed on the surfaces of phthalocyanine crystals. Compared to the traditional methods, no re-crystallization operation was carried out for our method. To the best of our knowledge, this is the first report on the solvothermal synthesis of metal-free phthalocyanine crystals with controllable crystal form adjusted by DBU/DBN in one step.

The quadrangular β phthalocyanine and multiply-laminated α phthalocyanine crystals could be synthesized via a solvothermal route by using DBU and DBN, respectively.     

Novel amphiphilic graft block azobenzene-containing copolymer with polypeptide block: synthesis,self-assembly and photo-responsive behavior

Well-defined amphiphilic graft block azobenzene-containing copolymer with polypeptide block was synthesized via a combination of copper-mediated atom transfer radical polymerization (ATRP), ring-opening polymerization and click reaction. The alkyne-terminated poly[6-(4-methoxy-azobenzene-4′-oxy)hexyl methacrylate] (PAzoMA) was synthesized by ATRP with a bromine-containing alkyne bifunctional initiator, and the azido-terminated poly(γ-2-chloroethyl-l-glutamate) (PCELG) was synthesized by ROP of γ-2-chloroethyl-l-glutamate-N-carboxyanhydride (CELG-NCA), then the two homopolymers were conjugated by click reaction to afford block azobenzene-containing copolymer PAzoMA-b-PCELG. The chloro groups in PCELG block were transformed into azido groups via azide reactions, and the alkyne-terminated MPEG was grafted to the polypeptide block to afford the final product PAzoMA-b-poly((l-glutamate)-graft-methoxy poly(ethylene glycol)) (PAzoMA-b-(PELG-g-MPEG)) by click reaction. Giant vesicles (micrometer size) were obtained from the amphiphilic graft block copolymer PAzoMA-b-(PELG-g-MPEG) through a solution self-assembly due to the rigid PAzoMA chains and polypeptide chains with the α-helical structure. The investigation of the photo-isomerization behavior of PAzoMA-b-(PELG-g-MPEG) in solution and in vesicular solution showed trans–cis isomerization in solution was quicker than that in vesicular solution and azobenzene J-aggregates in the vesicle solution were only observed. The formation mechanisms of the vesicles were also explored. The research results may provide guidelines for the study of complex copolymers containing different types of rigid chains.

Giant vesicles (micrometer size) were prepared from novel amphiphilic graft block azobenzene-containing copolymer with polypeptide block synthesized via a combination of ATRP, ROP and click reaction.