UV-curing behavior and adhesion performance of polymeric photoinitiators blended with hydrogenated rosin epoxy methacrylate for UV-crosslinkable acrylic pressure sensitive adhesives

UV-crosslinkable polyacrylates were synthesized for use as pressure sensitive adhesives (PSAs). These polyacrylates acted as polymeric photoinitiators due to the benzophenone incorporated into their backbones. Hydrogenated rosin epoxy methacrylate (HREM; based on hydrogenated rosin and glycidyl methacrylate) was also synthesized as a tackifier, and blended at different levels with the synthesized, UV-crosslinkable polyacrylates for use as PSAs. The effect of the new tackifier, HREM, on the properties of the UV-crosslinkable PSAs was examined in comparison with the properties exhibited by PSA/hydrogenated rosin blends. The characteristics of these PSA/tackifier blends were examined by Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and an advanced rheometric expansion system (ARES). In addition, the adhesion performance of the PSA blends was investigated using probe tack tests. DSC and ARES revealed all the PSA blends with HREM or hydrogenated rosin to be miscible at the molecular level. The glass transition temperature (T_g) of HREM was −25.6 °C, which is lower than that of other commercially available rosin tackifiers. FTIR revealed changes in the relative concentration of benzophenone groups in the PSAs at 1580 cm⁻¹, which demonstrated that the crosslinking efficiency is proportional to the benzophenone content and UV dose, but decreases with increasing hydrogenated rosin content. However, the reduced crosslinking reaction efficiency was improved in the PSA/HREM blends due to the low T_g of HREM which only slightly increased the T_g of the PSA blends. Moreover, the relative initial decrease in the probe tack of the PSA/HREM blends was lower than that of the PSA/hydrogenated rosin blends after UV irradiation.     

Thermal field-flow fractionation and multiangle light scattering of polyvinyl acetate with broad polydispersity and ultrahigh molecular weight microgel components

The challenging task of characterizing polydisperse polymer mixtures possessing ultrahigh molecular weight (MW) polymers and microgels in organic solvents is addressed with thermal field-flow fractionation (ThFFF) and multiangle light scattering-differential refractive index (MALS-dRI) detection. In initial experiments, a 350,000 g/mol poly(methyl methacrylate) (PMMA) standard is used to evaluate the effects of temperature gradient and temperature gradient programming on the measurements. dRI baseline fluctuations caused by temperature programming were minimized by using a mobile phase heater to thermostat connecting tubing. ThFFF–MALS-dRI is then used to separate and characterize a complex polyvinyl acetate (PVAc) sample containing ultrahigh MW polymers and microgels. The open channel design employed by ThFFF allowed the PVAc sample to be analyzed with minimal sample preparation. Unfiltered PVAc sample showed components with MWs close to 10⁹ g/mol and root mean square radius r_rms values approaching 400 nm. The same sample, filtered through a 0.5 μm pore-size membrane, yielded a MW that was at least one order of magnitude lower. These results demonstrated that the common practice of prefiltering polymer samples prior to analysis can lead to erroneously low average MWs and polydispersities. The accuracy of MW and r_rms calculated using standard light scattering equations developed for small scattering molecules and relatively high wavelengths is also examined.     

涡旋混合辅助超声提取-高效液相色谱法同时测定水基胶中的苯系物和邻苯二甲酸酯类增塑剂

采用涡旋混合辅助超声提取样品的前处理方式,应用高效液相色谱法同时测定水基胶中的苯系物和5种邻苯二甲酸酯类增塑剂.基于样品基质的属性,采用漩涡混合器混合样品,使样品均匀分散于萃取溶剂中,再使用超声萃取的方式提取目标物.研究了不同萃取溶剂、提取方式及其条件对目标物检测结果的影响.优选了色谱分析所用的流动相、检测波长、流量和进样量等条件.在优化条件下,5种苯系物和5种邻苯二甲酸酯类增塑剂的峰面积与其质量浓度呈线性关系,相关系数(R2)都大于0.997,回收率分别在91.9%~111.4%和88.3%~121.4%之间,相对标准偏差RSD分别在1.41%~2.80%和1.24%~2.94%之之间.检出限分别在0.3~0.6 mg/kg和0.2~0.3 mg/kg之间.研究表明方法简便、耗溶剂量少、准确,与现行标准方法具有较好的吻合性,适合于同时测定水基胶中的苯系物和5种邻苯二甲酸酯类增塑剂.     

A novel synthetic method for preparation of some folates

An improved method was developed for preparation of 5,6,7,8-tetrahydrofolic acid (THF) and calcium-5-methyltetrahydrofolate (5-MTHF-Ca) by reduction of folic acid using KBH₄ catalyzed by Pb(NO₃)₂. The yields of THF and 5-MTHF-Ca were 56.5 and 42.7 %, respectively. A convenient method for measurement of THF and 5-MTHF-Ca using liquid chromatography–mass spectrometry (LC–MS) was also established, enabling analysis of those folates within 10 min without application of gradient elution.     

Multidetector Thermal Field‐Flow Fractionation as a Novel Tool for the Microstructure Separation of Polyisoprene and Polybutadiene

For the first time, it is demonstrated that thermal field‐flow fractionation (ThFFF) is an efficient tool for the fractionation of polyisoprene (PI) and polybutadiene (PB) with regard to molecular microstructure. ThFFF analysis of 1,4‐ and 3,4‐PI as well as 1,4‐ and 1,2‐PB samples in tetrahydrofuran (THF), THF/cyclohexane, and cyclohexane reveals that isomers of the same polymer family having similar molar masses exhibit different Soret coefficients depending on microstructure for each solvent. The separation according to microstructure is found to be based on the cooperative influence of the normal and the thermal diffusion coefficient. Of the three solvents, cyclohexane has the greatest influence on the fractionation of the isomers. In order to determine the distribution of isomeric structures in the PI and PB samples, the samples are fractionated by ThFFF in cyclohexane and subsequently analyzed by ¹H NMR. The isomeric distributions determined from NMR data correlate well with ThFFF retention data of the samples and thus further highlight the unique fractionating capabilities of ThFFF. The interplay of the normal and thermal diffusion coefficients that are influenced by temperature and the mobile phase opens the way to highly selective fractionations without the drawbacks of column‐based separation methods.

     

Nanostructured latexes made by a sequential multistage emulsion polymerization

A series of linear and lightly crosslinked nanostructured latices was prepared by a sequential multistage semicontinuous emulsion polymerization process alternating styrene (S) and n‐butyl acrylate (BA) monomer feeds five times, that is ten stages, and vice versa, along with several control latices. Transmission electron micrographs of the RuO₄‐stained cross sections of nanostructured and copolymer latex particles and films showed that their particle morphologies were not very different from each other, but the nanostructured latex particles were transformed into a nanocomposite film containing both polystyrene (PS) and poly(n‐butyl acrylate) (PBA) nanodomains interconnected by their diffuse polymer mixtures (i.e. interlayers). The thermal mechanical behaviors of the nanostructured latex films showed broad but single T_gs slightly higher than those of their counterpart copolymer films. These single T_gs indicated that their major component phases were the diffuse interlayers and that they behaved like pseudopolymer alloys. The minimum film formation temperatures of nanostructured latices capped with PBA and PS, respectively, were 15 °C lower than and equal to those of their counterpart copolymer latices, but their T_gs were about 10 °C higher. Consequently, nanostructured latices enabled us to combine good film formation with high strengths for adhesives and coatings applications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2826–2836, 2006     

Environmental PAH analysis by gas chromatography–atmospheric pressure laser ionization–time-of-flight–mass spectrometry (GC-APLI-MS)

The application of polycyclic aromatic hydrocarbon (PAH) analysis by gas chromatography coupled with atmospheric pressure laser ionization and mass spectrometry (GC-APLI-MS) to environmental samples was investigated in the study. The limit of detection for 40 PAH in a standard mixture was 5–100 fg, demonstrating GC-APLI-MS to be a highly sensitive technique and more sensitive by a factor of 100–3,500 compared to GC-MS. Acenaphthylene and cyclopenta[cd]pyrene were not detectable <2,500 fg per injection. To make use of this very high PAH sensitivity, the technique was applied to samples of environmental interest with limited available sample amounts such as particulate matter (PM), soot and a sample from a bioaccumulation test with Lumbriculus variegatus. First, special sample preparation was necessary and ultrasonic extraction proved to be suitable, if a thorough clean-up was performed and plastic materials avoided. By GC-APLI-MS and GC-MS, 224 and 28 single PAH compounds were detected in PM, about 1,000 and 15 in birch soot, and 9 and 2 in worm tissue, respectively, revealing the enormous potential of the method. The selectivity of GC-APLI-MS was shown for a crude oil where >2,200 PAH were detected without any sample preparation.     

Electrochemical immunoassay for the prostate specific antigen using ceria mesoporous nanospheres

We report on a sensitive electrochemical immunoassay for the prostate specific antigen (PSA). An immunoelectrode was fabricated by coating a glassy carbon electrode with multiwalled carbon nanotubes, poly(dimethyldiallylammonium chloride), CeO₂ and PSA antibody (in this order) using the layer-by-layer method. The immunosensor is then placed in a sample solution containing PSA and o-phenylenediamine (OPD). It is found that the CeO₂ nanoparticles facilitate the electrochemical oxidation of OPD, and this produces a signal for electrochemical detection of PSA that depends on the concentration of PSA. There is a linear relationship between the decrease in current and the concentration of PSA in the 0.01 to 1,000 pg mL^?1 concentration range, and the detection limit is 4 fg mL^?1. The assay was successfully applied to the detection of PSA in serum samples. This new differential pulse voltammetric immunoassay is sensitive and acceptably precise, and the fabrication of the electrode is well reproducible. Figure

A novel electrochemical immunoassay for prostate specific antigen (PSA) was developed. Ceria (CeO₂) mesoporous nanospheres facilitated the electrochemical oxidation of o-phenylenediamine (OPD). The developed immunoassay has high sensitivity and can be successfully applied for the detection of PSA in serum samples     

Reduced sample recovery in liquid chromatography at critical adsorption point of high molar mass polystyrene

Low sample recovery may represent an important drawback in liquid chromatography at the critical adsorption point (LC-CAP) if the critical eluent is not carefully fitted to the system. So far, this problem was often overlooked and only few experimental examples can be found in literature. We showed that in the case of polystyrene (PS) in a tetrahydrofuran (THF)/n-hexane critical mixed eluent, PS with molar masses higher than 100 kg mol⁻¹ were not eluted from a tandem of two columns packed by bare silica gels with 30 nm and 100 nm pore size, respectively. The polymer trapped within the columns was well recovered after injection of a small volume of pure THF as demonstrated using 2D chromatography. We studied PS conformations by means of small angle neutron scattering and found that the THF/n-hexane critical eluent is in fact a theta solvent for PS. By replacing it by a CH₂Cl₂/n-hexane critical mixture, which is a good solvent for PS, the limits of reduced sample recovery was displaced towards far higher molar masses. Thus, thermodynamic quality of eluent - theta or good solvent - plays an important role on the phenomenon of sample recovery.     

Sample preparation for the chromatographic determination of ecdysteroids using solid-phase extraction methods

The ecdysteroids are hormones widely distributed in insects and crustaceans, where they are involved in the regulation of moulting. Methods of sample preparation based on solid-phase extraction of the ecdysteroids from biological samples for subsequent chromatographic analysis are reviewed. Most methods use an initial partition of the sample (or extract) between water or aqueous methanol with a non-polar solvent to remove lipids. The aqueous portion is applied to a cartridge containing C₁₈-bonded silica gel. Removal of polar impurities is then effected, followed by recovery of the ecdysteroids from the cartridge and chromatographic analysis. Results are given for the use of phenylboronic acid (PBA) bonded to silica gel as a means of obtaining a much more specific method of extraction. The PBA phase was shown to have a high affinity for ecdysteroids containing a 20,22-cis-diol but not for compounds possessing only a 2,3-cis-diol.     

Polystyrene latices containing dodecanamide‐modified poly(propyleneimine) dendrimers

Polymerization of styrene in aqueous dispersions of the dodecanamide derivative of poly(propyleneimine) dendrimer DAB‐dendr‐(NH₂)₆₄ and sodium dodecyl sulfate (SDS) produced stable latices. With initial SDS concentrations of 10 mM or less, molar ratios of SDS to dendrimer end groups ranging from 2.3:1 to 9.5:1, and less than 10 wt % of SDS relative to styrene, the polystyrene latices had diameters of 30–60 nm and coefficients of variation of diameters of less than 10% when measured by transmission electron microscopy. Higher concentrations of SDS gave more polydisperse latices. The polystyrene latices formed with SDS and the dodecanamide‐modified dendrimer were almost the same size and polydispersity as those formed with SDS and the parent primary amine dendrimer DAB‐dendr‐(NH₂)₆₄. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 597–605, 2003     

Dithiadiazolium salts as initiators for the cationic ring-opening polymerization of tetrahydrofuran

Mono-, bis- and tris-(1,3,2,4-dithiadiazolium) salts [R-(CNSNS ⁺)_n]_n+[AsF^-₆]_n (R = aryl, n = 1, 2, 3) were found to initiate the cationic ring-opening polymerization of tetrahydrofuran (THF) at room temperature to give clear gels from which the pure polymer was precipitated. 1,3,2,4-Dithiadiazolium cations associated with the hard [AsF₆]^- anion thus constitute a new class of cationic polymerization initiators. The poly(THF) formed by initiation with 1,3,2,4-dithiadiazolium cation was characterized by gel permeation chromatography, infrared spectrophotometry, and ¹³C-NMR spectroscopy. Number-average molecular weights of 198 700 g mol^-1 (polydispersity 1.96) and 190 000 g mol^-1 (polydispersity 1.61) were obtained using [PhCNSNS ] [AsF₆] and [C₆H₃-1,3,5-(CNSNS )₃][AsF₆]₃, respectively, as initiators. The use of multifunctional dithiadiazolium salts as initiators suggests that they may be useful in the preparation of starburst and dendritic polymers. © 1992 John Wiley & Sons, Inc.     

Influence of fluorination on the characterization of fluorotelomer-based acrylate polymers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The relative degree of fluorotelomer-based acrylate polymers (FTACPs) fluorination was demonstrated to influence the sample preparation protocol for matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry. A homologous series of FTACPs were synthesized from fluorotelomer and hydrocarbon acrylates of different chain lengths, which varied the ratio of perfluorinated to hydrogenated carbons (R_F/R_H). The solubility of FTACPs in tetrahydrofuran (THF) and chloroform was observed to decrease for highly fluorinated FTACPs (R_F/R_H > 0.5) promoting FTACP aggregation. No dependence on the degree of fluorination was observed for the solubility of FTACPs in the fluorinated solvents α,α,α-trifluorotoluene (TFT) or dichloropentafluoropropanes (HCFC-225). For FTACPs with a low degree of fluorination such as poly(8:2 FTAC-co-HDA) (R_F/R_H = 0.375), MALDI-ToF analysis was successful using a conventional sample preparation protocol with THF, and dithranol (Dith) matrix. Conversely, the poor solubility of the highly fluorinated poly(8:2 FTAC-co-BA) (R_F/R_H = 1.5) in THF resulted in mass discrimination. Several fluorinated sample preparation protocols were evaluated for poly(8:2 FTAC-co-BA) using TFT and HCFC-225, and decafluoroazobenzene (DFAB) or 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) matrices. The high volatility of HCFC-225 decreased FTACP pooling during solvent evaporation in comparison to the less volatile TFT, and improved the quantity of detectable signals. MALDI-ToF analysis of poly(8:2 FTAC-co-BA) in a 95:5 HCFC-225:methanol with DCTB being the best sample preparation protocol for highly fluorinated FTACPs in this study producing the highest number of observable signals. Employing a fluorinated sample preparation offers the capability of analyzing other highly fluorinated polymers that are not compatible with conventional sample preparations.     

Polymerization by alkaline earth metal compounds—II Polymerization of 2- and 4-vinylpyridine,styrene and butadiene by triphenylmethyl derivatives of calcium,strontium and barium

Investigations are reported on polymerizations of 2- and 4- vinylpyridine, styrene and butadiene by a series of related alkaline earth metal initiators, Ph₃CMX(THF)_n (M = Ca, Ba, X = Cl, n = 2; M = Ca, X = Br, n = 4; M = Sr, X = Cl, n = 4; M = Sr, X = Br, n = 5) in tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME) at various temperatures and in the absence of solvent. The polymers have been examined by GPC and aspects of their microstructures determined by ¹³C and/or ¹H NMR spectroscopy and, for polybutadiene, i.r. spectroscopy. Poly-2-vinylpyridine produced by Ph₃CMX(THF)_n is rich in isotactic content; the isotacticity is higher for polymer formed in THF than DME solution, falls with change of initiator in the order M = Ca > Sr > Ba and, in DME, is greater when X = Br. The tacticities of poly-4-vinylpyridine and polystyrene are similar to those obtained from related organometallic initiators. The 1,4-content of polybutadiene decreases with initiator Ph₃CMX(THF)_n in the order M = Ba > Sr > Ca; the trans-1,4 structure generally predominates except when M = Ba from which cis-1,4 links are formed in comparable amounts.     

Reaction of thianthrene cation radical with grignard reagents : Evidence for electron transfer and trapping of alkyl radicals by the thianthrene cation radical

Reactions are reported between RMgCl and thianthrene cation radical perchlorate (Th^.+ClO^-₄) suspended in ether and tetrahydrofuran (THF). In ether solution reactions R = Bu, s-Bu, t-Bu, 5-hexenyl, and cyclopentylmethyl. Major products were the alkane, the alkene R(-H) in some cases, and, in the cases of R = Bu, 5-hexenyl, and cyclopentylmethyl, the 5-alkylthianthrenium perchlorate (ThR⁺ClO^-₄). When 5-hexenylMgCl was used a mixture of 5-(5-hexenyl)- and 5-(cyclopentylmethyl)thianthrenium per-chlorates in the ratio of approximately 2 was obtained. Since the ratio of 5-hexenyl/cyclopentylmethyl in the Grignard reagent was 10.4, it is concluded that the C₆ sulfonium ions were formed by radical trapping by Th^.+ after single electron transfer from Grignard to cation radical had occurred, thus allowing for cyclization of 5-hexenyl radical. Formation of ThBu⁺ClO^-₄ is attributed to the trapping of butyl radical by Th^·+, while formation of RH and R(-H) is in all cases also attributed to alkyl radical reactions. Reactions in THF(R = Me, i-Pr, Bu, s-Bu, t-Bu, Ph) led almost exclusively to RH and Th. Polymerization of THF was also initiated and took place slowly giving rise to low molecular weight poly(THF). By using THF-d₈, as solvent for reaction between BuMgCl and Th^.+, it was possible to find Bu groups (¹H-NMR) in the poly(THF-d₈). Polymerization of THF is attributed, in some cases (R = Me, Bu), to alkyl-cation transfer from ThR⁺ to THF. In other cases initiation of polymerization by R⁺ and THF(-H)⁺ is considered.     

Synthesis of N,N-diethyldithiocarbamate functionalized 1,4-polyisoprene, from natural rubber and synthetic 1,4-polyisoprene

N,N-Diethyldithiocarbamate functionnalized 1,4-polyisoprenes were prepared from 1,4-polyisoprenes (natural or synthetic). The syntheses were performed by nucleophilic addition of N,N-diethyldithiocarbamate salts upon oxirane rings of epoxidized units according to a SN2 mechanism with ring opening. Studies on model molecules of epoxidized 1,4-polyisoprene units (1,2-epoxy-1-methylcyclohexane and 4,5-epoxy-4-methyloctane) were previously achieved to develop the procedure. The best yields were obtained at low temperature in polar medium, and more especially in water with sodium N,N-diethyldithiocarbamate (DEDT-Na) as reagent. A diastereospecific addition was noted when reaction was performed in water with DEDT-Na. Afterwards, the developed procedure was successfully generalized to epoxidized synthetic polyisoprenes and epoxidized natural rubber (in THF, then in latex medium). Excellent results were obtained in latex medium with epoxidized natural rubber (ENR) latices. As with the models, a diastereospecific addition of sodium N,N-diethyldithiocarbamate trihydrate onto epoxidized 1,4-polyisoprene units of ENR was observed at the condition to bring the latex medium to pH 8 before introduction of DEDT-Na. Influence of temperature, drc, and DEDT-Na concentration were successively examined to determine the best conditions of the addition on ENR latices.     

Influence of MgCl2 on Grignard reagent composition in tetrahydrofuran. III

Reaction of [MgCl₂(THF)₂] with [NBu₄][BF₄] yields the compounds [NBu₄][MgCl₄] (IV) and [Mg(THF)₆][BF₄]₂ (V). After addition of dioxane the reaction equilibrium shifts in the opposite direction. The formation of [MgCl₂(C₄H₈O₂)₂] in solution does not require the presence of MgCl₂. This compound may be formed in the reaction of dioxane with the ionic or molecular species formed by the magnesium atom in solution. The [NBu₄][BF₄] salt also reacts with the Grignard reagent to produce compound IV which confirms that there is a new equilibrium between [Mg(R)X(THF)_n] and [MgR₂(THF)₂], [MgCl₄]²⁻ and [Mg(THF)₆]²⁺. Bis(tetrahydrofuran)magnesium dichloride, because of its reactivity is only stable in Grignard reagent. For that reason the composition of the Grignard reagent in solution is best described as an equilibrium between [Mg(R)X(THF)_n] and [(THF)₄Mg(μ-Cl)₂MgR₂] and [RMg₂(μ-Cl)₃(THF)₅] rather than as a Schlenk equilibrium.     

A novel quantification strategy of transferrin and albumin in human serum by species-unspecific isotope dilution laser ablation inductively coupled plasma mass spectrometry (ICP-MS)

Species-specific (SS) isotope dilution analysis with gel electrophoresis (GE)-laser ablation (LA)-ICP-MS is a promising technique for the quantification of particular metal-binding proteins in biological samples. However, unavailable isotopically enriched spike and metal losses in GE separation are main limitations for SS-isotope dilution PAGE-LA-ICP-MS. In this study, we report for the first time the absolute quantification of transferrin (Tf) and albumin (Alb) in human serum by non-denaturing (native) GE combined with species-unspecific isotope dilution mass spectrometry (IDMS). In order to achieve a homogeneous distribution of both protein and isotope-enriched spike (simulated isotope equilibration), immersing the protein strips with ³⁴S spike solution after gel electrophoresis was demonstrated to be an effective way of spike addition. Furthermore, effects of immersion time and ³⁴S spike concentration were investigated to obtain optimal conditions of the post-electrophoresis isotope dilution method. The relative mass of spike and ablated sample (m_sp/m_sam) in IDMS equation was calculated by standard Tf and Alb proteins, which could be applied to the quantification of Tf and Alb in ERM-DA470k/IFCC for method confirmation. The results were in agreement with the certified value with good precision and small uncertainty (1.5–3%). In this method, species-specific spike protein is not necessary and the integrity of the heteroatom-protein could be maintained in sample preparation process. Moreover, the application of species-unspecific isotope dilution GE-LA-ICP-MS has the potential to offer reliable, direct and simultaneous quantification of proteins after conventional 1D and 2D gel electrophoretic separations.     

Reaction between magnesium chloride and iron(III) chloride in tetrahydrofuran. The crystal structure of Di-μ-chlorodichloroiron(II) tetrakis(tetrahydrofuran)magnesium(II)

The direct reaction between magnesium chloride and iron(III) chloride in tetrahydrofuran (THF) yields the [MgCl(THF)₅][FeCl₄] complex (I). It is unstable and in reacting with THF undergoes reduction to produce the complex FeCl₂(μ-Cl)₂Mg(THF)₄ (II). Crystals of (II) are orthorhombic, space group Pbcn, with a = 9.18(1) b = 16.30(2) c = 15.85(1)Å. The structure was solved by the heavy atom method and refined by least squares to R = 0.053 for 1194 observed diffractometer data. The molecule is heterobimetallic with an Fe…Mg distance 3.455(3)ǎ and the Fe and Mg atoms are bridged by two Cl atoms.     

Synthesis of chiral pyrrolidine and pyrrole derivatives through the chemoselective Dieckmann reaction of α,β-aminodiesters

α,β-Aminodiesters were allowed to react with t-BuOK in THF at −78°C. The chemoselectivity of the Dieckmann cyclization was controlled by the nature of the substituents R₃ and R₄, allowing the preparation of pyrrolidine or pyrrole derivatives.