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- Linear and crosslinked copolymers of p-tert-butylcalix[4]arene derivatives and styrene: new synthetic approaches to polymer-bound calix[4]arenesPublication . Mendes, Ana R.; Gregório, Carla C.; Barata, Patrícia; Costa, Alexandra I.; Prata, José V.As an extension of our previous studies concerning the free radical copolymerisation of 25,26,27-tripropoxy-28-(4-vinyl-benzyloxy)-p-tert-butylcalix[4]arene (3) with styrylic monomers, we report herein on the synthesis and characterisation of new terpolymers derived from 3, styrene and divinylbenzene, having nominal crosslinking degrees ranging from 4% to 40% wt. The terpolymers exhibited good thermal stabilities (DSC) and were prepared in good yields. Depending on the reaction conditions (dilution degree and aqueous phase to porogen ratio), materials with identical nominal crosslinking but otherwise differentiated morphologies and swelling abilities were obtained. In a related study, the radical polymerisation of styrene was carried out in the presence of a novel calix[4]arene derivative 4, bearing two distal benzyl–vinyl groups in the lower rim. It is shown that, albeit the presence of two phenolic groups within the calixarene moiety which could have functioned as inhibitors of the free radical polymerisation, the macrocycle was able to take part in the copolymerisation reaction, yielding new soluble and crosslinked polymers. In both cases, no pendant vinyl groups were found in the polymeric materials. The probable mechanisms underlying their formation are discussed.
- The synthesis of novel polymer-bound calix[4]arenesPublication . Barata, Patrícia; Costa, Alexandra I.; Granja, Paulo; Prata, José V.Two simple and efficient procedures for the synthesis of linear and cross-linked polymers containing hydrophobic and hydrophilic lower rim derivatives of p-tert-butylcalix[4]arenes are described. The target polymers were prepared either through the direct attachment of 25,26,27,28-tetrahydroxy-p-tert-butylcalix[4]arene (1), or its tripropoxy (2) and tetracarboxymethoxy (4) derivatives, to light cross-linked Merrifield's resins (1–2% DVB) or via radical addition copolymerisation of 25,26,27-tripropoxy-28-(4-vinyl-benzyloxy)-p-tert-butylcalix[4]arene (11) with styrene and a cross-linking agent.
- Comparative study of the copolymerization kinetics of mono and divinylbenzyl p-tert-butylcalix[4]arene derivatives and styrenePublication . Costa, Alexandra I.; Barata, Patrícia; Prata, José V.A study of the copolymerization kinetics of 25,27-bis(4-vinyl-benzyloxy)-26,28-dihydroxy-p-tert-butylcalix[4]arene (1) and 25,26,27-tripropoxy-28-(4-vinyl-benzyloxy)-p-tert-butylcalix[4]arene (2) with styrene (St) was undertaken. The radical copolymerizations were carried outin THF in thepresence ofbenzoyl peroxideat 758C for a certain period. Six molar feed ratios, ranging from 1:1 to 1:20 (1 or 2 to St), were used to calculate the reactivity parameters. The copolymer composition was determined by FT-IR spectroscopy using a Beer’s law plot obtained from the corresponding homopolymers. The reactivity ratio calculations were performed with the linearization methods of Fineman-Ross (F-R) and Kelen-Tu ¨do ¨s (K-T), assuming the validity of the so-called terminal model. In the copolymerization of the monoene 2, similar reactivity ratios were found for the comonomers (ca. 1.2; K-T). On the other hand, the reactivity ratios calculated for the copolymerization of 1 with St yielded rSt 5 0.67 and rcalix 5 3.0 (K-T method). The higher reactivity of monomer 1 as compared to styrene is discussed in connection with our previously postulated cyclocopolymerization route.
- Studies towards the living polymerisation of phenylethynyl-calix[4]arene compounds with rhbased ternary catalytic systemsPublication . Costa, Alexandra I.; Prata, José V.The living polymerisation of mono and difunctional phenylethynylcalix[4]arene compounds 1 and 2 by Rh(I) ternary catalytic systems (TCS) was examined. Two TCS were tentatively prepared in situ, adapting known methodologies: (1) Rh(CuCPh)(norbornadiene)(PPh3) and (2) Rh(C(Ph)CvCPh2)(norbornadiene)(PPh3). Using the first TCS, the conjugated polymers poly 1 and poly 2 could be obtained in very good yields (77–86%), in short reaction times and freed from low-molecular-weight products, only when NEt3 was used as a co-catalyst. With the second TCS, excellent results were obtained. Indeed,thiscatalyticsystemprovedtobequiteefficientin thepolymerisation ofcalix[4]arenes 1and 2, affording the correspondent poly 1 and poly 2 essentially in almost quantitative yields (by GPC analysis), under appropriate conditions. The living nature of the polymerisation has been proved. For instance, in the case of calix[4]arene 1, the Mn of the polymer obtained at high monomer conversion increased proportionally with the [1]:[Rh] molar ratio in the feed, keeping [1]o constant, thus showing that irreversible chain transfer or termination reactions did not occur to a major extent. Under the most favourable conditions {[1]:[Rh] 5 50 and [2]:[Rh] 5 50}, the polydispersities of poly 1 and poly 2 were kept in a narrow range (1.16–1.30).
- Novel laser-induced luminescence resulting from benzophenone/O-propylated p-tert-butylcalix[4]arene complexes: a diffuse reflectance studyPublication . Ferreira, Luis F. Vieira; Ferreira, Margarida R. Vieira; Silva, José P. da; Machado, Isabel Ferreira; Oliveira, Anabela S.; Prata, José V.Laser-induced room temperature luminescence of air-equilibrated benzophenone/O-propylated p-tert-butylcalix[4]arene solid powdered samples revealed the existence of a novel emission, in contrast with benzophenone/p-tertbutylcalix[4]arene complexes, where only benzophenone emits. This novel emission was identified as phosphorescence of 1-phenyl-1,2-propanedione, which is formed as the result of an hydrogen atom abstraction reaction of the triplet excited benzophenone from the propoxy substituents of the calixarene. Room temperature phosphorescence was obtained in air-equilibrated samples in all propylated hosts. The decay times of the benzophenone emission vary greatly with the degree of propylation, the shortest lifetimes being obtained in the tri- and tetrapropylated calixarenes. Triplet–triplet absorption of benzophenone was detected in all cases, and is the predominant absorption in the p-tert-butylcalix[4]arene case, where an endo-calix complex is formed. Benzophenone ketyl radical formation occurs with the O-propylated p-tert-butylcalix[4]arenes hosts, suggesting a different type of host/guest molecular arrangement. Diffuse reflectance laser flash photolysis and gas chromatography–mass spectrometry techniques provided complementary information, the former about transient species and the latter regarding the final products formed after light absorption. Product analysis and identification clearly show that the two main degradation photoproducts following laser excitation in the propylated substrates are 1-phenyl1,2-propanedione and 2-hydroxybenzophenone, although several other minor photodegradation products were identified. A detailed mechanistic analysis is proposed. While the solution photochemistry of benzophenone is dominated by the hydrogen abstraction reaction from suitable hydrogen donors, in these solid powdered samples, the α-cleavage reaction also plays an important role. This finding occurs even with one single laser pulse which lasts only a few nanoseconds, and is apparently related to the fact that scattered radiation exists, due to multiple internal reflections possibly trapping light within non-absorbing microcrystals in the sample, and is detected until at least 20 µs after the laser pulse. This could explain how photoproducts thus formed could also be excited with only one laser pulse.
- Diffuse reflectance studies of β-phenylpropiophenone and benzophenone inclusion complexes with calix[4], [6] and [8]arenesPublication . Ferreira, Luís F. Vieira; Ferreira, Margarida R. Vieira; Oliveira, Anabela S.; Branco, T. J. F.; Prata, José V.; Moreira, J. C.The formation of inclusion complexes of β-phenylpropiophenone and benzophenone with p-tert-butylcalix[4], [6] and [8]arenes (powdered solid samples) was studied with the use of diffuse reflectance techniques. Experimental evidence was obtained for inclusion in all cases. The benzophenone n → π* absorption band exhibits hypsochromic shifts with an increase of the calixarene ring aperture, providing evidence for an increase in cavity polarity. β-Phenylpropiophenone exhibits strong room temperature phosphorescence in contrast to its solution behaviour, and this is a clear evidence for the inclusion of this probe within the calixarene molecules. Calix[4]arene and calix[8]arene provide a more rigid environment to β-phenylpropiophenone whereas calix[6]arene appears to be more flexible. Transient absorption spectra of inclusion complexes with benzophenone (1 : 1 mol : mol, excited at 355 nm) show major triplet formation in the calix[4]arene case, while the ketyl radical of benzophenone is formed in the calix[6]arene case and is predominant in the calix[8]arene case. Phenoxyl radical transient absorption also becomes evident in the last two cases. For β-phenylpropiophenone (1 : 1 mol : mol) phenoxyl radicals are formed but no evidence was found for triplet or ketyl radical formation of this ketone in the calix[4]arene case. However, strong support for the ketyl radical of the aryl alkyl ketone was found for calix[6]arene and calix[8]arene inclusion complexes.
- Intramolecular addition of acyldiazenecarboxylates onto double bonds in the synthesis of heterocyclesPublication . Prata, José V.; Clemente, Dina-Telma S.; Prabhakar, Sundaresan; Lobo, Ana M.; Mourato, Isabel; Branco, Paula SérioAppropriate aryl-substituted unsymmetrical azodicarbonyl compounds, generated from bishydrazides by oxidation, undergo intramolecular cyclisations to furnish a variety of useful heterocycles such as N-substituted oxindoles, carbostyrils, benzazepinones, benzazocinones, benzimidazolones, benzoxazinones and pyrazolones in varying degrees of efficiency. Methods are described to remove the N-acyl groups from the heteroaromatic compounds. Under mildly acidic conditions where equal opportunities are available for an ipso or a normal cyclisation it is the former process that occurs preferentially. Evidence is presented in favour of a C-to-C migration in the ipso product for the formation of a methoxy-substituted carbostyril derivative. One of the spiro substances is shown to participate in dienone–phenol rearrangement to provide the corresponding quinolone–phenol in high yield.
- Novel fluorescent (p-Phenylene ethynylene)-Calix[4]arene- based polymer: design, synthesis, and propertiesPublication . Costa, Alexandra I.; Ferreira, Luís F. V.; Prata, José V.A novel fluorescent (p-phenylene ethynylene)-calix[4]arene-based polymer (CALIX-PPE) has been successfully synthesized by cross-coupling polymerization of bis-calix[4]arene 1 with 1,4-diethynylbenzene. The polycondensation was carried out in toluene/NEt3 at 35 8C for 24 h, using PdCl2(PPh3)2/CuI as the catalytic system, furnishing CALIX-PPE in excellent isolated yields (higher than 95%, several runs). The yellow polymer is freely soluble in several nonprotic organic solvents. The GPC trace of the isolated polymer showed a monomodal distribution and a number-average molecular weight of 23,300 g mol-1 (Mw/Mn¼ 2.05). No evidence was found in the structural analysis (FTIR and 1H/13C NMR) regarding the formation of alkyne homocoupled segments along the polymer chain. For comparative purposes, the syn- thesis of an analogous poly(p-phenylene ethynylene) containing p-t-butyl-phenoxy- methyl side chains (TBP-PPE) was also undertaken. A great similarity was found between the photophysical properties of CALIX-PPE and TBP-PPE in solution (UV–vis and laser induced luminescence), clearly demonstrating their unique de- pendence on the structure and conformation of the conjugated PPE backbone. The fluorescence spectra of polymers are of nearly identical shape, displaying their maxi- mum emission around 420 nm. The calculated solution photoluminescence quantum yields of CALIX-PPE and TBP-PPE are of similar magnitude (Ø/F(CALIX-PPE) = 0.43; Ø/F(TBP-PPE) = 0.51).
- Radical cyclopolymerization of a divinylbenzyl-p-tert-butylcalix[4]arene derivativePublication . Costa, Alexandra I.; Barata, Patrícia; Prata, José V.The synthesis and characterization of a new homopolymer (poly 1), obtained in the course of the radical polymerization of 25,27-bis-(4-vinyl-benzyloxy)-26,28-dihydroxy-p-tert-butylcalix[4]arene (1), is described. Homopolymerization of 1 in THF, using BPO or thermal initiation, afforded soluble polymers in good isolated yields (60–90%). Gel permeation chromatography (GPC) profiles showed unimodal distributions for all the analyzed polymers, which is indicative that chain branching reactions did not occur to a major extent. Molecular weights (Mn) ranging from 30,000 to 60,000 g mol−1 were reached within a 8 h period, when the reactions were conducted at 0.06–0.5 mol% of BPO or thermally initiated, showing relatively narrow polydispersity indexes (1.5–2.0). The structure of the polymers was deduced upon analysis of their 1H NMR and FT-IR spectra, which, in conjunction with GPC and solubility data led to their formulation as cyclopolymers.
- Synthesis and Rh(I)‐catalyzed polymerization of 1,3‐diphenylyne–calix[4]arene compounds: novel conjugated, calixarene‐based polymersPublication . Costa, Alexandra I.; Prata, José V.The synthesis of two 1,3‐bis(4‐ethynylbenzyloxy)calix[4]arenes, 5,11,17,23‐tetrakis(1,1‐dimethylethyl)‐25,27‐bis(4‐ethynylbenzyloxy)‐26,28‐dihydroxycalix[4]arene (1 ) and 25,27‐bis(4‐ethynylbenzyloxy)‐26,28‐dihydroxycalix[4]arene (2 ), was accomplished through Sonogashira coupling of appropriate calixarene derivatives. Methods for the polymerization of these bifunctional building blocks with Rh(I) as a catalyst, leading ultimately to conjugated polymers having calix[4]arene units incorporated into the main chain, were explored. Calixarenes 1 and 2 were efficiently polymerized with rhodium‐based initiators and afforded the conjugated polymers poly{5,11,17,23‐tetrakis(1,1‐dimethylethyl)‐25,27‐bis(4‐ethynylbenzyloxy)‐26,28‐dihydroxycalix[4]arene} (poly 1 ) and poly{25,27‐bis(4‐ethynylbenzyloxy)‐26,28‐dihydroxycalix[4]arene}. Depending on the conditions, high conversions and good yields were obtained. The effects of adding cocatalysts (NHEt2 and/or PPh3) were studied in connection with the number‐average molecular weight and the molecular weight distribution of the resultant polymer (poly 1 ) and tentatively correlated with the formation of low‐molecular‐weight materials. A catalytic system containing triphenylphosphine as the sole additive ([Rh(nbd)Cl]2; [Rh]/[PPh3] = 0.5) proved to be the best for the polymerization of p ‐tert ‐butylcalixarene compound 1 . Linear polymers having high number‐average molecular weights (up to 1.1 × 105 g mol−1) with low polydispersities were produced under these conditions. For debutylated homologue 2 , its polymerization was best carried out in the absence of any added cocatalyst. A cyclopolymerization route, comprising the intramolecular ring closing of the calix[4]arene pendant ethynyl groups followed by an intermolecular propagation step, is advanced to explain the results.