Jiří Brus

Cage-like structure formation during sol-gel polymerization of glycidyloxypropyltrimethoxysilane

Abstract The sol–gel polymerization of 3-glycidyloxypropyltrimethoxysilane (GTMS) was followed by size exclusion chromatography and 29Si NMR. Extensive non-random cyclization under formation of polyhedral cycles – cubic cages – predominates at the beginning of the reaction. Structure growth of polysilsesquioxanes proceeds by combining the incompletely condensed cage frameworks. The extent of the cage formation increases with dilution and the amount of water and depends appreciably on a catalyst. The cage fraction was isolated from a reaction mixture using preparative size exclusion chromatography and identified by 29Si NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). High content of polyhedral cages prevents gelation of the trifunctional GTMS monomer. Reaction of pendant epoxy groups is much slower; however, at a late reaction stage the epoxy hydrolysis can be significant. Under some conditions, like base catalysis, polysilsesquioxane clusters are crosslinked by intermolecular condensation of SiOH with hydrolyzed epoxy groups and the system gels. The cage with epoxide functionalities may serve as a rigid precursor of crosslinking.

Structure of silicon oxycarbide glasses derived from poly(methylsiloxane) and poly[methyl(phenyl)siloxane] precursors

Abstract The preparation of silicon oxycarbide glasses and structure changes of polysiloxane precursors during heat-treatment in oxidative and nitrogen atmosphere are studied by 13 C and 29 Si magic angle spinning (MAS) NMR, Fourier-transform infrared (FTIR) spectroscopy and thermogravimetry analysis (TG). The reaction mechanisms in inert and oxidation conditions are discussed with respect to the chemical composition of the used precursors. We focused on the formation of new silicon oxycarbide units as well as a highly condensed aromatic carbon phase, the structure of which is discussed. A correlation of thermo-oxidative properties with chemical structure of prepared materials as well as with conditions of their preparation is given.

Solution and cross-polarization/magic angle spinning NMR investigation of intramolecular coordination SnN in some organotin(IV) C,N-chelates

Abstract An intramolecular donor/acceptor SnN bonding connection in a set of triphenyl- and diphenyl-(halogeno)tin(IV) C,N-chelates, Ph2XSnL, where Ph=C6H5, X=Ph, Cl or Br and L1=2-(dimethylaminomethyl)phenyl-, C6H4(CH2NMe2)-2, and L2=2,6-bis[(dimethylaminomethyl)phenyl]-, C6H3(CH2NMe2)2-2,6, respectively, was studied by 119Sn, 15N, 13C and 1H NMR spectroscopy in solution of non-coordinating solvent (CDCl3) and by 119Sn cross-polarization/magic angle spinning NMR techniques in the solid-state. The existence of SnN coordination bonds was confirmed in studied compounds and their strengths were evaluated through the values of NMR spectra parameters of nuclei directly involved in SnN connection, namely by characteristic changes of chemical shifts δ(119Sn) and δ(15N) and values of J(119Sn, 13C) and J(119Sn, 15N) coupling constants. The set was extended by compound [2,6-C6H3(CH2NMe2)2]PhSnCl2 (5a), that is the decomposition product of compound [2,6-C6H3(CH2NMe2)2]Ph2SnCl (5). This 5a was characterized by NMR spectroscopy and its structure was estimated by X-ray diffraction techniques.

Solid-State NMR Study of Phase Separation and Order of Water Molecules and Silanol Groups in Polysiloxane Networks

Homogeneity and structure of organically modified polysiloxane networks prepared by sol-gel co-condensation, as well as location and nature of water molecules and silanol groups were studied by 1D and 2D solid-state NMR. 1H–29Si and 1H–1H interatomic distances were estimated from variable contact-time CP/MAS experiments, 1H NMR chemical shifts and off-resonance WISE NMR. A structure model of these networks is proposed and discussed. The fraction of proton-inaccessible units Q4 in the networks decreases with increasing amounts of dimethylsiloxane (D) and methylsiloxane (T) units. In contrast to systems prepared by co-condensation of tetraethoxysilane (TEOS) with dimethyl(diethoxy)silane (DMDEOS), proton-inaccessible units form essential fraction in networks prepared by co-condensation of TEOS with methyl(triethoxy)silane (MTEOS). The proton-accessible part of the networks with high O/Si ratios is nano-heterogeneous phase, which is composed of water containing Qi particles separated by copolymer domains. The overall homogeneity and uniformity of binding sites around silanol groups increases by co-condensation TEOS with DMDEOS or MTEOS, while the amount of physisorbed water as well as the hydrogen bond strength decreases, as compared with neat silica gel prepared by polycondensation of TEOS.

Novel “soft” biodegradable nanoparticles prepared from aliphatic based monomers as a potential drug delivery system

The search for new biomaterials intended for biomedical applications has considerably intensified in recent years. Herein, the synthesis and characterization of a new aliphatic biodegradable copolyester named PBS/PBDL (poly(butylene succinate-co-butylene dilinoleate)) is reported. Surfactant-free, narrowly distributed, nanosized spherical particles (RH < 60 nm) have been produced from the biodegradable material by applying a single-step nanoprecipitation protocol. Their structure was characterized in detail by employing a variety of scattering techniques and transmission electron microscopy (TEM). Combined SLS and DLS measurements suggested that the nanoparticles comprise a porous core conferring a non-compact characteristic. Their porosity enables water to be entrapped which is responsible for their pronounced stability and relatively fast degradation as followed by size exclusion chromatography (SEC). The polymeric nanoparticles could be loaded with the hydrophobic model drug paclitaxel (PTX) with an encapsulation efficiency of ∼95% and drug loading content of ∼6–7% wdrug/wpolymer. The drug release was followed by HPLC and scattering measurements (DLS, SLS and SAXS). The drug encapsulation and release modifies the inner structure of the nanoparticles, which holds a large amount of entrapped water in the drug-free condition. PTX encapsulation leads to replacement of the entrapped water by the hydrophobic model drug and to shrinking of the nanoparticles, probably due to favorable drug–polymer hydrophobic interactions. Cell viability experiments demonstrated that the nanoparticles are biocompatible and non-toxic, making them potentially useful for applications in nanomedicine.

The multifunctional role of ionic liquids in the formation of epoxy-silica nanocomposites

This work addresses the use of ionic liquids (ILs) as additives for formation of epoxy-silica nanocomposites, via the simultaneous sol–gel process and epoxy network build-up. The application of different methylimidazolium based ILs allows controlling the silica structure and modifying interphase interaction, thus producing hybrids with diverse morphologies and improved mechanical properties. Both the anionic and cationic components of the ILs affected the hybrid formation and the final properties. The application of 1-decyl-3-methylimidazolium tetrafluoroborate ionic liquid together with HCl as an acid catalyst promotes both hydrolysis and condensation in the sol–gel process as well as the self-assembly ordering of the IL. This system produces a very fine hybrid morphology with well dispersed silica nanodomains and a significantly increased rubbery modulus due to physical crosslinking by the ordered domains of decyl-substituents.

Copolymerization of tetraethoxysilane and dimethyl(diethoxy)silane studied by 29Si NMR and ab initio calculations of 29Si NMR chemical shifts

Abstract Copolymerization of tetraethoxysilane (TEOS) and dimethyl(diethoxy)silane (DMDEOS) was studied by means of the 29Si NMR spectroscopy and ab initio quantum chemical calculations. 29Si NMR spectra of reaction mixtures of TEOS and DMDEOS were effectively measured by a slightly modified DEPT experiment, in which the pulse sequence is divided into two parts and all pulses on protons are replaced by selective pulses. Quantum chemical ab initio calculation of structures and 29Si NMR chemical shifts of some reaction products were used in signal assignment. The assignment of the signals to corresponding structure units is not yet unambiguous; however, a semiquantitative analysis of relations in the reaction mixture of TEOS and DMDEOS could be done. Although the reactivity of the DMDEOS monomer is much higher in comparison with TEOS, the arising gel is not strictly phase separated and copolymerization of both monomers occurs. In the first stages of the process, resulting oligomers are composed of 70% of DMDEOS structure units. In the final stages of polycondensation, domains of the TEOS structure units are formed, which can act as linking units between cyclic oligomers.

Multifunctional ATRP macroinitiators for the synthesis of graft copolymers

Abstract Multifunctional ATRP macroinitiators, polystyrene with 1-(2-bromopropionyloxy)ethyl or 1-(2-bromoisobutyryloxy)ethyl groups in the benzene rings and poly[4-methylstyrene- co -4-(bromomethyl)styrene], were synthesized. All the functionalized polymers were characterized by IR, 1 H and 13 C NMR spectroscopy and by size exclusion chromatography.

Structural and Surface Properties of Novel Polyurethane Films

In this work, novel polycarbonatediol- and polybutadienediol-based polyurethanes (PU) with promising properties were prepared and compared with an analogous polypropyleneglycol-based product. Hexamethylenediisocyanate was used as isocyanate (NCO) component. All three PU systems were subsequently modified by the incorporation of two different nanofillers (Montmorillonite clays): “Cloisite 15A” and “Bentonite for organic systems.” The PUs were prepared in form of film coatings and characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and wide angle X-ray scattering (WAXS). Their chemical microstructure was checked by 13C-NMR spectroscopy.

Solid-state NMR study of structure, size and dynamics of domains in hybrid siloxane networks

Structure, size and dynamics of domains of hybrid siloxane networks prepared by copolymerization of tetraethoxysilane (TEOS) and dimethyl(diethoxy)silane (DMDEOS) were studied by several techniques of solid-state 1 H and 29 Si NMR spectroscopy as well as by ab initio quantum chemical calculations. T1r ( 1 H) relaxation times, dipolar dephasing and spin diffusion experiments confirmed the existence of nanoheterogeneous system with random, bicontinuous morphology. The size of domains of TEOS homopolymer is about 1.3‐2.8 nm, the size of copolymer phase being larger, ca. 4.0‐6.8 nm. Quantum chemical ab initio calculations of geometry and the principal values of 29 Si NMR chemical shift tensor and the isotropic chemical shift confirmed our structural predictions that copolymers with polycyclic structure units are formed. The accord of calculated values with those experimentally determined was sufficiently good.q 2000 Elsevier Science Ltd. All rights reserved.