Novel polyisobutylene/poly(dimethylsiloxane) bicomponent networks. I. Synthesis and characterization

The synthesis of novel polyisobutylene (PIB)/poly(dimethylsiloxane) (PDMS) bicomponent networks is described. The synthesis strategy (see Figure 1) was to prepare well-defined and -characterized allyl-tritelechelic polyisobutylenes [ϕ(PIB—C—C=C)₃] and SiH-ditelechelic poly(dimethylsiloxanes) (HSi–PDMS–SiH) and then crosslink these moieties by hydrosilation. The ϕ(PIB—C—C=C)₃ was prepared by living isobutylene polymerization followed by end-quenching with allyltrimethylsilane, whereas the HSi–PDMS–SiH was obtained by equilibrium polymerization of octamethylcyclotetrasiloxane and tetramethyldisiloxane. The detailed structures of the starting polymers were characterized by GPC and ¹H-NMR spectroscopy. A series of PIB/PDMS bicomponent networks of varying compositions and average molecular weights between crosslinks (M _c) of ∼ 20,000 g/mol were assembled. Optimum crosslinking conditions were defined in terms of H₂PtCl₆ catalyst concentration, nature of solvent, time, temperature, and stoichiometry of ∼ CH₂CH=CH₂/∼SiH groups, allowing for the convenient synthesis of well-defined model bicomponent networks. Swelling studies and elemental analysis confirm the correctness of the synthetic strategy. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1891–1899, 1998     

Structure of a self-assembled hydrogen-bonded ‘living’ main chain liquid crystalline polymer

A main chain hydrogen-bonded liquid crystalline polymer was formed by melt mixing two complementary components, A and B, which in their individual states do not exhibit liquid crystallinity. The structure of the polymer and the thermal stability of its mesophase were studied using synchrotron radiation SAXS/WAXS/DSC at Daresbury (UK) and by variable temperature Fourier transform infrared. The chain extension, or “polymerization” process, was accelerated at the point when the polymer formed a liquid crystalline phase upon cooling from the isotropic melt. The polymer has an aabb chain structure and forms a smectic layer with a length of the A-B repeating unit. The hydrogen-bonded main chain polymer studied here is a monotropic liquid crystal. Above 150°C, it exhibits kinetic stabilization of its monotropic smectic phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1617–1624, 1998     

Chemical Synthesis of a Circular Protein Domain: Evidence for Folding-Assisted Cyclization

Extremely fast cyclization of the linear polypeptide precursor 1 takes place to form 2 . The reaction appears to be assisted by the native fold of 1 , which positions the reactive ends in close proximity. The circular topology has no influence on the folding or function of 2 .     

A new Na/Mg inverse crown ether

A `missing' member of the inverse crown ether family, namely μ₄‐oxo‐tetrakis(μ‐2,2,6,6‐tetra­methyl­piperidinido)­di­mag­nes­ium­(II)­disodium(I), [Na₂Mg₂O(C₉H₁₈N)₄], has been synthesized by blocking the alternative aromatic metallation route via the use of sterically hindered 1,3,5‐mesityl­ene as a solvent. [Na₂Mg₂O(NR₂)₄] (NR₂ is 2,2,6,6‐tetra­methyl­piperidinide) is shown to form a cationic planar eight‐membered ring with alternating metal and N atoms, which captures at its core an oxide guest that lies on an inversion centre [principal dimensions: Na—O = 2.2405 (11) Å, Na—N = 2.445 (3) and 2.572 (3) Å, Mg—O = 1.8673 (9) Å, and Mg—N = 2.032 (2) and 2.063 (2) Å].     

Star–block polymers of multiple polystyrene-b-polyisobutylene arms radiating from a polydivinylbenzene core

The synthesis together with mechanical property and rheological characterization of novel star–block copolymers comprising multiple polystyrene (PSt)-b-polyisobutylene (PIB) arms emanating from polydivinylbenzene (PDVB) cores are described. The synthesis strategy involved the preparation of PSt-b-PIB-Cl^t (i.e., diblocks fitted with a tert-chlorine terminus at the PIB end) by sequential living block polymerization of St and IB, ionizing the -Cl^t terminus by TiCl₄ at room temperature, and linking the PSt-b-PIB^⊕ prearms by DVB. Molecular characterization was effected mainly by triple detector GPC including refractive index (RI)-, UV-, and laser light scattering (LLS)-GPC traces. Evidence for intra- and intermolecular reactions between individual star–blocks is presented and a comprehensive mechanism to the final product is proposed. The stress–strain behavior of star–blocks has been studied and is compared with those of linear triblocks (i.e., two-arm stars) of similar arm molecular weights and composition in the 25–70°C range. The mechanical properties of star–blocks are invariably superior to those of the triblocks over the entire temperature range. The rheological behavior of star–blocks and linear triblocks has been compared in terms of dynamic viscosity at various frequencies. Star–blocks exhibit significantly lower melt viscosities than their linear counterparts, which signals improved processing behavior. We have also compared select rheological properties of the commercially available PSt-b-(hydrogenated-1,4-polybutadiene)-b-PSt thermoplastic elastomer (Kraton G 1650) with those of PIB-based linear triblocks and multiarm star–blocks of similar glassy/rubbery compositions. The melt viscosities of PIB-based triblocks and star–blocks were significantly lower than that of Kraton G over the entire frequency range investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2235–2243, 1999     

Interaction energies of ZnO in acrylic polymer lattices

Water‐based acrylic polymers are frequently used as binders in ceramic materials that contain ZnO as a major component. Thin flexible ceramic films used in semiconductor elements are prepared from ceramic powder, polymer binder, dispergant and plasticizer. In the present work, the chemical reaction of acrylic acid, a part of the polymer, and ceramic powder (ZnO) has been studied by semi‐empirical calculations and Fourier transform infrared spectroscopy (FTIR). Systematic evaluation of model structures using varying contents of acrylic acid showed that ZnO adhesion increases as the carboxylate content in the polymer system increases. Interaction energy surfaces provide a quantitative insight into the spatial distribution of ZnO particles in various model structures. These results are also in good agreement with our infrared spectroscopy measurements.     

A study of Src SH2 domain protein-phosphopeptide binding interactions by electrospray ionization mass spectrometry

The noncovalent binding of various peptide ligands to pp60^src (Src) SH2 (Src homology 2) domain protein (12.9 ku) has been used as a model system for development of electrospray ionization mass spectrometry (ESI-MS) as a tool to study noncovalently bound complexes. SH2 motifs in proteins are critical in the signal transduction pathways of the tyrosine kinase growth factor receptors and recognize phosphotyrosine-containing proteins and peptides. ESI-MS with a magnetic sector instrument and array detection has been used to detect the protein-peptide complex with low-picomole sensitivity. The relative abundances of the multiply charged ions for the complex formed between Src SH2 protein and several nonphosphorylated and phosphorylated peptides have been compared. The mass spectrometry data correlate well to the measured binding constants derived from solution-based methods, indicating that the mass spectrometry-based method can be used to assess the affinity of such interactions. Solution-phase equilibrium constants may be determined by measuring the amount of bound and unbound species as a function of concentration for construction of a Scatchard graph. ESI-MS of a solution containing Src SH2 with a mixture of phosphopeptides showed the expected protein-phosphopeptide complex as the dominant species in the mass spectrum, demonstrating the method’s potential for screening mixtures from peptide libraries.