Arborescent

Abstract: The synthesis of novel arborescent (arb; randomly branched, “tree-like,” and often called “hyperbranched”) block copolymers comprised of rubbery polyisobutylene (PIB) and glassy polystyrene (PSt) blocks (arb-PIB-b-PSt) is described. The syntheses were accomplished by the use of arb-PIB macroinitiators (prepared by the use of 4-(2-methoxyisopropyl) styrene inimer) in conjunction with titanium tetrachloride (TiCl4). The effect of reaction conditions on blocking of St from arb-PIB was investigated. Purified block copolymers were characterized by 1H NMR spectroscopy and Size Exclusion Chromatography (SEC). arb-PIB-b-PSt with 11.7–33.8 wt % PSt and Mn = 468,800–652,900 g/mol displayed thermoplastic elastomeric properties with 3.6–8.7 MPa tensile strength and 950–1830% elongation. Samples with 26.8–33.8 wt % PSt were further characterized by Atomic Force Microscopy (AFM), which showed phase-separated mixed spherical/cylindrical/lamellar PSt phases irregularly distributed within the continuous PIB phase. Dynamic Mechanical Thermal Analysis (DMTA) and solvent swelling of arb-PIB-b-PSt revealed unique characteristics, in comparison with a semicommercial PSt-b-PIB-b-PSt block copolymer. The number of aromatic branching points of the arb-PIB macroinitiator, determined by selective destruction of the linking sites, agreed well with that calculated from equilibrium swelling data of arb-PIB-b-PSt. This method for the quantitative determination of branching sites might be generally applicable for arborescent polymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1811–1826, 2005

Abstract: Scanning force microscopy has been employed to visualize the molecular organization of arborescent graft polystyrenes with different branching densities as monolayer films cast on mica. The apolar polymers were depicted as distinct hexagonally packed globuli whose size was consistent with molecular dimensions obtained from viscosity and dynamic light scattering measurements. Distortions of the long-range order were observed and attributed to the inherent polydispersity of the polymers. The layer thickness, i.e., the particle diameter in the z-direction, depended on the branching density and indicated formation of a prolate shape in the vicinity of the flat substrate after evaporation of the solvent. Molecules with a high branching density (Mb ∼ 500 g/mol, average molecular weight between grafting points) recovered a spherical geometry after annealing above the glass transition temperature. In the case of a lower branching density, e.g., Mb = 2000 g/mol, the “pancake” structure remained stable upon anneali...

Abstract: Cascade-branched, “graft on graft” polybutadienes, denoted arborescent graft polymers, were prepared by hydrosilylation of polybutadiene vinyl groups with chlorodimethylsilane, followed by grafting with living polybutadiene chains. Repeated hydrosilylation−anionic grafting cycles led to a treelike outward growth, yielding well-defined, spherically shaped macromolecules. The molecular weight of the materials increased dramatically with each grafting cycle, since polymer chains rather than small molecules were used as building blocks. The materials were characterized by means of gel permeation chromatography, static light scattering measurements, and viscosimetry.