A technique is described for the preparation of arborescent copolymers containing poly(2vinylpyridine) (P2VP) segments. 2-Vinylpyridine is first polymerized with 1,1-diphenyl-2-methylpentyllithium in tetrahydrofuran in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA). The graft copolymers are obtained by titration of the P2VP anions with a solution of a chloromethylated polystyrene substrate. Copolymers incorporating either short (M w ≈ 5000) or long (Mw ≈ 30 000) P2VP side chains were prepared by grafting onto linear, comb-branched (G0), G1, and G2 chloromethylated arborescent polystyrenes. Branching functionalities ranging from 14 to 3880 and molecular weights ranging from 8.2 × 10 4 to 6.7 × 10 7 were obtained for the copolymers, while maintaining a low apparent polydispersity (Mw/Mn ≈ 1.06-1.15) after grafting. Characterization data for these materials from size exclusion chromatography and light scattering indicate that they have a highly compact structure. Dynamic light scattering results show that the arborescent poly(2-vinylpyridine) copolymers expand much more in solution than the linear homologous polymers when protonated with HCl. This is attributed to the higher charge density attained in the branched copolymers.
The autopolymerization of styrene in the presence of TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy free radical), with and without organic acids, such as benzoic acid and camphorsulfonic acid, was studied. Broad polydispersities are obtained in the absence of acid, whereas narrower polydispersities are obtained in their presence. The significance of these results for the synthesis of narrow polydispersity polystyrene by the TEMPO-mediated living polymerization process is discussed.
Toward improved understanding of the dilute-solution properties of arborescent polystyrenes, new measurements are reported for osmotic second virial coefficients and for intrinsic viscosities in three common organic solvents. As observed for other branched polymers, branching decreases the second virial coefficient in good solvents and lowers the theta temperature for a polymer-solvent system. For generation-zero arborescent polystyrene in meth)ilcyclohexane, the theta temperature is 36±2°C.The spherical topology of these polymers allows a correspondence between intrinsic viscosity and seconeJ virial coefficient that holds in good solvents; this correspondence improves with decreasing branch molecular weight.The osmotic-pressure data are interpreted using a colloid-like thermodynamic framework using a van der Waals-type• equation of state. The reference state is the hard sphere and the perturbation is given by an attraction decaying with the sixth power of the center-to-center distance between polymers. The hard-sphere diameter is obtained from intrinsic.,viscosity data. Predicted and observed osmotic second virial coefficients are in good agreement.
Small-angle neutron scattering was used to characterize the structure of arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers dissolved in methanol-d4 (CD 3 OD). A radial density profile based on a power law functional form provided a good fit to the scattering data. While a model with homogeneous density profiles in the core and shell, respectively, and with a size distribution (a polydisperse core -shell model) also fits the data comparably well, the extra parameters required for this fit are difficult to justify on the basis of the data. In addition, unconstrained fits using the core -shell model failed to converge to values of the overall molecular size and molecular weight which agreed with values determined from independent light scattering measurements which leads to the conclusion that the power law model is a more appropriate function for describing the radial density function of these molecules. The density profile from either model showed that the polystyrene core of the molecules is not collapsed. Values of the second virial coefficient, A 2 ; have been calculated from Zimm plots and it was found that A 2 decreased as a function of generation to close to zero for the highest generation (i.e. highest molecular weight) polymers. Finally, it was found that the radius of gyration of the polymers increases with the molecular weight according to the scaling relationship, R g , M v w with v ¼ 0:24^0:04: q
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