No abstract

The radical polymerization of styrene and n-butyl acrylate is demonstrated to proceed under controlled conditions between 125 and 130 °C in the presence of either a 1,5-dimethyl-3-phenyl-6-oxoverdazyl radical or a 1,5-dimethyl-3-ethyl-6-oxoverdazyl radical producing polymers with polydispersity indices in the 1.2−1.3 range. While polymerizations initiated with benzoyl peroxide or 1,1-azobis(cyanocyclohexane) in the presence of verdazyl radical were unsuccessful, polymerizations initiated with a styrene/verdazyl unimolecular initiator proceeded in a living fashion, although quite slowly. An increase in polymerization rate was obtained with a 1,5-dimethyl-6-oxoverdazyl radical, producing higher yields of well-defined polymers. The livingness of the resulting styrene and n-butyl acrylate homopolymers is illustrated with chain extension reactions to make well-defined diblock copolymers. These results open a new front in the development of living-radical polymerization processes, and the ability to manipulate the verdazyl structure offers the opportunity to further control and modify this process.

A new method is presented for the preparation of arborescent copolymers containing polyisoprene (PIP) segments. The technique uses acetyl coupling sites randomly distributed on polystyrene substrates. Isoprene was polymerized with sec-butyllithium in tetrahydrofuran to yield polyisoprenyllithium, and the living polymer was titrated with an acetylated substrate to generate a copolymer. The grafting yield was maximized at 25 °C using 5 equiv of LiCl to attenuate the reactivity of polyisoprenyllithium. Arborescent copolymers were synthesized by grafting PIP side chains with a weight-average molecular weight M w of either 5000 (PIP5) or 30 000 (PIP30) onto linear, comb-branched (G0), G1, and G2 acetylated polystyrenes. The copolymers with short (PIP5) side chains contain 84−91% w/w polyisoprene. For long (PIP30) side chains, the polyisoprene content varies from 92 to over 97% w/w. The graft copolymers exhibit a geometric increase in branching functionality and molecular weight for successive generations (f w = 11−4000, M w = 6.5 × 104−2.5 × 107 for the PIP5 series, and f w = 11−1530, M w = 3.2 × 105−4.9 × 107 for the PIP30 series). A narrow molecular weight distribution (M w /M n = 1.06−1.09) was maintained after grafting. Film formation by the arborescent copolymers was investigated using tapping mode atomic force microscopy after spin-casting from different solvents. When heptane, a solvent selective for the polyisoprene segments, was used, phase separation between the polystyrene core and the polyisoprene shell was clearly visible in phase contrast imaging, even for copolymers with longer PIP side chains. In nonselective solvents (toluene and chloroform) phase contrast was reduced, presumably due to enhanced mixing of the polystyrene and polyisoprene phases.

The radius of gyration (R g ) was determined as a function of generation number for arborescent polystyrenes with two different side chain mass average molecular mass (M w ≈ 5000, 5K, versus 30 000, 30K) by small-angle neutron scattering (SANS) measurements. The R g values obtained were analyzed in terms of the Zimm-Stockmayer model for randomly branched polymers, the scaling relation R g ∝ M w V , and the expansion factor R s ) (R g ) goodsolvent /(R g ) Θsolvent . The R g and scaling exponent V ) 0.26 ( 0.01 found for G0 through G3 polymers with 5K side chains in cyclohexane-d correspond to the values predicted by the Zimm-Stockmayer model. The R g for G0 through G3 polymers with 30K side chains deviate from the model with V ) 0.32 ( 0.02, corresponding to V ) 0.33 expected for hard spheres. Deuterated polystyrene (PS-d) side chains were grafted onto G2 and G3 polystyrene (PS) cores. These copolymers, G2PS-graft-PS-d and G3PS-graft-PS-d, were characterized as spheres with a well-defined PS core-PS-d shell structure by the SANS contrast matching method. The shape and the segment radial density profile of the core and shell for GPS-graft-PS-d were determined based on P(r) and ∆F(r) obtained by indirect Fourier transformation and deconvolution methods (P(r), pair distance distribution function and ∆F(r) ) F(r) -F(solvent), scattering length density contrast profile).