Synthesis of Arborescent Polystyrene-graft-polyisoprene Copolymers Using Acetylated Substrates

Abstract

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.

Phase-Segregated Dendrigraft Copolymer Architectures

“…Conversely, in nonselective solvents (toluene and chloroform) the phase contrast was reduced, presumably due to enhanced mixing of the polystyrene and polyisoprene components. This clearly confirms the occurrence of phase separation between the polystyrene core and the polyisoprene shell of the molecules on the nanometric scale, but also that the extent of phase separation achieved strongly depends on the selectivity of the solvent used in film preparation [30]. While AFM imaging provides direct evidence for phase segregation within the individual styrene-isoprene graft copolymer molecules, characterization results obtained by other methods were also consistent with a heterogeneous morphology.…”

“…Arborescent copolymers containing polyisoprene (PIP) segments were obtained by coupling polyisoprenyllithium "living" chains with either chloromethylated [29] or acetylated [30] polystyrene substrates according to the procedures outlined in Scheme 2. The synthesis from chloromethylated substrates proceeded most efficiently after capping of the chains with 1,1-diphenylethylene and coupling at −30 °C.…”

“…Film formation by the isoprene copolymers on mica surfaces was investigated using tapping mode atomic force microscopy (AFM) after spin-casting from different solvents [30]. Measurements in the phase contrast mode are ideal to probe phase separation in these systems, because the glassy polystyrene core and the rubbery polyisoprene shell should lead to significant phase changes in the signal.…”

Arborescent polymers and other dendrigraft polymers: A journey into structural diversity

“…17 This reaction was obviously extended easily to the synthesis of polyisoprene and poly(2-vinylpyridine) copolymers by substituting the capped polystyryl anions with the corresponding ''living'' homopolymers in the coupling reaction. 18 The latest developments in our laboratory concern new methods for the synthesis of arborescent homo-and copolymers and the physical characterization of the copolymers. Thus a technique was developed recently for the synthesis of arborescent homopolymers from polyisoprene chains by partial epoxidation of the polyisoprene substrates and coupling with polyisoprenyl anions in the presence of lithium halides.…”

A gaussian model for substrates of entangled cross‐linked poly(ethylene glycol) in biomedical applications

“…Both chloromethyl1 and acetyl11 functionalities have been used as coupling sites for the preparation of arborescent styrene homopolymers on a small (10–15 g) scale. Copolymers were also obtained on a small scale by grafting other macroanions onto arborescent polystyrene substrates 12, 13…”