Impact of Polymer Microstructure on the Self-Assembly of Amphiphilic Polymers in Aqueous Solutions

A new series of amphiphilic polymers (amphipols) with varied molecular characteristics was prepared, and their properties in aqueous media were examined by static and dynamic light scattering techniques. These polymers are short poly(sodium methacrylate) chains of various molecular weights and tacticities, modified with different degrees of n-octylamine as copolymers of two distinct hydrophobe distribution sequences (random vs multiblocky). To synthesize the parent poly(methacrylic acid) (PMAA) prior to hydrophobic modification, tert-butyl methacrylate was polymerized under the controlled conditions of atom transfer radical polymerization (ATRP) to yield after deprotection the syndiotactic-rich PMAA of targeted molar masses (12−28 kg mol-1) and low polydispersity indexes (1.08−1.19). Under similar conditions of ATRP and deprotection, a well-defined isotactic-rich PMAA was obtained from triphenylmethyl methacrylate. The amphipol carrying octyl side chains randomly distributed along the polymer main chain was produced by coupling the parent PMAA with n-octylamine in an organic medium (N-methylpyrrolidone). In contrast, the coupling reaction of PMAA in aqueous media, with the n-octylamine solubilized by sodium dodecyl sulfate, gave the amphipols bearing octyl groups distributed in a multiblocky fashion. The highly controlled hydrophobe distribution sequence and polymer tacticity were confirmed by 1H and 13C NMR spectroscopic techniques. All polymers in aqueous solutions form nanoparticles with the structure strongly determined by the polymer microstructure and composition. In the case of random graft amphipol, the polymer self-assembles and preferentially forms small aggregates of 1−2 polymer chains on average with a hydrodynamic radius of ∼3 nm. In cases of the multiblocky graft amphipols, well-defined nanoscaled self-assemblies are formed but from multiple polymer chains (aggregation number = ∼17), with a drastic increase in the hydrodynamic radius (∼13 nm). Comparing to the effects due solely to the hydrophobe distribution sequence, the increments in structural parameters of the amphipol self-assemblies are only slightly enhanced when concurrently improving the polymer isotacticity or increasing the polymer molar mass. All results point to the critical impact of hydrophobe distribution sequence on the self-assembly of methacrylate-based amphipols in aqueous solutions.