Effect of Cyclization on the Association Behavior of Block Copolymers in Aqueous Solution. Comparison of Oxyethylene/Oxypropylene Block Copolymers Cyclo-P₃₄E₁₀₄ and E₅₂P₃₄E₅₂

Abstract

Triblock copolymer E52P34E52 was prepared by sequential anionic polymerization of propylene oxide followed by ethylene oxide. Some of this copolymer was cyclized via acetal closure to yield a cyclic diblock copolymer, cyclo-P34E104, which was characterized by gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy. Light scattering methods were used to study the micellization and micellar properties of the two copolymers: dynamic light scattering for hydrodynamic properties of the micelles, static light scattering for micellar molar mass and critical micellization conditions. The gelation of concentrated solutions was also studied. The critical micelle concentrations (cmc) of the two copolymers were similar for solutions at 40−50 °C, but the temperature dependence of the cmc was smaller for the linear copolymer. The cyclic copolymer in solution at 50−55 °C formed micelles with larger association numbers and hard-sphere volumes than its linear triblock counterpart.

Effects of block architecture and composition on the association properties of poly(oxyalkylene) copolymers in aqueous solution

“…This work has been extended [49][50][51] in tandem with work on the preparation of large cyclic poly(oxyethylene)s 52) . Ring closure is via an acetal linkage, 1OCH 2 O1, and the formula of the cyclic copolymer prepared from a triblock EBE (cyclo-BE) can be written schematically as:…”

“…Examples of intensity fraction distributions of log r h, app are shown in Fig. 1 for a 40 g N dm -3 solution of copolymer E 52 P 34 E 52 at various temperatures 51) . The intensity distribution at 25 8C indicates a molecular solution (r h,app at the peak L 2 nm), that at 50 8C indicates a micellar solution (r h,app at the peak L 7 nm), and that at 40 8C is consistent with a solution just above its cmt.…”

“…Just two sets of cmc results are available for cyclic poly-(oxyalkylene) copolymers 49,51) , as indicated in Tab. 4.…”

“…4, is notably lower for the cyclic copolymers than for their linear counterparts. This effect has been attributed to reduced exposure of the hydrophobic block to water when the cyclic copolymer is in its unassociated molecular state 49,51) . The behaviour of the cmc itself reflects the fact that it is determined by the Gibbs energy, and so by entropic as well as enthalpic effects.…”

Biodegradable and thermosensitive micelles of amphiphilic polyaspartamide derivatives containing aromatic groups for drug delivery

“… Determined by DLS in water. The CMT was defined as the temperature at which the scattering curve left the baseline established for the polymer solution at low temperatures …”