No abstract

The aqueous crystallization-driven sphere-to-rod transition of poly(lactide)-b-poly(acrylic acid), PLA-b-PAA block copolymers, with a short homochiral PLA core forming block and a 10 times longer (in terms of degree of polymerization) PAA corona forming block is presented. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) is utilized to follow the kinetics of the transition and wide angle X-ray diffraction (WAXD) to confirm the correlation between degree of crystallinity and morphology. Studies at different concentrations and solvent mixtures provide valuable information regarding the nucleation and growth mechanism of the system, showing that the micelle dynamics are a key aspect of the assembly process. Furthermore, the in situ crystallization-driven cylinder formation during the acrylate ester hydrolysis reaction is demonstrated. Finally, we report that the micelle morphology can be switched between cylinders and spheres by facilitating or blocking the crystallization of the core block, demonstrating a simple method to control the morphology of the resultant assembly.

A series of poly(L-lactide)-b-poly(acrylic acid) (PLLA-b-PAA) diblock copolymers with a range of hydrophobic or hydrophilic block lengths were designed in order to tune the size of the resultant cylindrical micelles using a crystallization-driven self-assembly (CDSA) approach. The precursor poly(L-lactide)-b-poly(tetrahydropyran acrylate) (PLLA-b-PTHPA) was synthesized by a combination of ring-opening polymerization (ROP) and reversible additionfragmentation chain transfer (RAFT) polymerization. The CDSA process was carried out in a tetrahydrofuran/water (THF/H 2 O) mixture during the hydrolysis of PTHPA block at 65 °C using an evaporation method. A majority of PLLA-b-PAA diblock copolymers resulted in the formation of cylindrical micelles with narrow size distributions (L w /L n < 1.30) as determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Furthermore, the length of PLLA block was found to control the length of the resultant cylindrical micelles while the length of PAA block governed their widths. Synchrotron small-angle X-ray scattering (SAXS) further proved that the length increase of these cylinders was a consequence of the decreasing PLLA block lengths. The crystalline core nature of these cylinders was characterized by wide-angle X-ray diffraction (WAXD), and the relative core crystallinity was calculated to compare different samples. Both the hydrophobic weight fraction and the relative core crystallinity were found to determine the geometry of the formed PLLA-b-PAA cylindrical micelles. Finally, changing the pH conditions of the CDSA process was found to have no significant effect on tuning the resultant dimensions of the cylinders.

The effect of covalently attaching 4-(dimethylamino)pyridine (DMAP) functionality to the hydrophobic core of a polymeric micelle in water has been investigated in the context of acylation reactions employing non-water-soluble substrates. For this purpose a novel temperature-responsive polymeric micelle has been synthesized using reversible addition−fragmentation chain transfer (RAFT) polymerization techniques. The reactivity of the tethered organocatalyst within the nanostructure was found to be extremely high, improving in some cases the acylation rates up to 100 times compared to those for unsupported DMAP in organic solvents. Moreover, the catalytic nanoreactors have been demonstrated to be capable of reuse up to 6 times while maintaining high activity.