Reversible addition-fragmentation chain transfer polymerization has been utilized to polymerize 2-hydroxypropyl methacrylate (HPMA) using a water-soluble macromolecular chain transfer agent based on poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC). A detailed phase diagram has been elucidated for this aqueous dispersion polymerization formulation that reliably predicts the precise block compositions associated with well-defined particle morphologies (i.e., pure phases). Unlike the ad hoc approaches described in the literature, this strategy enables the facile, efficient, and reproducible preparation of diblock copolymer spheres, worms, or vesicles directly in concentrated aqueous solution. Chain extension of the highly hydrated zwitterionic PMPC block with HPMA in water at 70 °C produces a hydrophobic poly(2-hydroxypropyl methacrylate) (PHPMA) block, which drives in situ self-assembly to form well-defined diblock copolymer spheres, worms, or vesicles. The final particle morphology obtained at full monomer conversion is dictated by (i) the target degree of polymerization of the PHPMA block and (ii) the total solids concentration at which the HPMA polymerization is conducted. Moreover, if the targeted diblock copolymer composition corresponds to vesicle phase space at full monomer conversion, the in situ particle morphology evolves from spheres to worms to vesicles during the in situ polymerization of HPMA. In the case of PMPC(25)-PHPMA(400) particles, this systematic approach allows the direct, reproducible, and highly efficient preparation of either block copolymer vesicles at up to 25% solids or well-defined worms at 16-25% solids in aqueous solution.

ABSTRACT. Well-defined poly(lauryl methacrylate-benzyl methacrylate) (PLMA-PBzMA) diblock copolymer nanoparticles are prepared in n-heptane at 90°C via reversible addition-fragmentation chain transfer (RAFT) polymerization. Under these conditions, the PLMA macromolecular chain transfer agent (macro-CTA) is soluble in n-heptane, whereas the growing PBzMA block quickly becomes insoluble. Thus this dispersion polymerization formulation leads to polymerization-induced self-assembly (PISA). 10Using a relatively long PLMA macro-CTA with a mean degree of polymerization (DP) of 37 or higher leads to the formation of well-defined spherical nanoparticles of 41 to 139 nm diameter, depending on the DP targeted for the PBzMA block. In contrast, TEM studies confirm that using a relatively short PLMA macro-CTA (DP = 17) enables both worm-like and vesicular morphologies to be produced, in addition to the spherical phase. A detailed phase diagram has been elucidated for this more asymmetric diblock 15 copolymer formulation, which ensures that each phase can be targeted reproducibly.1 H NMR spectroscopy confirmed that high BzMA monomer conversions (> 97 %) were achieved within 5 h, while GPC studies indicated that reasonably good blocking efficiencies and relatively low diblock copolymer polydispersities (M w /M n < 1.30) were obtained in most cases. Compared to prior literature reports, this allmethacrylic PISA formulation is particularly novel because: (i) it is the first time that higher order 20 morphologies (e.g. worms and vesicles) have been accessed in non-polar solvents and (ii) such diblock copolymer nano-objects are particularly relevant to potential boundary lubrication applications for engine oils.

Diblock copolymers of vinyl ethers with two thermosensitive segments possessing different phase separation temperatures (TPS) have been synthesized by sequential living cationic copolymerization. Examples include 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and 2-methoxyethyl vinyl ether (MOVE), which were sequentially polymerized using a cationogen/Et1.5AlCl1.5 initiating system in the presence of tetrahydrofuran to give diblock copolymers with a very narrow molecular weight distribution. When an aqueous solution of a diblock copolymer (EOEOVE/MOVE ) 200/400) was heated, four different viscoelastic stages were observed: clear liquid (sol, e40 °C), transparent gel (42-55 °C), hot clear liquid (sol, 57-63 °C), and opaque mixture by phase separation (>63 °C). Micelle formation during physical gelation was confirmed by the change in particle diameter. The temperatures of the first and third transitions corresponded to each T PS for two segments, whereas that of the second transition was dependent on factors such as structure (sequence, composition, and molecular weight distribution) and physical properties (concentration, additives). Freeze-fracture transmission electron microscopy revealed the physical gels to consist of a regular arrangement of spherical micelles with controlled size. Experimental SectionMaterials. 2-(2-Ethoxy)ethoxyethyl vinyl ether (EOEOVE) was prepared from 2-(2-hydroxy)ethoxyethyl vinyl ether (BASF),

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Stimuli-responsive diblock copolymers with a thermosensitive segment and a hydrophilic segment have been synthesized via sequential living cationic copolymerization, which involves a poly(vinyl ether) with oxyethylene pendants exhibiting LCST-type phase separation in water and a poly(hydroxyethyl vinyl ether) segment. Highly sensitive and reversible thermally induced micelle formation and/or physical gelation were observed with such diblock copolymers. For example, the flow behavior of an aqueous solution of a diblock copolymer varied from Newtonian to non-Newtonian and plastic flow within a very narrow temperature range. TEM and SANS studies showed that the observed change in viscosity was due to the formation of a macrolattice with body-centered-cubic symmetry of spherical micelles in aqueous solution. The critical temperature of micelle formation and/or physical gelation could be varied by altering the combination of two segments in the diblock copolymer. On the basis of these results, several systems incorporating various patterns of physical gelation behavior have been developed, and a strategy for constructing stimuli-responsive systems with block copolymers was established.

Stimuli-responsive ABC triblock copolymers with three segments with different phase-separation temperatures were synthesized via sequential living cationic copolymerization. The triblock copolymers exhibited sensitive thermally induced physical gelation (open association) through the formation of micelles. For example, an aqueous solution of EOVE 200 -b-MOVE 200 -b-EOEOVE 200 [where EOVE is 2-ethoxyethyl vinyl ether, MOVE is 2-methoxethyl vinyl ether and EOEOVE is 2-(2-ethoxy)ethoxyethyl vinyl ether; the order of the phase-separation temperatures was poly(EOVE) (20°C) Ͻ poly(EOEOVE) (41°C) Ͻ poly(MOVE) (70°C)] underwent multiple reversible transitions from sol (Ͻ20°C) to micellization (20 -41°C) to physical gelation (physical crosslinking, 41-64°C) and, finally, to precipitation (Ͼ64°C). At 41-64°C, the physical gel became stiffer than similar diblock or ABA triblock copolymers of the same molecular weight. Furthermore, the ABC triblock copolymers exhibited Weissenberg effects in semidilute aqueous solutions. In sharp contrast, another ABC triblock copolymer with a different arrangement, EOVE 200 -b-EOEOVE 200 -b-MOVE 200 , scarcely exhibited any increase in viscosity above 41°C. The temperatures of micelle formation and physical gelation corresponded to the phase-separation temperatures of the segment types in the ABC triblock copolymer. No second-stage association was observed for AB and ABA block copolymers with the same thermosensitive segments found in their ABC counterparts.

Thermosensitive random copolymers of hydrophilic and hydrophobic monomers are synthesized via living cationic copolymerization. The synthesis starts with the copolymerization of isobutyl vinyl ether (IBVE) and 2-(tert-butyldimethylsilyloxy)ethyl vinyl ether (BMSiVE) using the cationogen/Et1.5AlCl1.5 initiating system in the presence of an added base to give a copolymer with a narrow molecular weight distribution. The relative reactivities determined by the Fineman−Ross method indicate that the product copolymer has a highly random sequence distribution. Subsequent desilylation gave a well-defined amphiphilic random copolymer of hydrophobic (IBVE) and hydrophilic units (2-hydroxyethyl vinyl ether:  HOVE). On heating, an aqueous solution of the product random copolymer undergoes thermally induced phase separation at a critical temperature. This phase separation is quite sensitive and reversible on heating and cooling. The randomness of the sequence distribution is indispensable to realizing such highly sensitive phase separation. For example, diblock copolymers are soluble and form micelles over a wide range of temperature, whereas copolymers with both block and random segments exhibit only slightly sensitive phase separation behavior with hysteresis. The critical temperature of the random copolymer presented here can be controlled by the composition of IBVE and HOVE. In addition to the composition, the structure of the hydrophobic repeating unit is a major factor determining the critical temperature.