Block copolymer polymersomes offer considerable access for applications in a variety of fields; however, the traditional cosolvent self-assembly method can only produce polymersomes at a low solids content (typically <1%). Recently, an in situ growth method, termed polymerization-induced self-assembly (PISA), has been developed to allow the preparation of polymersomes at high solids (10–50%). Synthesis and self-assembly of block copolymers occur simultaneously in PISA, and therefore, morphological evolution occurs throughout the polymerization. It is highly desirable to provide mechanistic insights into morphological evolution that enables one to rationally synthesize a variety of morphologies. Herein, we demonstrate that the further growth of polymersomes in aqueous PISA can be conveniently driven by temperature via two different pathways: (i) at high temperatures from polymersomes with a thin membrane to polymersomes with a thick membrane and (ii) at low temperatures from spherical polymersomes to tubular and donut-like polymersomes. We show that both the hydrodynamic diameter and membrane thickness of polymersomes increase during PISA at high temperatures, indicating that the membrane of polymersomes grows inward and outward simultaneously as the polymerization proceeds. Furthermore, careful characterization of samples withdrawn during the kinetic study at low temperatures by transmission electron microscopy and dynamic light scattering reveals various intermediate morphologies that provide important insights into the formation of tubular and donut-like polymersomes. We expect that this study not only provides mechanistic insights into the morphological evolution of PISA but also expands the scope of PISA for the preparation of a variety of structures.
Enzyme catalysis is a mild, efficient, and selective technique that has many applications in organic synthesis as well as polymer synthesis. Here, a novel enzyme-catalysis-induced reversible addition-fragmentation chain transfer (RAFT)-mediated dispersion polymerization for preparing AB diblock copolymer nano-objects with complex morphologies at room temperature is described. Taking advantage of the room-temperature feature, it is shown that pure, worm-like polymer nano-objects can be readily prepared by just monitoring the viscosity. Moreover, it is demonstrated that inorganic nanoparticles and proteins can be loaded in situ into vesicles by this method. Finally, a novel oxygen-tolerant RAFT-mediated dispersion polymerization initiated by enzyme cascade reaction that can be carried out in open vessels is developed. The enzyme-initiated RAFT dispersion polymerization provides a facile platform for the synthesis of various functional polymer nano-objects under mild conditions.
In this communication, we developed the first well-controlled Z-RAFT (RAFT = reversible addition− fragmentation chain transfer) mediated polymerization-induced self-assembly (PISA) formulation based on photoinitiated RAFT dispersion polymerization of tert-butyl acrylate (tBA) in ethanol/water (60/40, w/w) at room temperature using a Z-type macromolecular chain transfer agent (macro-CTA). Polymerizations proceeded rapidly via the exposure of visible-light irradiation (405 nm, 0.45 mW/cm 2 ) with high monomer conversion (>95%) being achieved within 1 h. A variety of polymer nano-objects (spheres, worms, and vesicles) with narrow molar mass distributions were prepared by this Z-RAFT mediated PISA formulation. Silver nanoparticles were loaded with the vesicles via in situ reduction, which can be used as a catalyst for the reduction of methylene blue (MB) in the presence of NaBH 4 . Finally, gel permeation chromatography (GPC) analysis demonstrated that the corona block and the core-forming block could be cleaved by treating with excess initiator. This novel PISA formulation will greatly expand the scope of PISA and provide more mechanistic insights into the PISA research.
Reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization-induced self-assembly (PISA) has served as a versatile platform for the large-scale preparation of block copolymer nano-objects with a diverse set of morphologies. However, almost all PISA formulations are focused on the syntheses of linear block copolymers rather than star block copolymers. Owing to the asymmetrical structure of the RAFT agent, herein, we report a direct comparison between Z-RAFT (the nonfragmenting group attached to the core) and R-RAFT (the fragmenting group attached to the core) strategies for preparing well-defined star block copolymer nano-objects via RAFTmediated PISA. We showed that the Z-RAFT strategy is a more suitable strategy for the preparation of star block copolymer nano-objects without compromising the control over the molecular weight and morphology. A binary mixture of Z-type and R-type tetrafunctional macro-RAFT agents was used to control the morphologies of star block copolymer assemblies. The effect of numbers of the RAFT group on polymerization kinetics and morphologies of block copolymer nano-objects was also investigated in detail. Finally, (ABC) 4 four-arm star triblock copolymer vesicles were prepared by seeded RAFT dispersion polymerization of solvophobic and solvophilic monomers using (AB) 4 four-arm star diblock copolymer vesicles as seeds. This research not only expands the scope for preparing well-defined star block copolymer nano-objects but also provides important insights into the effect of the polymer architecture on RAFT-mediated PISA.
Front Cover: In article number https://doi.org/10.1002/marc.201700871, Jianbo Tan, Li Zhang, and co‐workers describe a novel enzyme‐catalysis‐induced reversible addition‐fragmentation chain transfer (RAFT)‐mediated dispersion polymerization for preparing AB diblock copolymer nano‐objects with complex morphologies, performed at room temperature. Taking advantage of the oxygen‐tolerant feature of enzyme cascade catalysis, block copolymer nano‐objects with various morphologies are also prepared in open vessels.
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