Robert B. Grubbs

Nitroxide-Mediated Radical Polymerization: Limitations and Versatility

Since its introduction in the early 1990s, nitroxide-mediated radical polymerization (NMP) has been widely adopted for the preparation of a panoply of new polymer architectures. While NMP provides a number of advantages for the preparation of specific types of polymers, several inherent limitations with NMP have led to the more widespread use of other reversible-deactivation radical polymerization (RDRP) methods, chiefly atom transfer radical polymerization (ATRP) and reversible addition-fragmentation-chain transfer (RAFT) polymerization, in polymer synthesis. This short review discusses strategies that have been adopted for dealing with the difficulties of NMP and also surveys the use of NMP to prepare new polymers and copolymers over the past several years.

Reversible Restructuring of Aqueous Block Copolymer Assemblies through Stimulus-Induced Changes in Amphiphilicity

ABC triblock copolymers with a hydrophilic-thermomorphic-hydrophobic block sequence can assemble into distinct structures in water above and below the lower critical solution temperature for the thermomorphic block. Characterization of aqueous solutions of a representative poly(ethylene oxide)-block-poly(N-isopropylacrylamide)-block-polyisoprene copolymer by light scattering and transmission electron microscopy shows that small spherical micelles are formed at low temperatures and that large vesicles are favored at higher temperatures. The transformation from micelles to vesicles is reversible but slow, occurring over several weeks.

Roles of Polymer Ligands in Nanoparticle Stabilization

Both inorganic nanoparticles and polymers have become ubiquitous components of nanostructured systems. The development of an understanding of the interactions between these components will be of crucial importance in the further elaboration of these systems. This review touches upon two aspects of control over surface chemistry in inorganic nanoparticles with polymer ligands: (1) the effect of polymer ligands on accessibility of the inorganic nanoparticle surface to other species, such as is necessary in catalysis; and (2) the design of polymer ligands that can be modified to enable compatibility of the entire polymer‐coated nanoparticle with a broad variety of other materials.

Thermally Induced Changes in Amphiphilicity Drive Reversible Restructuring of Assemblies of ABC Triblock Copolymers with Statistical Polyether Blocks

ABC triblock copolymers in which a block with stimulus-dependent solvophilicity resides between solvophilic and solvophobic end blocks can undergo reversible transitions between different thermodynamically stable assemblies in the presence or absence of stimulus. As a new example of such a copolymer system, thermoresponsive poly(ethylene oxide)-b-poly(ethylene oxide-stat-butylene oxide)-b-poly(isoprene) (E-BE-I) triblock copolymers with narrow molecular weight distributions (M(w)/M(n): 1.05-1.18) were prepared by sequential living anionic and nitroxide-mediated radical polymerizations. The specific copolymers examined (9.0 ≤ M(n) ≤ 14.4 kg/mol, 14% ≤ wt % isoprene ≤35%) form near-spherical aggregates with narrow size distributions at 25 °C. The thermoresponsive behavior of these polymers was studied by applying cloud point, DLS, and TEM measurements to a representative polymer, E(2.3)BE(5.3)I(2.3). The transformation of polymer aggregates from spherical micelles to vesicles (polymersomes) at elevated temperatures was detected by DLS and TEM studies, both with and without cross-linking of polymer assemblies. The rate of transformation with E-BE-I systems is more rapid than that observed for poly(ethylene oxide)-b-poly(N-isopropylacrylamide)-b-poly(isoprene) assemblies, suggesting that interchain hydrogen bonding of responsive blocks after dehydration plays an important role in the kinetics of aggregate rearrangement.

Chemically enhancing block copolymers for block-selective synthesis of self-assembled metal oxide nanostructures.

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-block-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base-acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template.

Polymerization of Tellurophene Derivatives by Microwave‐Assisted Palladium‐Catalyzed ipso‐Arylative Polymerization

We report the synthesis of a tellurophene-containing low-bandgap polymer, PDPPTe2T, by microwave-assisted palladium-catalyzed ipso-arylative polymerization of 2,5-bis[(α-hydroxy-α,α-diphenyl)methyl]tellurophene with a diketopyrrolopyrrole (DPP) monomer. Compared with the corresponding thiophene analog, PDPPTe2T absorbs light of longer wavelengths and has a smaller bandgap. Bulk heterojunction solar cells prepared from PDPPTe2T and PC71 BM show PCE values of up to 4.4%. External quantum efficiency measurements show that PDPPTe2T produces photocurrent at wavelengths up to 1 µm. DFT calculations suggest that the atomic substitution from sulfur to tellurium increases electronic coupling to decrease the length of the carbon-carbon bonds between the tellurophene and thiophene rings, which results in the red-shift in absorption upon substitution of tellurium for sulfur.

Magnetic Hydrogels from Alkyne/Cobalt Carbonyl-Functionalized ABA Triblock Copolymers

A series of alkyne-functionalized poly(4-(phenylethynyl)styrene)-block-poly(ethylene oxide)-block-poly(4-(phenylethynyl)styrene) (PPES-b-PEO-b-PPES) ABA triblock copolymers was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. PESn[Co2(CO)6]x-EO800-PESn[Co2(CO)6]x ABA triblock copolymer/cobalt adducts (10-67 wt % PEO) were subsequently prepared by reaction of the alkyne-functionalized PPES block with Co2(CO)8 and their phase behavior was studied by TEM. Heating triblock copolymer/cobalt carbonyl adducts at 120 °C led to cross-linking of the PPES/Co domains and the formation of magnetic cobalt nanoparticles within the PPES/Co domains. Magnetic hydrogels could be prepared by swelling the PEO domains of the cross-linked materials with water. Swelling tests, rheological studies and actuation tests demonstrated that the water capacity and modulus of the hydrogels were dependent upon the composition of the block copolymer precursors.

One-step synthesis of alkoxyamines for nitroxide-mediated radical polymerization

An alkoxyamine that is an effective initiator for the controlled polymerization of styrene and isoprene has been prepared by the reaction of 2-methyl-2-nitrosopropane with two equivalents of radicals derived from 1-bromoethylbenzene.

Facile Synthesis of Novel Polyethylene-Based A-B-C Block Copolymers Containing Poly(methyl methacrylate) Using a Living Polymerization System

Ethylene-propylene-methyl methacrylate (MMA) and ethylene-hexene-MMA A-B-C block copolymers with high molecular weight (>100,000) are synthesized using fluorenylamide-ligated titanium complex activated by modified methylaluminoxane and 2,6-di-tert-butyl-4-methylphenol for the first time. After diblock copolymerization of olefin is conducted completely, MMA is added and activated by aluminum Lewis acid to promote anionic polymerization. The length of polyolefin and poly (methyl methacrylate) (PMMA) is controllable precisely by the change of the additive amount of olefin and polymerization time, respectively. A soft amorphous polypropylene or polyhexene segment is located between two hard segments of semicrystalline polyethylene and glassy PMMA blocks.

ipso-Arylative polymerization as a route to π-conjugated polymers: synthesis of poly(3-hexylthiophene)

ipso-Arylative cross-coupling with two 3-hexylthiophene derivatives, (5-bromo-4-hexylthiophen-2-yl)diphenylmethanol and 2-(5-bromo-4-hexylthiophen-2-yl)propan-2-ol, has been used to prepare poly(3-hexylhiophene) (P3HT) as a model conjugated polymer. P3HT with number-average molecular weights ranging from 8–20 kg mol−1 (Đ 1.4–2.2) was prepared from 5-bromo-4-hexylthiophen-2-yl)diphenylmethanol with a Pd(OAc)2/PCy3/Cs2CO3 catalyst system. Only oligomerization of 2-(5-bromo-4-hexylthiophen-2-yl)propan-2-ol (Mn ≈ 3 kg mol−1) was observed under similar conditions. Studies with model compounds suggest that side reactions involving end-group loss limit ultimate molecular weights.