Makoto Ouchi

Selective Radical Addition with a Designed Heterobifunctional Halide: A Primary Study toward Sequence-Controlled Polymerization upon Template Effect

A ruthenium(II)-catalyzed, highly selective, quantitative radical addition of an alkene, methacrylic acid (MAA), has been achieved by using a template halide (2) containing a built-in amine group as a recognition site for the carboxyl group of the substrate. The specific ionic binding of MAA by the amine template (1:1 molar ratio) led to preferential formation of the 1:1 MAA-2 adduct, whereas a similar halide without a template induced MAA oligomerization even in the presence of an externally added amine. A competitive radical addition of MAA versus its ester form [methyl methacrylate (MMA)] on the halide further demonstrated that the substrate selectivity [k(MAA)/k(MMA)] for 2 is enhanced more than 10 times by the intramolecular introduction of the template relative to the result for the nontemplate halide. These specificities are most likely triggered by the specific interaction (recognition) of the carboxyl group in MAA via the acid-selective template amine, which is implanted in the close vicinity of the radical addition site in 2. These results intimate possibility of control over the repeat-unit sequence in precision polymerization.

Template-Assisted Selective Radical Addition toward Sequence-Regulated Polymerization: Lariat Capture of Target Monomer by Template Initiator

Surprisingly high monomer selectivity was demonstrated in competitive radical addition with two kinds of methacrylates carrying sodium and ammonium cation. Crucial is size-specific recognition by a lariat crown ether embedded close to the reactive halide in a designer template initiator. Especially, a combination with an active ruthenium catalyst led to outstanding selectivity at low temperature. This template system will open the way to unprecedented sequence-regulated polymerization.

A strategy for sequence control in vinyl polymers via iterative controlled radical cyclization

There is a growing interest in sequence-controlled polymers toward advanced functional materials. However, control of side-chain order for vinyl polymers has been lacking feasibility in the field of polymer synthesis because of the inherent feature of chain-growth propagation. Here we show a general and versatile strategy to control sequence in vinyl polymers through iterative radical cyclization with orthogonally cleavable and renewable bonds. The proposed methodology employs a repetitive and iterative intramolecular cyclization via a radical intermediate in a one-time template with a radical-generating site at one end and an alkene end at the other, each of which is connected to a linker via independently cleavable and renewable bonds. The unique design specifically allowed control of radical addition reaction although inherent chain-growth intermediate (radical species) was used, as well as the iterative cycle and functionalization for resultant side chains, to lead to sequence-controlled vinyl polymers (or oligomers).

Alternating Sequence Control for Carboxylic Acid and Hydroxy Pendant Groups by Controlled Radical Cyclopolymerization of a Divinyl Monomer Carrying a Cleavable Spacer.

By utilizing features of the hemiacetal ester (HAE) bond: easy formation from vinyl ether and carboxylic acid and easy cleavage into different functional groups (-COOH and -OH), we achieved control of the alternating sequence of two functional pendant groups of a vinyl copolymer. Methacrylate- and acrylate-based vinyl groups were connected through HAE bonds to prepare a cleavable divinyl monomer, which was cyclo-polymerized under optimized conditions in a ruthenium-catalyzed living radical polymerization. Subsequent cleavage of the HAE bonds in the resultant cyclo-pendant led to a copolymer consisting of alternating methacrylic acid and 2-hydroxyethyl acrylate units as analyzed by 13 Cu2005NMR spectroscopy. The alternating sequence of -COOH and -OH pendants specifically provided a lower critical solution temperature (LCST) in an ether solvent, which was not observed with the random copolymer of same composition ratio.

Sequence Analysis for Alternating Copolymers by MALDI-TOF-MS: Importance of Initiator Selectivity for Comonomer Pair.

A special initiator for metal-catalyzed living radical polymerization facilitates sequence analyses by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) of alternating copolymers from styrene and maleimide derivatives. The initiator is a malonate-based alkyl halide (DEMM-Br), in which two ester groups are attached on the carbon neighboring to bromide, and poor electron density of the radical species allows determination of next unit to the initiator in resultant alternating copolymers due to the selective initiation to styrene derivative. Thanks to the well-defined α-end group, sequence of the oligomeric products via radical copolymerization of PMS and EMI with DEMM-Br can be more simply analyzed by MALDI-TOF-MS, and indeed the following are clarified: the crossover propagation is almost perfectly controlled regardless of the injection ratio; a minor error event of the disordered alternating sequence containing St-St sequential unit could take place; the minor error can be suppressed with an excess amount of maleimide.

A convergent approach to ring polymers with narrow molecular weight distributions through post dilution in ring expansion cationic polymerization

In this paper, we demonstrate a convergent approach to convert “fused” ring chains obtained via ring expansion cationic polymerization of vinyl ether with a hemiacetal ester (HAE)-based ring initiator (1) into “sing” ring ones of narrow MWDs. In the ring-expansion propagation process, the propagation can be controlled without any side reactions but an intermolecular counteranion exchange reaction between HAE bonds between propagating ring polymers ineluctably occurs resulting in broad molecular weight distributions (MWDs) composed of “fused” ring chains with multiple HAE bonds as well as a “sing” ring chain with one HAE bond. Hence, after monomer conversion reached over 95%, the polymerization solution was diluted (i.e., post dilution) without deactivation of an employed Lewis acid activator (i.e., SnBr4) to induce intramolecular counteranion exchange in the fused ring chain. Size exclusion chromatography (SEC) curves of the product eventually became almost unimodal, though a small shoulder peak from the fused ring remained. Importantly, the HAE bond in ring chains still survived even after the post dilution process, which was confirmed by 1H NMR, and the retention of the ring structure was also supported by an acidolysis experiment where the apparent peak top molecular weight (Mp) was increased in SEC due to topology conversion from a ring to linear. The approach was proved to be effective even for higher molecular weight ring polymers that were prepared with a higher [monomer]/[1] ratio as well as ring block copolymers.

Periodic introduction of a Hamilton receptor into a polystyrene backbone for a supramolecular graft copolymer with regular intervals

The Hamilton receptor group (–DADDAD–; D = hydrogen donor; A = hydrogen acceptor) was periodically introduced into a polystyrene backbone, starting from the ruthenium-catalyzed living radical polymerization of styrene with the Hamilton receptor-based bifunctional initiator. The carbon–halogen bonds at both terminals of the resultant unimodal polystyrene (PSDADDAD) carrying the –DADDAD– units at the center position were activated with a ruthenium catalyst to promote the radical–radical coupling reaction between chains, and thereby –DADDAD– units were multiply introduced into the polymer backbone with regular intervals [(PSDADDAD)n]. Upon mixing of the polymer with the end-functionalized poly(methyl methacrylate) (PMMA) carrying a “complementary” hydrogen bonding site (ADADA: PMMAADADA) in CDCl3, supramolecular graft copolymers were formed through the complementary hydrogen bonding between the terminal ADADA site of the guest polymers and the DADDAD site at the periodic positions in the polystyrene backbone. The differential scanning calorimetry (DSC) analysis showed the influence of periodicity in the grafting arm positions on the enthalpic transition of the supramolecular copolymers.