Tandem Kumada-Tamao catalyst-transfer condensation polymerization and Suzuki-Miyaura coupling for the synthesis of end-functionalized poly(3-hexylthiophene).

Successive Kumada-Tamao catalyst-transfer condensation polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene and Suzuki-Miyaura end-functionalization with pinacol arylboronate in one pot afforded poly(3-hexylthiophene) (P3HT) with a base-sensitive functional group at both ends. The use of poly(methyl methacrylate) (PMMA) bearing a boronic acid ester moiety at one end enabled one-pot synthesis of PMMA-b-P3HT-b-PMMA triblock copolymer.

Intramolecular Palladium Catalyst Transfer on Benzoheterodiazoles as Acceptor Monomers and Discovery of Catalyst Transfer Inhibitors.

Intramolecular catalyst transfer on benzoheterodiazoles was investigated in Suzuki-Miyaura coupling reactions and polymerization reactions with t Bu3 PPd precatalyst. In the coupling reactions of dibromobenzotriazole, dibromobenzoxazole, and dibromobenzothiadiazole with pinacol phenylboronate, the product ratios of monosubstituted product to disubstituted product were 0/100, 27/73, and 89/11, respectively, indicating that the Pd catalyst undergoes intramolecular catalyst transfer on dibromobenzotriazole, whereas intermolecular transfer occurs in part in the case of dibromobenzoxazole and is predominant for dibromobenzothiadiazole. The polycondensation of 1.3 equivalents of dibromobenzotriazole with 1.0 equivalent of para- and meta-phenylenediboronates afforded high-molecular-weight polymer and cyclic polymer, respectively. In the case of dibromobenzoxazole, however, para- and meta-phenylenediboronates afforded moderate-molecular-weight polymer with bromine at both ends and cyclic polymer, respectively. In the case of dibromobenzothiadiazole, they afforded low-molecular-weight polymers with bromine at both ends. Addition of benzothiadiazole derivatives interfered with catalyst transfer in the coupling reactions.

Development of Helical Aromatic Amide Foldamers with a Diphenylacetylene Backbone.

We designed and synthesized aromatic polyamides with a diphenylacetylene backbone, α-DPA and ß-DPA, bearing (S)-α- and (S)-ß-methyl-substituted triethyleneglycol (TEG) side chains, respectively, and examined their conformations in solution. Both polymers exhibit strong, solvent polarity-dependent circular dichroism spectra, which indicated that they take helical conformations in low-polarity solvents. The spectra were mirror images, depending on the chiral position of the side chains. Thus, the polyamide α-DPA bearing (S)-α-methyl-substituted TEG groups takes a left-handed helical conformation, while the polyamide ß-DPA with (S)-ß-methyl-substituted TEG groups takes a right-handed helical conformation. The difference in the screw sense of α-DPA and ß-DPA would be caused by the steric interaction between the main chain and the side chain, as observed in poly(p-benzamide) possessing (S)-ß-methyl-substituted TEG side chains (ß-PA) because the large cavity of the helical structure of DPA would disturb the solvophobically induced helical folding. Detailed conformational analyses of the oligoamides 6-12 with ß-methyl-substituted TEG groups were conducted. Theoretical calculations indicated that the oligoamides with ß-methyl-substituted TEG groups exist in a helical conformation with a cavity of 7 Å in diameter. The 1H NMR spectra of the oligomers revealed interactions with small anions such as chloride and acetate anions and with pyridinium cations.

Intramolecular Catalyst Transfer on a Variety of Functional Groups between Benzene Rings in a Suzuki-Miyaura Coupling Reaction.

Suzuki-Miyaura coupling reaction of BrC6 H4 -X-C6 H4 Br 1 (X=CH2 , CO, N-Bu, O, S, SO, and SO2 ) with arylboronic acid 2 was investigated in the presence of tBu3 PPd precatalyst and CsF/[18]crown-6 as a base to establish whether or not the Pd catalyst can undergo catalyst transfer on these functional groups. In the reaction of 1 (X=CH2 , CO, N-Bu, O, and SO2 ) with 2, aryl-disubstituted product 3 (Ar-C6 H4 -X-C6 H4 -Ar) was exclusively obtained, indicating that the Pd catalyst undergoes catalyst transfer on these functional groups. On the other hand, the reaction of 1 e (X=S) and 1 f (X=SO) with 2 afforded only aryl-monosubstituted product 4 (Ar-C6 H4 -X-C6 H4 -Br) and a mixture of 3 and 4, respectively, indicating that S and SO interfere with intramolecular catalyst transfer. Furthermore, we found that Suzuki-Miyaura polycondensation of 1 (X=CH2 , CO, N-Bu, O, and SO2 ) and phenylenediboronic acid 5 in the presence of tBu3 PPd precatalyst afforded high-molecular-weight polymer even when excess 1 was used. The polymers obtained from 1 (X=CH2 , N-Bu, and O) and 5 turned out to be cyclic.

Exclusive Synthesis of Poly(3-hexylthiophene) with an Ethynyl Group at Only One End for Effective Block Copolymerization.

Well-controlled synthesis of ethynyl-functionalized poly(3-hexylthiophene) (P3HT) is crucial for preparation of block copolymers containing the P3HT segment by means of click coupling reaction. A well-known chain end modification method, in which Kumada-Tamao catalyst-transfer polymerization is quenched with ethynylmagnesium chloride, under various conditions is re-examined, but in all cases not only P3HT with an ethynyl group at one end but also P3HT di-ethynylated at both ends is obtained. Accordingly, Sonogashira coupling reaction of P3HT having H/Br ends with trimethylsilylacetylene is tried, followed by removal of the trimethylsilyl group, and it is found that this protocol affords exclusively P3HT with an ethynyl group at one end. This post end-modification method is applied to the synthesis of an amphiphilic diblock copolymer of P3HT and poly(2-ethyl-2-oxazoline) (PEtOx) by means of click reaction between ethynylated P3HT and PEtOx with an azide group at one end, and the product is confirmed to be free from contamination with triblock copolymer. Micellization of this block copolymer is confirmed in tetrahydrofuran (THF)/water and THF/methanol mixtures.

Photodeprotectable N-Alkoxybenzyl Aromatic Polyamides.

N-alkoxybenzyl aromatic polyamides were synthesized by polycondensation of N-alkoxybenzyl aromatic diamine with equimolar dicarboxylic acid chloride in the presence of 2.2 equiv. of pyridine at room temperature for 2 days. The obtained polyamides were mainly cyclic polymers, as determined by means of matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, and showed higher solubility in organic solvents than unprotected aromatic polyamides. Photodeprotection of N-alkoxybenzyl aromatic polyamide film containing photo acid generator (PAG) proceeded well under UV irradiation (5 J/cm²), followed by heating at 130 °C for 15 min. The nature of the polymer end groups of N-alkoxybenzyl aromatic polyamides was found to be crucial for photodeprotection reactivity. These polymers are promising candidates for photosensitive heat-resistant materials for fine Cu wiring formation by electroless Cu plating of high-density semiconductor packaging substrates.

Unusual cyclic polymerization through Suzuki-Miyaura coupling of polyphenylene bearing diboronate at both ends with excess dibromophenylene.

The Suzuki-Miyaura coupling polymerization of p-dibromophenylene and m-phenylenediboronic acid ester, as well as m-dibromophenylene and p-phenylenediboronic acid ester, and the combination of two meta-phenylene monomers in the presence of the t-Bu3PPd(0) catalyst selectively afforded cyclic polyphenylenes with polyphenylene bearing boronate moieties at both ends when excess dibromophenylene was used.

Mechanistic Investigation of Catalyst-Transfer Suzuki-Miyaura Condensation Polymerization of Thiophene-Pyridine Biaryl Monomers with the Aid of Model Reactions.

We have investigated the requirements for efficient Pd-catalyzed Suzuki-Miyaura catalyst-transfer condensation polymerization (Pd-CTCP) reactions of 2-alkoxypropyl-6-(5-bromothiophen-2-yl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (12) as a donor-acceptor (D-A) biaryl monomer. As model reactions, we first carried out the Suzuki-Miyaura coupling reaction of X-Py-Th-X' (Th=thiophene, Py=pyridine, X, X'=Br or I) 1 with phenylboronic acid ester 2 by using tBu3 PPd0 as the catalyst. Monosubstitution with a phenyl group at Th-I mainly took place in the reaction of Br-Py-Th-I (1 b) with 2, whereas disubstitution selectively occurred in the reaction of I-Py-Th-Br (1 c) with 2, indicating that the Pd catalyst is intramolecularly transferred from acceptor Py to donor Th. Therefore, we synthesized monomer 12 by introduction of a boronate moiety and bromine into Py and Th, respectively. However, examination of the relationship between monomer conversion and the Mn of the obtained polymer, as well as the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra, indicated that Suzuki-Miyaura coupling polymerization of 12 with (o-tolyl)tBu3 PPdBr initiator 13 proceeded in a step-growth polymerization manner through intermolecular transfer of the Pd catalyst. To understand the discrepancy between the model reactions and polymerization reaction, Suzuki-Miyaura coupling reactions of 1 c with thiopheneboronic acid ester instead of 2 were carried out. This resulted in a decrease of the disubstitution product. Therefore, step-growth polymerization appears to be due to intermolecular transfer of the Pd catalyst from Th after reductive elimination of the Th-Pd-Py complex formed by transmetalation of polymer Th-Br with (Pin)B-Py-Th-Br monomer 12 (Pin=pinacol). Catalysts with similar stabilization energies of metal-arene η2 -coordination for D and A monomers may be needed for CTCP reactions of biaryl D-A monomers.

Alternating Intramolecular and Intermolecular Catalyst-Transfer Suzuki-Miyaura Condensation Polymerization: Synthesis of Boronate-Terminated π-Conjugated Polymers Using Excess Dibromo Monomers.

The Suzuki-Miyaura coupling polymerization of dibromoarene 1 and arylenediboronic acid (ester) 2 with a Pd catalyst having a high propensity for intramolecular catalyst transfer is reported. The polymerization of excess 1 with 2 affords high-molecular-weight π-conjugated polymer having boronic acid (ester) moieties at both ends, contrary to Flory's principle. This unstoichiometric polycondensation behavior is accounted for by intramolecular transfer of the Pd catalyst on 1. In the polymerization of 1 and 2 having different aryl residues, high-molecular-weight polymer is obtained when the stronger donor aromatic is used as the dibromo monomer and the weaker donor or acceptor aromatic is used as diboronic acid (ester) monomer. The pinacol boronate moieties at both ends of the obtained poly(p-phenylene) (PPP) can be converted to benzoic acid ester, hydroxyl group, and bromine. Furthermore, the reaction of the pinacol boronate-terminated PPP with poly(3-hexylthiophene) (P3HT) having bromine at one end yields a triblock copolymer of P3HT-b-PPP-b-P3HT.

Structural requirements for palladium catalyst transfer on a carbon-carbon double bond.

Intramolecular transfer of (t)Bu3PPd(0) on a carbon-carbon double bond (C═C) was investigated by using Suzuki-Miyaura coupling reaction of dibromostilbenes with aryl boronic acid or boronic acid esters in the presence of various additives containing C═C as a model. Substituent groups at the ortho position of C═C of stilbenes are critical for selective intramolecular catalyst transfer and may serve to suppress formation of the bimolecular C═C-Pd-C═C complex that leads to intermolecular transfer of (t)Bu3PPd(0).

Influence of the boron moiety and water on suzuki-miyaura catalyst-transfer condensation polymerization.

Although water promotes Suzuki-Miyaura coupling reaction, it also induces side reactions such as deboronation and dehalogenation. Therefore, Suzuki-Miyaura polymerization of triolborate halothiophene monomer 1 with (t) Bu3 PPd(o-tolyl)Br (2) in dry tetrahydrofuran (THF) is investigated. However, the resultant poly(3-hexylthiophene) (P3HT) shows a broad molecular weight distribution and uncontrolled polymer ends. Model reactions of a number of boron reagents 3 with 2,5-dibromothiophene (4) in the presence or absence of water indicate that intramolecular transfer of the catalyst is hardly affected by the boron moiety of 3, whereas it is hindered in the absence of water. Indeed, polymerization of 1 with 2 in H2 O/THF affords P3HT with a narrower molecular weight distribution and controlled tolyl/H ends, as compared to the reaction in dry THF.

Co-crystallization phase transformations in all π-conjugated block copolymers with different main-chain moieties.

Driven by molecular affinity and balance in the crystallization kinetics, the ability to co-crystallize dissimilar yet self-crystallizable blocks of a block copolymer (BCP) into a uniform domain may strongly affect its phase diagram. In this study, we synthesize a new series of crystalline and monodisperse all-π-conjugated poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-(2-ethylhexyl)thiophene) (PPP-P3EHT) BCPs and investigate this multi-crystallization effect. Despite vastly different side-chain and main-chain structures, PPP and P3EHT blocks are able to co-crystallize into a single uniform domain comprising PPP and P3EHT main-chains with mutually interdigitated side-chains spaced in-between. With increasing P3EHT fraction, PPP-P3EHTs undergo sequential phase transitions and form hierarchical superstructures including predominately PPP nanofibrils, co-crystalline nanofibrils, a bilayer co-crystalline/pure P3EHT lamellar structure, a microphase-separated bilayer PPP-P3EHT lamellar structure, and finally P3EHT nanofibrils. In particular, the presence of the new co-crystalline lamellar structure is the manifestation of the interaction balance between self-crystallization and co-crystallization of the dissimilar polymers on the resulting nanostructure of the BCP. The current study demonstrates the co-crystallization nature of all-conjugated BCPs with different main-chain moieties and may provide new guidelines for the organization of π-conjugated BCPs for future optoelectronic applications.

Scope of controlled synthesis via chain-growth condensation polymerization: from aromatic polyamides to π-conjugated polymers.

Conventional condensation polymerization proceeds in a step-growth polymerization manner, in which the generated polymers possess a broad molecular weight distribution, and control over molecular weight and polymer end groups is difficult. However, the mechanism of condensation polymerization of some monomers has been converted from step-growth to chain-growth by means of activation of the polymer end group, either due to the difference in substituent effects between the monomer and the polymer, or due to successive intramolecular transfer of catalyst to the polymer end. In this article, we review recent developments in chain-growth condensation polymerization (CGCP) in these two areas. The former approach has yielded many architectures containing aromatic polyamides and aromatic polyethers, with unique properties. In the latter case, the mechanism, catalysts, and initiators of Ni- and Pd-catalyzed coupling polymerizations leading to poly(alkylthiophene)s and poly(p-phenylene)s have been extensively investigated. Other well-defined π-conjugated polymers, such as polyfluorenes, n-type polymers, and alternating aryl polymers, have also been synthesized by means of catalyst-transfer condensation polymerization. Many π-conjugated polymer architectures prepared by utilizing catalyst-transfer condensation polymerization are not covered in this article.

Precision Synthesis of n-Type π-Conjugated Polymers in Catalyst-Transfer Condensation Polymerization.

Recent developments in catalyst-transfer condensation polymerization, which proceeds in a chain-growth polymerization manner, have made it possible to synthesize well-defined π-conjugated polymers with controlled molecular weight and low polydispersity, as well as block copolymers and gradient copolymers. However, catalyst-transfer condensation polymerization has been limited to the polymerization of donor monomers (such as thiophene) for the synthesis of p-type π-conjugated polymers. Here, we highlight several recent advances in catalyst-transfer condensation polymerization leading to n-type π-conjugated polymers. The Kumada-Tamao coupling polymerization of Grignard pyridine monomers yields well-defined poly(pyridine-3,5-diyl) and poly(pyridine-2,5-diyl) with a broad molecular weight distribution. Monomers consisting of strong acceptor and weak donor moieties also undergo catalyst-transfer polymerization; well-defined poly(fluorene benzothiaziazole) was obtained by Suzuki-Miyaura coupling polymerization and poly(bithiophene naphthalene diimide) was obtained by an unusual Ni-catalyzed coupling polymerization of an anion radical generated from a bromothiophene naphathalene diimide bromothiophene monomer and activated zinc.

Precision synthesis of poly(3-hexylthiophene) from catalyst-transfer Suzuki-Miyaura coupling polymerization.

(t)Bu(3) PPd(Ph)Br (1)-catalyzed Suzuki-Miyaura coupling polymerization of 2-(4-hexyl-5-iodo-2-thienyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) was investigated. Monomer 2 was polymerized with 1 at 0 °C in the presence of CsF and 18-crown-6 in THF containing a small amount of water to yield P3HT with a narrow molecular weight distribution and almost perfect head-to-tail regioregularity. The M(n) values increased up to 11,400 g · mol(-1) in proportion to the feed ratio of 2 to 1. The MALDI-TOF mass spectra showed that P3HT with moderate molecular weight uniformly had a phenyl group at one end and a hydrogen atom at the other, indicating involvement of a catalyst-transfer mechanism. Successive 1-catalyzed polymerization of fluorene monomer 3 and then 2 yielded a well-defined block copolymer of polyfluorene and P3HT.

Synthesis of Well-Defined Polybenzamide-block-Polystyrene by Combination of Chain-Growth Condensation Polymerization and RAFT Polymerization.

Well-defined diblock copolymers composed of poly(N-octylbenzamide) and polystyrene were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene with a polyamide chain transfer agent (CTA) prepared via chain-growth condensation polymerization. Synthesis of a dithioester-type macro-CTA possessing the polyamide segment as an activating group was unsatisfactory due to side reactions and incomplete introduction of the benzyl dithiocarbonyl unit. On the other hand, a dithiobenzoate-CTA containing poly(N-octylbenzamide) as a radical leaving group was easily synthesized, and the RAFT polymerization of styrene with this CTA afforded poly(N-octylbenzamide)-block-polystyrene with controlled molecular weight and narrow polydispersity.

Successive chain-growth condensation polymerization for the synthesis of well-defined diblock copolymers of aromatic polyamide and aromatic polyether.

Well-defined diblock condensation copolymers composed of an aromatic polyamide and an aromatic polyether have been synthesized by means of successive chain-growth condensation polymerizations. Polymerization of a polyamide monomer with an orthogonally difunctional initiator is accompanied with side reactions. On the other hand, polymerization with a monofunctional initiator afforded well-defined polyamide, which has been converted into a macroinitiator by introduction of a terminal 4-fluorobenzophenone unit. Well-defined diblock copolymers are obtained by polymerization of a polyether monomer in the presence of this macroinitiator.