Introduction

Aliphatic polyesters are among the most used biodegradable polymers in medical applications and have been extensively investigated. Ester linkages are frequently encountered in nature and hence it is expected that synthetic polymers containing such linkages and an appropriate structure would be environmentally degradable. One of the common synthetic methods towards aliphatic polyesters is a metal-catalyzed ring-opening polymerization (ROP) of lactones, such as β-lactones, δ-lactones or ε-lactones having a four-, six- or seven-membered ring. Nevertheless, this synthetic strategy is less attractive for an industrial application due to the multi-step and hence expensive syntheses of the lactones. Another shortcoming of such a process is its incompetence to produce polyesters by ROP of γ-lactones, five-membered ring cyclic esters, in practical reaction conditions: rapid back-biting immediately takes place to come back to the original γ-lactone due to the thermodynamic stability of its five-membered ring structure. Only a limited numbers of copolymerization with methyl methacrylate, δ-valerolactone, and ε-caprolactone has been reported for unsubstituted γ-lactones with low incorporation ratio of the γ-lactone (2-43%).

As another approach to polyester, carbonylative polymerization of oxirane, that is a three-membered cyclic ether, is known to provide an equivalent of poly(β-lactone). Furukawa et al. reported the first example of copolymerization of oxirane with carbon monoxide (CO) using AlEt3/Co(acac)3 (acac = acetylacetonate) and AliBu3/H2O/Co(acac)3. Recently, several other catalyst systems, such as Co2(CO)8/pyridines, Na[Co(CO)4]/pyridines, and acylcobalt complexes, have been reported as active catalysts in the alternating copolymerization of propylene oxide with CO.

In this study, four acetylcobalt complexes, AcCo(CO)3[P(p-tolyl)3] (p-tolyl = p-Me-C6H4-) (1), AcCo(CO)3[P(OPh)3] (2), AcCo(CO)3[P(NMe2)3] (3), and AcCo(CO)2(dppp) [dppp = 1,3-bis(diphenylphosphino)propane] (4) were synthesized, characterized, and examined as catalysts for the unprecedented carbonylative polymerization of oxetanes. Here, the first example of carbonylative polymerization of oxetanes, corresponding to the ring-opening unit of γ-lactone was successfully achieved to provide up to 63% of the incorporation of the 4-hydroxyalkanoate unit into the polymer.

Syntheses of acetylcobalt complexes

Acetylcobalt complexes 1-4 were synthesized as potential catalysts for the oxetane/CO copolymerization. According to a literature, treatment of Na[Co(CO)4] with MeI and P(p-tolyl)3 under CO atmosphere provided a phosphine complex 1. Other acetylcobalt complexes, containing phosphite ligand 2, phosphorous triamide ligand 3, and bidentate phosphine ligand 4, were also prepared in a process similar to that for 1. All of complexes were characterized by 1H, 13C, and 31P NMR as well as FT-IR, and elemental analyses. The X-ray single crystal analyses reveal that the complex adopts a trigonal bipyramidal conformation, with the acetyl and phosphorous ligands occupying the apical site, except for complex 4, at which the dppp ligand occupies apical-equatorial site. The IR study indicates that electron density on cobalt center of the complexes is in the order of 4>3>1>2.

Reactions of oxetanes with CO

With complexes 1-4 in hand, the copolymerization of oxetanes 5 with CO was investigated (Scheme 1). Oxetane 5a was readily prepared from commercially available 3-hydroxymethyl-3-methyloxetane. Copolymers containing both ester and ether units were obtained with complexes 1 and 2, but no reaction proceeded with complexes 3 or 4. The reaction of 5a with CO using complex 1 (5a/Co = 200) under 8.0 MPa of CO at 100 °C for 12 h gave copolymer with activity of 300 g・(mol of Co)-1・h-1, ester/ether ratio of 27/73, and Mn of 4,900 g/mol (PDI = 1.5). The catalytic activity and the ester/ether ratio increased at higher temperature in the range of 100 °C to 140 °C, while the higher reaction temperature of 160 °C conversely reduced the ester unit formation. Thus, the copolymerization at 140 °C gave the highest incorporation of 4-hydroxyalkanoate unit (47%) into the polymer in this series. Molecular weights of the obtained copolymers decreased at higher reaction temperature in spite of an increase of the copolymer yields, most probably due to chain transfer. When compared at the same temperature, phosphite complex 2 exhibited the higher catalytic activity and produced the higher molecular weight copolymer than complex 1, while it provided the copolymer with lower ester-unit content. Temperature dependency of catalytic activity, ester/ether ratio, and molecular weight observed with 2 was similar to that with 1. The highest molecular weight of 7,800 g・mol-1 was obtained with 2 at 80 °C. The reaction of unsubstituted oxetane 5b with CO using 1 gave an oligomeric product with ester/ether ratio of 28/72. Copolymerization of ethyl ether 5c with CO using the same catalyst provided a copolymer with ester unit of 60%, higher than the copolymer derived from 5a under the same reaction condition, and Mn of 2,900 g・mol-1. Copolymerization with a longer alkyl chain at the alkoxy group, 5d, initiated by the same catalyst, provided only an oligomeric product with unexpectedly lower ester unit. Copolymerization of dimethyl-substituted oxetane 5e gave only trace amount of the copolymer. However, although 5c derived-copolymer was higher in ester unit, the yield was considerably low (6%).

Relation between electron density on Co and ester content in the copolymer

The difference in catalytic activity and the ester unit content are most likely due to the difference in electron density on cobalt center of the complexes. Scheme 2 describes a plausible mechanism of a desirable alternating incorporation of oxetane and CO to afford an ester unit (step→step b) and a competing ring-opening enchainment without carbonylation to give an ether unit (step→step c). The use of complexes with higher electron density on cobalt center would decelerate a cobaltate dissociation upon an attack of oxetane (step a), which accordingly would reduce the rate of the copolymerization. On the other hand, the higher electron density on the cobalt center is favorable for the nucleophilic attack of the dissociated cobaltate to the oxonium intermediate (step b), leading to the higher ester unit content.

In the discussion of the catalytic activity and the ester unit content based on the electron density of the cobalt center, it is necessary to consider how the cobalt complex behaves upon CO pressure. Thus, the complexes were exposed under 7.0 MPa of CO at 100 C and monitored by an in situ IR observation technique. Complex 1 was found to exist mostly as the phosphine adduct even after 3-hour exposure to CO. On the other hand, the CO exposure of complex 2 resulted immediately in the conversion to AcCo(CO)4. Accordingly, the higher electron density on the cobalt center of complex 1 should result in the lower catalytic activity and the higher ester unit content than AcCo(CO)4 which is derived from complex 2 under the reaction condition.

Because a ligand dissociation seems critical under the polymerization reaction conditions, copolymerization of 5a/CO in the presence of additional phosphorus ligands was examined. Addition of 1.0 equivalent either free P(p-tolyl)3 or P(OPh)3 ligand in the copolymerization of oxetane 5a and CO initiated by phosphine complex 1 increased the ester unit up to 60%, yet it sacrificed the catalytic activity and molecular weight. While a large excess of the P(OPh)3 up to 5.0 equivalent in the above copolymerization increased a slightly more ester unit, an equivalent excess of P(p-tolyl)3 deactivated the reaction. Complex 2 provided a similar phenomenon only in the addition of free P(OPh)3 in 1 to 1 ratio to the catalyst. It gave lower ester unit when P(p-tolyl)3 of 1.0 equivalent was added or gave only trace amount of the copolymer when the phosphite up to 5.0 equivalent was added. In the absence of any ligand, that is the reaction initiated by BnCOCo(CO)4 (6), a copolymer with ester/ether ratio of 29/71, and Mn of 3,900 g/mol (PDI = 1.9) was obtained in 37% yield. The above observations may be summarized as follows. The stronger electron donor P(p-tolyl)3 increases the ester content when compared to the weaker donor (thus weaker binder) P(OPh)3. Addition of extra amount of a phosphorus ligand seems to increase the ester content. Large excess of a free phosphorus ligand retards the reaction. This was likely the result of an equilibrium shift from a tetracarbonylcobaltate species to a phosphorous-ligated cobaltate one. In other words, the more Lewis acidic ligand-free cobalt species tend to give higher ether content while the less acidic cobalt species, achieved by the coordination of a phosphorus ligand is less active for the homopolymerization of 5a. As a result, copolymers with higher ester content were obtained in the presence of a proper amount of phosphorus ligands.

Conclusion

The first example of copolymerization of oxetanes with CO initiated by acetylcobalt complexes was reported. The copolymerization afforded polymers containing ester and ether units. The phosphorous ligands ligated to cobalt center were observed to control the ratio of ester/ether units, catalytic activity and molecular weight of the copolymer. The reaction with a phosphite complex 2, in which P(OPh)3 was instantaneously substituted by CO under the copolymerization condition, provided a copolymer with higher molecular weight, while the reaction with a P(p-tolyl)3 complex 1 gave a copolymer with higher ester unit content. In the presence of a free phosphorous ligand, a copolymer with 63% of ester content was achieved.

脂肪族ポリエステルは生分解性などの特性を活かし、広く用いられている。ポリエステルの合成法としては、環状エステルであるラクトンの開環重合が有効であり、4,6,7員環のラクトンの開環重合については多くの研究が報告されてきた。これに対し、5員環のラクトンは、熱力学的に極めて安定であるため、開環して重合するよりはポリマーがバックバイティングしてラクトンに分解する反応の方が優先してしまう。これを防ぐためには2.0 GPaといった超高圧が必要である。本研究では、コバルト触媒を用いてオキセタンと一酸化炭素を共重合させることで、5員環のラクトンに相当する4-ヒドロキシブタン酸ユニットをもつポリマーの合成に成功した。

第1章では、ポリエステルの物性とその合成法について概観についてまとめ、環状エーテルのカルボニル化によるポリエステル合成の意義を、カルボン酸とアルコールの重縮合や、ラクトンの開環に比較しながら記述した。

第2章では、環状エーテルと一酸化炭素の共重合に開始剤としてアシルコバルト種を選び、種々のリン配位子をもつ錯体を合成した。合成した錯体については、それぞれのコバルト中心上の電子密度について赤外吸収スペクトルと単結晶X線構造解析に基づいて比較検討をおこなった。その結果、単座のホスファイト錯体<単座のホスフィン錯体<単座のリントリアミド錯体<二座のホスフィン錯体の順、金属上の電子密度が上がっていることがわかった。次にこれらの錯体を用いて2-メチル-2-メトキシオキセタンと一酸化炭素の共重合反応を検討した。比較的コバルト上の電子密度の低い、単座のホスファイト錯体と単座のホスフィン錯体では重合反応が進行したが、他の錯体では反応がおこらなかった。得られたポリマーを核磁子共鳴ならびに質量分析により解析すると、オキセタンと一酸化炭素の交互共重合により生じる4-ヒドロキシブタン酸エステルユニットとともに、オキセタンが単独重合したポリエーテルユニットも存在していることがわかった。得られたポリマーのエステル部分を加水分解し、サイズ排除クロマトグラフィーで分子量別に単離すると、加水分解前のポリマーには、ポリエーテル部分、ポリエステル部分、両者の共存している部分がそれぞれ存在していたことがわかった。重合活性は電子不足であるホスファイト錯体の方が高く、エステル/エーテル比はホスフィン錯体の方が高かった。この違いを考察すべく、それぞれの錯体の一酸化炭素加圧条件での挙動を調べた。その結果、ホスフィン配位子は一酸化炭素加圧下でも配位しているのに対し、ホスファイト配位子は一酸化炭素と置換してコバルトカルボニル錯体を生じていることがわかった。

第3章では、上記第2章で得られた知見に基づき、リン配位子が配位した錯体の濃度を上げることで、エステルユニットを増やすことを検討した。すなわち、第2章で用いたコバルト錯体に対し、さらに過剰量のリン配位子を加えて重合反応をおこなった。その結果、最大63%の4-ヒドロキシブタン酸エステルユニットを含むポリマーが得られた。これまでに5員環ラクトンを他のモノマーと共重合させる試みはあったが、例えば他のラクトンとの共重合で代価上(98当量)の5員環ラクトンを用いても最大43%しか取り込まれなかったことと対照的である。さらに本章では、オキセタンと一酸化炭素が直接共重合しているのか、あるいはいったん5員環ラクトンを生成し、生じたラクトンが開環して重合しているのかについて調べた。その結果、2位に置換基をもつ2-メチル-2-メトキシオキセタンは直接共重合が主たるルートであるのに対し、無置換のオキセタンではラクトンの生成とその開環が確認された。

第4章では、コバルトカルボニルアニオンの対イオンが重合に与える影響を調べ、ラクトン生成とポリマー生成の二つのルートが、用いる対カチオンによって大きく左右されることを明らかにした。

第5章では、各章で得られた知見を総合し、今後の当該分野の方向性について論じた。2-ヒドロキシ-2-メチルオキセタンはプロパナールとホルムアルデヒドから容易に合成できるため、本研究は今後のポリエステル合成に大きなインパクトを与える結果として重要である。