Effects of solvents, additives, and π-allyl ligand structures on the polymerization behavior of diazoacetates initiated by π-allylPd complexes

Hiroaki Shimomoto*, Moemi Nakajima, Akihiro Watanabe, Hirokazu Murakami, Tomomichi Itoh and Eiji Ihara*
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan. E-mail: ihara@ehime-u.ac.jp; shimomoto.hiroaki.mx@ehime-u.ac.jp; Fax: +81-89-927-8547; Fax: +81-89-927-9949; Tel: +81-89-927-8547 Tel: +81-89-927-9949

Received 2nd November 2019 , Accepted 21st January 2020

First published on 22nd January 2020


In the polymerization of diazoacetates initiated by π-allylPd-based initiating systems, the effects of solvents, additives, and π-allyl ligand structures on the polymerization behavior were investigated. As a result, the polymerization in the presence of pyridine or its derivatives as an additive was found to afford a polymer with relatively narrow molecular weight distribution compared to that with π-allylPdCl alone. Furthermore, we have demonstrated that Pd complexes with π-allyl ligands with a variety of substituents are capable of polymerizing diazoacetates in a similar manner to the parent unsubstituted π-allylPdCl, and the tacticity of the resulting polymers is affected by the structure of the π-allyl ligands.


Introduction

For the synthesis of carbon–carbon main chain polymers, C1 polymerization has received much attention as a complementary approach to vinyl polymerization. A representative example of C1 polymerization is the polymerization of diazoacetates, in which the carbon backbone is built from one carbon unit with concomitant N2 elimination in each chain-growth step to yield carbon–carbon main chain polymers bearing an ester substituent on each main chain carbon atom [poly(alkoxycarbonylmethylene)s] (Scheme 1).1–6 In this polymerization technique, various functional groups (R) can be introduced as substituents of the ester, as is the case with the polymerization of (meth)acrylates. Functional ester substituents in poly(alkoxycarbonylmethylene)s are located on every main chain carbon atom, in contrast to poly(meth)acrylates, in which the ester side chain exists on every other main chain carbon atom. Thus, the poly(alkoxycarbonylmethylene)s possessing densely accumulated functional groups are anticipated to and have been indeed demonstrated to exhibit unique properties due to their structural characteristics of dense packing of the substituents around the polymer chain,7–24 compared to the vinyl polymer counterparts [i.e., poly(meth)acrylates]. Although poly(alkoxycarbonylmethylene)s bearing an ester side chain on each main chain carbon atom can be alternatively obtained by the radical polymerization of α,β-unsaturated diesters (dialkyl fumarate or maleate),25 there exist considerable limitations such as monomer scope, livingness, and the control of tacticity in radical polymerization.
image file: c9py01654f-s1.tif
Scheme 1 Polymerization of diazoacetates and polymerization of (meth)acrylates.

As an initiator for the polymerization of diazoacetates, several transition metal-based initiating systems have been reported to be capable of producing polymers. In particular, Rh(diene)-based initiating systems developed by de Bruin and coworkers are effective at obtaining polymers with extremely high molecular weights and high stereoregularity.26–29 However, the monomer scope is relatively narrow and controlled/living polymerization is still a challenge for Rh-based systems. Meanwhile, Pd-based initiating systems30–34 are effective at obtaining polymers from diazoacetates bearing various functional groups, although molecular weights of the polymers are lower than those of polymers obtained by the Rh(diene) complexes and tacticity of the polymers is atactic. In particular, π-allylPdCl-based systems32 are effective at polymerizing various diazoacetates with high initiating efficiency (Fig. 1).


image file: c9py01654f-f1.tif
Fig. 1 Effective Pd-based initiating systems for the polymerization of diazoacetates.

As for control of the polymerization, we have demonstrated that π-allylPdCl/NaBPh4 can polymerize diazoacetates bearing a cyclophosphazene unit in a controlled manner to give well-defined organic–inorganic hybrid homopolymers and block copolymers.9,14 In the polymerization system, a Pd–Ph initiating species is proposed to be generated by transmetalation from the borate BPh4 to the Pd center; replacing the Cl ligand of π-allylPdCl with the more nucleophilic Ph group is likely to promote uniform initiation to give a Ph-initiated polymer with relatively narrow molecular weight distribution. However, monomers that can be applied for the controlled polymerization are limited to diazoacetates bearing a bulky substituent. Meanwhile, Toste and coworkers35 recently demonstrated that π-allylPd(carboxylate) dimers, where the Cl atom of π-allylPdCl is replaced by the more nucleophilic carboxylates including acetate, methacrylate, and pivalate, are effective for controlled polymerization of diazoacetates, such as ethyl diazoacetate (EDA) and 2,2,2-trifluoroethyl diazoacetate bearing ester groups with a common size. For example, the polymerization of EDA in fluorinated solvents yielded polymers with narrow molecular weight distribution (Mw/Mn = 1.2–1.4) in high yield (ca. 90%). In addition, they proposed therein a polymerization mechanism with a dinuclear Pd propagating species based on theoretical calculations with the density functional theory. However, the π-allylPd(carboxylate) dimers are less stable compared to the commercially available π-allylPdCl dimer.36 On the other hand, Wu and coworkers recently reported that the addition of a sulfinamide bisphosphine ligand to the π-allylPdCl dimer enables the living polymerization of a variety of diazoacetates.22 However, the synthesis of the ligand is somewhat elaborate and the role of the ligand in achieving the living polymerization is unclear at present. In addition, in our newly found Pd-based initiating system, Pd(nq)2/NaBPh4 (nq = 1,4-naphthoquinone), we have demonstrated that π-allyl-type ligation of an anionic ligand in situ derived from nq with the assistance of the borate plays an essential role in bringing about an efficient initiating ability of the initiating system.34

The above-described results clearly indicated the general importance of the π-allyl-type ligand in the Pd-based initiators for diazoacetate polymerization. For example, as revealed in papers by us, Wu, and Toste, in the parent π-allylPdX(L) system, the choice of X (e.g., Cl, Ph, OAc) and L [π-allylPdX (dimeric form) or a sulfinamide bisphosphine ligand] exerts a crucial effect on the initiating ability, suggesting that a subtle difference around the coordination environment of the Pd-complexes should be critical for realizing high initiating ability. Accordingly, it should be reasonable to conduct a systematic study to examine the effect of solvents and additives on the diazoacetate polymerization by π-allylPdCl, which we indeed employed in this study. As for the choice of the solvent, because we have reported that THF as a solvent causes termination via coordination to the Pd center and ring-opening,32 the use of THF derivatives with less ring opening reactivity would improve the controllability of the polymerization. Thus, in this study, we first conducted the polymerization of EDA in a variety of solvents including THF derivatives and in the presence of a variety of additives such as amines and phosphines. Another point we focused on in this study is the steric effect of π-allyl ligands on the tacticity of the resulting polymers. It is reasonable to expect that the substituents on the carbon atoms on the π-allyl ligand should have some effects on the tacticity of the resulting polymers, although such investigation has not been reported so far. Thus, in this study, we used Pd-complexes with a variety of substituents on the π-allyl ligand for diazoacetate polymerization, and investigated the tacticity of the resulting polymers.

As a result, we have indeed found that the choice of solvents, additives, and π-allyl ligands has certain influences on the polymerization results, even though the extent is not so significant. However, because such a systematic study has not been conducted so far and the findings here will contribute to further improvement of the Pd-based initiating systems for this relatively new and important polymerization, the results of fundamental investigation described here have novelty and significance in the field of polymer chemistry.

Results and discussion

Effect of solvents on polymerization behavior

Attempting to improve the controllability of π-allylPd-based initiating systems for the polymerization of diazoacetates, we considered that the appropriate choice of solvents is crucial because we have demonstrated that tetrahydrofuran (THF) used as a solvent can terminate the π-allylPdCl-initiated polymerization of diazoacetates.32 We proposed therein a termination mechanism involving THF based on the results of MALDI-TOF-MS analyses; the Pd-propagating chain end nucleophilically attacks a coordinated THF followed by its ring-opening and protonolysis with an acidic quencher to give a –[CH2]4OH chain end (Scheme 2). The suppression of this termination would result in improved quality of control of the polymerization. Thus, first of all, we carried out the polymerization using various solvents other than THF used in the previous studies and investigated the effect of solvents on the polymerization behavior (Table 1), which has not been examined in the previous reports.
image file: c9py01654f-s2.tif
Scheme 2 A proposed termination mechanism for the polymerization of diazoacetates using π-allylPdCl-based initiating systems.
Table 1 Polymerization of EDA initiated with π-allylPdCl in various solventsa
Run Solvent Yieldb (%) Mn[thin space (1/6-em)]c Mw/Mn[thin space (1/6-em)]c
a Solvent = 2.0 mL, EDA = 1.2 mmol (used as a 2.1–3.1 M solution in CH2Cl2), [Pd] = 2[π-allylPdCl], [EDA]/[Pd] = ca. 100, at −20 °C for 13 h.b After purification with preparative GPC.c Determined by GPC (PMMA standards).d Toluene/THF (v/v = 1/1).
1 THF 64 6100 1.87
2 Dichloromethane Trace n.d. n.d.
3 Acetonitrile 15 4400 1.62
4 Toluene 25 2000 1.64
5 Toluene/THFd 51 5300 1.73
6 Diethyl ether 30 1900 1.98
7 Tetrahydropyran 38 3500 2.00
8 2-MeTHF 57 4900 1.92
9 3-MeTHF 66 5500 1.86
10 2,5-Me2THF 67 4300 1.78


First, dichloromethane, acetonitrile, and toluene were employed as solvents for the polymerization, because the use of these solvents along with THF was reported to be capable of producing high-molecular-weight polymers in the Rh-initiated polymerization.26 When dichloromethane was used as a solvent for the polymerization, π-allylPdCl did not produce polymers in contrast to the Rh-initiated polymerization (run 2). Considering that the polymerization system conducted in THF (run 1) contains a small amount of dichloromethane (ca. 20 vol%) because a dichloromethane solution of EDA was used for the polymerization, dichloromethane at least can be used as a co-solvent for the polymerization. Acetonitrile and toluene were much less effective as solvents for the polymerization compared to THF, giving polymers with significantly lower Mn and yield values (runs 3 and 4). Acyclic (diethyl ether) and six-membered cyclic (tetrahydropyran) ethers also yielded polymers with lower Mns in lower yield compared to those obtained in THF (runs 6 and 7). Judging from the results, THF plays a crucial role for the polymerization to proceed effectively. This is also supported by the result that a higher Mn polymer was obtained in a mixture of toluene and THF (v/v = 1/1) compared to that in toluene alone (run 5).

Then, we examined the polymerization behavior in a series of Me-substituted THF derivatives, 2-methyltetrahydrofuran (2-MeTHF) and 3-methyltetrahydrofuran (3-MeTHF), expecting that the presence of the Me groups would sterically prevent the nucleophilic attack of the propagating chain end to the carbon atom adjacent to oxygen in the coordinated THF described in Scheme 2. In addition, 2-MeTHF should be less favorable for the coordination with the Pd-propagating chain end compared to THF due to the steric effect and thus the undesired ring-opening could be suppressed. When 2-MeTHF and 3-MeTHF were used as solvents, a similar polymerization behavior was observed with respect to polymer yield, Mn, and Mw/Mn, to those of the polymer obtained in THF (runs 8 and 9). Fig. 2A shows the MALDI-TOF-MS spectrum of the product obtained by the polymerization in 3-MeTHF. On the basis of the comparison of one of the major peak clusters with simulated signal appearance, the set of signals is assigned to a Na+ adduct of the polymer with Cl and ring-opened 3-MeTHF [–CH2CH(CH3)CH2CH2OH or –CH2CH2CH(CH3)CH2OH] at the α- and ω-chain ends, respectively (Fig. 2B).37 The same set of signals was also observed in the MS spectrum of the product obtained in 2-MeTHF (Fig. S1). These results indicate that the termination in these Me-substituted THFs was not suppressed, contrary to our expectation.


image file: c9py01654f-f2.tif
Fig. 2 Part of the MALDI-TOF-MS spectra of the products obtained by the polymerization of EDA in 3-MeTHF (A, run 9 in Table 1) and in 2,5-Me2THF (C, run 10 in Table 1), theoretical isotopic distribution of a Na-adduct of the polymer [degree of polymerization (DP) = 15] bearing Cl and ring-opened 3-MeTHF at the α- and ω-chain ends, respectively (B), and theoretical isotopic distribution of a Na-adduct of the HO-initiated polymer (DP = 16) terminated by backbiting (D). Several minor sets of signals were assigned as shown in the ESI (Fig. S3).

Next, we conducted the polymerization in a disubstituted THF, 2,5-dimethyltetrahydrofuran (2,5-Me2THF), expecting that the steric effect of the substitution at both 2- and 5-positions of THF effectively suppresses the termination. Because we could not observe the set of signals assignable to a Na+ adduct of the polymer with –CH(CH3)CH2CH2CH(CH3)OH at the ω-chain end in the MALDI-TOF-MS spectrum of the product, the termination should be effectively suppressed by using the disubstituted THF. However, any improvement in the quality of control of the polymerization was not achieved: a polymer with Mn = 4300 was obtained in 67% yield (run 10). This is probably because other side reactions including termination and chain transfer reactions32 occurred in this case; indeed, in the MS spectrum, the main set of signals can be assignable to the polymer with a cyclic framework resulting from backbiting at its ω-chain end (Fig. 2C and D).38 Since we could not find a better solvent than THF with respect to the quality of control of the polymerization, we tentatively conclude that THF is the best solvent for this polymerization system.

Effect of additives on polymerization behavior

Next, the polymerization was carried out in the presence of various additives including phosphines and amines, which are employed as good σ-donating ligands to increase the electron density of transition metal complexes (Table 2). When triphenylphosphine (PPh3) was added to the polymerization system ([PPh3]/[Pd] = ca. 1.05/1), no product was obtained at −20 °C (run 2). Instead, at a higher temperature of 30 °C (run 3), the polymerization of EDA proceeded to give a polymer with a similar Mn and yield to those of the polymer obtained in the absence of additives at −20 °C. The polymerization using tricyclohexylphosphine (PCy3) with higher basicity as an additive also proceeded at 30 °C (run 4) to give a polymer. The addition of phosphines did not lead to a significant improvement in the polymerization behavior.
Table 2 Polymerization of EDA initiated with π-allylPdCl in the presence of various additivesa
Run Additive Temp. (°C) Solvent Yieldb (%) Mn[thin space (1/6-em)]c Mw/Mn[thin space (1/6-em)]c
a Solvent = 2.0 mL, EDA = 1.2 mmol (used as a 2.1–3.1 M solution in CH2Cl2), [Pd] = 2[π-allylPdCl], [EDA]/[Pd] = ca. 100, [additive]/[Pd] = ca. 1.05, polymerization period = 13 h.b After purification with preparative GPC.c Determined by GPC (PMMA standards).
1 None −20 THF 64 6100 1.87
2 PPh3 −20 THF Trace n.d. n.d.
3 PPh3 30 THF 60 6000 1.74
4 PCy3 30 THF 49 7300 1.73
5 Pyridine −20 THF 67 6000 1.37
6 Pyridine −20 α,α,α-Trifluorotoluene 11 3200 1.60
7 Pyridine −20 2,5-Me2THF 26 2300 2.06
8 2-Methylpyridine −20 THF 52 5000 1.32
9 3-Methylpyridine −20 THF 45 4400 1.41
10 DMAP −20 THF 48 6400 1.48
11 2,6-Lutidine −20 THF 64 5100 1.64
12 2,2′-Bipyridine −20 THF 17 4700 1.61
13 L1 rt THF 47 7400 1.72


On the other hand, the addition of pyridine or its derivatives was found to be effective at improving the polymerization behavior. By adding pyridine with a ratio of [pyridine]/[Pd] = ca. 1.05/1, the Mw/Mn value of the obtained polymer became obviously narrow (Mw/Mn = 1.37) compared to the product obtained with π-allylPdCl alone (Mw/Mn = 1.87), while maintaining moderate yield (run 5). Fig. 3 shows SEC curves of the products obtained by the polymerization of EDA in the absence and presence of pyridine. A sharp rise at the onset in the latter curve suggests that fast initiation relative to propagation occurred for the polymerization in the presence of pyridine. In addition, a significantly different peak appearance was observed in MALDI-TOF-MS analyses (Fig. 4A and B). In the MS spectrum of the product obtained in the absence of pyridine, several sets of signals with similar intensities were observed, suggesting that there existed several modes of initiation and/or termination in the polymerization. In sharp contrast, one set of signals was predominantly detected in the MS spectrum of the product obtained in the presence of pyridine. The dominant set of signals can be assigned to the polymer with Cl and a ring-opened THF at the α- and ω-chain ends, respectively (Fig. 4B and C).32 These results suggest that the initiation from Pd–Cl is much more efficient on monomeric π-allylPdCl(pyridine) than on the dimeric (π-allylPdCl)2.


image file: c9py01654f-f3.tif
Fig. 3 SEC curves of the products obtained by the polymerization of EDA in the absence (left; run 1 in Table 2) and presence (right; run 5 in Table 2) of pyridine.

image file: c9py01654f-f4.tif
Fig. 4 Part of the MALDI-TOF-MS spectra of the polymers obtained by the polymerization of EDA in the absence (A, run 1 in Table 2) and presence of pyridine (B, run 5 in Table 2), and theoretical isotopic distribution of a Na-adduct of the polymer bearing Cl and ring-opened THF at the α- and ω-chain ends, respectively (C).

On the other hand, tailing at the lower Mn region still remains in the SEC curve of the polymer obtained in the presence of pyridine, which would be ascribed to the occurrence of the termination by THF before monomer consumption, as confirmed by MALDI-TOF-MS analyses (Fig. 4B and C). Interestingly, the termination was predominant in the presence of pyridine. The addition of pyridine derivatives such as 2-methylpyridine, 3-methylpyridine, and 4-dimethylaminopyridine (DMAP) showed a similar effect to pyridine (runs 8–10), while the use of 2,6-dimethylpyridine (2,6-lutidine) with a lower nucleophilicity due to the steric hindrance of the two methyl groups exhibited a significantly less effect in lowering Mw/Mn values (run 11). The polymerization in the presence of 2,2′-bipyridine (a bidentate pyridyl ligand) instead of pyridine produced a polymer in much lower yield (run 12).

In order to avoid the undesired termination with ring-opening of THF, we conducted the polymerization in α,α,α-trifluorotoluene, which was recently reported to be an efficient solvent for the polymerization of diazoacetates initiated by π-allylPd(carboxylate) dimers. However, a lower Mn polymer was obtained in low yield (run 6); this result is similar to the reported one for the polymerization initiated by the π-allylPdCl dimer alone in α,α,α-trifluorotoluene.35 A similar result was obtained when 2,5-Me2THF was used as a solvent for the polymerization with pyridine as an additive (run 7).

In addition, inspired by the recent work reported by Wu and coworkers,22 where controlled polymerization was achieved by the combination of π-allylPdCl and a sulfinamide bisphosphine ligand although the role of the ligand including the coordination mode has not been demonstrated in the report, we conducted the polymerization of EDA initiated by π-allylPdCl in the presence of L1, which has a substructure of the sulfinamide bisphosphine ligand used by Wu and coworkers (Scheme 3). It was reported that the treatment of the π-allylPdCl dimer with L1 gave 2, whose structure was revealed by X-ray structure analysis to have the coordination of a P atom in L1 on Pd and the intramolecular H-bonding interaction between NH and Cl, in the literature.39 The polymerization of EDA with the in situ-generated 2 in THF with a feed ratio of [EDA]/[Pd] = 100 did not give a polymer with narrow molecular weight distribution (run 13). This result suggests that the sulfinamide-containing bidentate phosphine structure plays a crucial role in achieving the controlled polymerization.


image file: c9py01654f-s3.tif
Scheme 3 Preparation of 2 by treatment of π-allylPdCl dimer with L1.

Furthermore, along with the aforementioned in situ technique, isolated Pd complexes with a well-defined structure (3–5) were employed for the polymerization (Chart 1 and Table 3). When the PPh3-ligated π-allylPdCl (3)40 was used as a catalyst, the polymerization of EDA did not proceed at −20 °C while a polymeric product was obtained at a higher temperature (runs 1 and 2), as is the case with the corresponding in situ technique (polymerization with π-allylPdCl in the presence of ca. 1.05 equivalents of PPh3). With the pyridine-ligated π-allylPdCl (4),41 a polymer with relatively narrow molecular weight distribution was obtained in a similar manner to the in situ approach (run 3). On the other hand, the chelate-type Pd complex 5,42 where the dissociation of the pyridine–Pd bond would be suppressed due to the chelating structure, did not produce polymers (run 4). The reactivity was not improved even at a higher temperature (run 5).


image file: c9py01654f-c1.tif
Chart 1 Structures of isolated Pd complexes with a P- or N-based ligand.
Table 3 Polymerization of EDA using isolated π-allylPd complexes with a P- or N-based liganda
Run Pd complex Temp. (°C) Yieldb (%) Mn[thin space (1/6-em)]c Mw/Mn[thin space (1/6-em)]c
a In THF (2.0 mL), EDA = 1.2 mmol (used as a 2.1–3.1 M solution in CH2Cl2), [Pd] = 2[π-allylPdCl], [EDA]/[Pd] = ca. 100, polymerization period = 13 h.b After purification with preparative GPC.c Determined by GPC (PMMA standards).
1 3 −20 Trace n.d. n.d.
2 3 rt 62 6700 1.51
3 4 −20 62 6900 1.31
4 5 −20 Trace n.d. n.d.
5 5 rt Trace n.d. n.d.


As a whole, we have demonstrated that the presence of pyridine brings about the improvement in lowering molecular weight distribution in the π-allylPdCl-catalyzed polymerization of diazoacetates, although ideal controlled/living polymerization has not been achieved. Another advantage for the polymerization with pyridine is that the obtained polymers have relatively well-defined and reactive end groups (Cl and OH at the α- and ω-chain ends, respectively) and thus can be used as a prepolymer for postpolymerization modification; for instance, atom transfer radical polymerization of vinyl monomers from the Cl group43 and ring-opening polymerization of cyclic monomers including lactide from the OH group44 should produce unprecedented triblock copolymers with a poly(alkoxycarbonylmethylene) middle block.

Effect of π-allyl ligand structures on polymerization behavior

Polymerization with isolated Pd complexes having a π-allyl ligand. In order to investigate the effect of the ligand structures on the polymerization behavior, we carried out the polymerization of EDA using various Pd complexes with a variety of π-allyl ligands (Chart 2 and Table 4). When (2-methylallyl)PdCl dimer (6)45 was used for the polymerization, both yield and Mn values of the resulting polymer were almost identical to those of the product obtained with π-allylPdCl (1) (runs 1 and 2). The cinnamyl- and β-pinenyl-substituted Pd complexes (7[thin space (1/6-em)]46 and 8[thin space (1/6-em)]47) can also polymerize EDA to give polymers in moderate yields (runs 3 and 4), although the Mn value of the product obtained with 7 was rather low (Mn = 2700). On the other hand, the indenyl-substituted Pd complex 9[thin space (1/6-em)]48 did not produce polymers (run 5). This might be attributed to the lower stability of the Pd complex with an indenyl ligand.49 The polymerization using the CF3COO-bridged dinuclear π-allylPd complex 10 afforded a polymer as is the case with the Cl-bridged dinuclear π-allylPd complexes. In the MALDI-TOF-MS spectrum, the set of signals that correspond to the polymer bearing a CF3COO group at its α-chain end were not observed, suggesting that the lower nucleophilic CF3COO– group cannot cause initiation (Fig. S4). Because the main set of signals can be assignable to the polymer with a OH group at the α-chain end, the polymerization would be initiated from a Pd–OH species which is probably generated via σ-bond metathesis of 10 and adventitious water.32 When 11 with an N-heterocyclic carbene ligand was used as a catalyst, a similar polymerization behavior was observed to that with the PPh3-ligated π-allylPd complex 3; polymerization proceeded at a higher temperature (50 °C) to give a polymer (runs 7 and 8) but retarded at −20 °C. With the above results, we have demonstrated that a series of Pd complexes having a π-allyl ligand have initiating ability for the polymerization of diazoacetates although no remarkable improvement in reactivity was observed.
image file: c9py01654f-c2.tif
Chart 2 Structures of Pd complexes with a π-allyl-based ligand.
Table 4 Polymerization of EDA using Pd complexes with a π-ligand coordinated in an η3-fashiona
Run Pd complex Temp. (°C) Yieldb (%) Mn[thin space (1/6-em)]c Mw/Mn[thin space (1/6-em)]c
a In THF (2.0 mL), EDA = 1.2 mmol (used as a 2.1–3.1 M solution in CH2Cl2), [Pd] = 2[π-allylPdCl], [EDA]/[Pd] = ca. 100, polymerization period = 13 h.b After purification with preparative GPC.c Determined by GPC (PMMA standards).
1 π-AllylPdCl (1) −20 64 6100 1.87
2 6 −20 65 6200 1.84
3 7 −20 65 2700 1.98
4 8 −20 53 4600 1.86
5 9 −20 Trace n.d. n.d.
6 10 −20 69 4200 1.71
7 11 −20 Trace n.d. n.d.
8 11 50 30 7200 1.59


Polymerization with in situ generated Pd complexes having a π-allyl ligand. Next, we carried out the polymerization of EDA with in situ-generated π-allyl-based Pd complexes prepared by the oxidative addition of an allyl compound to a Pd(0) complex (Fig. 5). By utilizing this methodology, a variety of π-allyl ligands are able to be easily screened without the isolation of well-defined complexes. The polymerization using the in situ-generated complex prepared from the reaction of Pd2(dba)3·CHCl3 [dba = (E,E)-dibenzylideneacetone] with ca. 1.05 equivalents of allyl chloride (A1) proceeded to give a polymer, while the polymerization without the addition of allyl chloride, namely Pd2(dba)3·CHCl3 alone did not give a polymer. The Mn (5300), Mw/Mn (1.78), and yield (67%) values of the polymer obtained by the in situ prepared complex are similar to those for the polymer obtained by the corresponding isolated Pd complex 1 (Mn = 6100, Mw/Mn = 1.87, yield = 64%). This result indicates that π-allylPdCl was formed in situ from Pd2(dba)3·CHCl3 and A1. A similar result was obtained in the polymerization with the in situ-generated complex prepared from Pd2(dba)3·CHCl3 and allyl trifluoroacetate (A2) [Mn = 6300, Mw/Mn = 1.88, yield = 74% (in situ-generated Pd complex) vs. Mn = 4200, Mw/Mn = 1.71, yield = 69% (isolated Pd complex 10; run 6 in Table 4)]. Also, allyl bromide (A3), which could yield a Br-bridged Pd complex in situ, gave a polymer. On the other hand, when allyl acetate (A4) was used as an allyl compound, no polymeric product was obtained in contrast to the case with A1–A3. Considering that the corresponding isolated Pd complex, π-allylPd(acetate) dimer, has been reported to be able to polymerize diazoacetates in a quasi-living/controlled manner to give polymers with low Mw/Mn,35 the inactivity of the in situ Pd2(dba)3·CHCl3/allyl acetate initiating system could be ascribed to either the low ability of the oxidative addition of allyl acetate to Pd(0) or the low stability or activity of the π-allylPd(acetate) generated in situ in the presence of dba. In the case with A5 and A6, which could give a π-allylPd complex having an electron-withdrawing group (Y = Cl and CO2CH3) on the 2-position of the π-allyl ligand, polymeric products were obtained, although the Mn and yield values were rather low. When A7 with the electron-donating OCH2OCH3 group was employed as an allyl compound for the polymerization, a polymer with a slightly higher Mn (8800) was obtained compared to those obtained with Pd complexes with the unsubstituted π-allyl ligand. The increase in Mn might be ascribed to the chelating coordination of the methoxymethyl moiety on the 2-position of the π-allyl ligand to Pd, because the use of A8 also gave a higher Mn polymer. As an allyl compound, A9 bearing a bulky substituent was also effective for the polymerization although no significant change in polymerization behavior was observed. Again, we have demonstrated that a series of in situ generated π-allylPd complexes can initiate the polymerization of diazoacetate although the reactivity was not largely dependent on the steric and electronic effects of substituents on the π-allyl ligands.
image file: c9py01654f-f5.tif
Fig. 5 Polymerization of EDA initiated with Pd2(dba)3·CHCl3/allyl compound systems. In THF (2.0 mL), EDA = 1.2 mmol (used as a 2.1–3.1 M solution in CH2Cl2), [Pd] = 2[π-allylPdCl], [EDA]/[Pd] = ca. 100, [allyl compound]/[Pd] = ca. 1.05, at −20 °C for 13 h. a[thin space (1/6-em)]After purification with preparative GPC. bDetermined by GPC (PMMA standards).
Stereoregularity of the resulting polymers. It is noteworthy that the polymer tacticity changed slightly depending on the π-allyl ligand structures. Fig. 6 shows a part of 1H and 13C NMR spectra of polymers obtained by various Pd complexes with a π-allyl-based ligand. The signals at around 2.8–3.7 ppm in the 1H NMR spectra and the signals at around 169–173 ppm in the 13C NMR spectra are assignable to the main chain methine protons and the side chain carbonyl carbons, respectively. The peak appearances should be dependent on tacticity, and the broad and split signals indicate that the polymers obtained in this study are all atactic polymers, while a highly stereoselective EDA polymer obtained by the Rh(diene) initiator exhibits a sharp signal at approximately 3.2 ppm in the 1H NMR spectrum and 171 ppm in the 13C NMR spectrum.27,34 The spectra in Fig. 6 show a different split pattern depending on the substituent on the π-ligands, and these spectra can be categorized into two patterns (category A and category B). The difference in peak appearance between the two patterns can be seen in the relative intensities among three peaks centered at 3.0, 3.2, and 3.4 ppm in 1H NMR and 170, 171, and 172 ppm in 13C NMR. For example, in 1H NMR, the relative intensity of the peak at 3.4 ppm to that at 3.2 ppm is slightly but apparently lower in the spectra in category A than those in category B. On the other hand, the relative intensity of the peak at 3.0 ppm to that at 3.2 ppm is slightly but apparently higher in the spectra in category A than those in category B, where a peak valley between the peaks is more clearly seen in category A. In a similar manner, in 13C NMR, the relative intensities of the peak at 172 ppm to that at 171 ppm and the peak at 170 ppm to that at 171 ppm are lower and higher, respectively, in the spectra in category A than those in category B. These peak shapes should reflect tacticity of the polymer, which would be attributed to the steric environment around the Pd center. Judging from the results that the Pd complexes with an unsubstituted allyl ligand (1, 10 and 11) and the Pd complex with 2-methylallyl ligand (6) produced polymers categorized in category A while 7 with a Ph group on the 1-posion of the allyl group and the β-pinenyl-ligated Pd complex 8 yielded polymers categorized in category B, structures of 1 or 3-positions of the π-allyl ligands are likely to play an important role in governing the polymer tacticity. This is supported by the fact that the in situ-generated π-allyl-based Pd catalysts prepared from Pd2(dba)3·CHCl3 and A1–A9, which all have no substituent on the 1- and 3-positions of the π-allyl ligands, produced polymers with a very similar tacticity to those obtained with 1, 6, 10, and 11 (Fig. S5).
image file: c9py01654f-f6.tif
Fig. 6 1H and 13C NMR spectra of polymers obtained by various Pd complexes recorded in CDCl3 at 50 °C (1H) or 25 °C (13C).

Conclusions

In conclusion, we investigated the effects of solvents, additives, and structures of the π-allyl-base ligands on polymerization behavior. As a result, we have demonstrated that the addition of pyridine afforded a polymer with significantly narrow molecular weight distribution compared to π-allylPdCl alone. In addition, we have revealed that the tacticity of polymers was influenced by the π-allyl ligand structures, especially substituents at the 1- or 3-position on the π-allyl ligands. Although there is still a great challenge for achieving ideal controlled/living and stereospecific polymerization of diazoacetates, the results obtained in this study gave us fundamental and important information for the polymerization with π-allylPd-based initiating systems. Based on the new findings described here, we are now trying to further improve the performance of the Pd-based initiating systems.

Experimental section

Materials

Tetrahydrofuran (THF, Junsei Chemical, 99.5%+) was dried over Na/K alloy and distilled before use. Toluene (Kanto Chemical, >99.5%, dehydrated Super Plus grade) was further purified using Glass Contour MINI (Nikko Hansen & Co.). Diethyl ether (Nacalai Tesque, >98.0%), 2-methyltetrahydrofuran (2-MeTHF, Tokyo Chemical Industry, >98.0%), 3-methyltetrahydrofuran (3-MeTHF, Tokyo Chemical Industry, >95.0%), and 2,5-dimethyltetrahydrofuran (2,5-Me2THF, Tokyo Chemical Industry, >98.0%) were dried over CaH2 and distilled before use. Dichloromethane (Junsei Chemical, 99.5%+), tetrahydropyran (Tokyo Chemical Industry, >98.0%), and α,α,α-trifluorotoluene (Tokyo Chemical Industry, >98.0%) were dried over CaH2. Acetonitrile (FUJIFILM Wako Pure Chemical, super dehydrated, >99.8%), triphenylphosphine (PPh3, Nacalai Tesque, 98%), tricyclohexylphosphine (PCy3, Tokyo Chemical Industry, ca. 18% in toluene), pyridine (Kanto Chemical, dehydrated, >99.5%), 2-methylpyridine (Tokyo Chemical Industry, >98.0%), 3-methylpyridine (Tokyo Chemical Industry, >98.0%), 4-dimethylaminopyridine (DMAP, Wako Pure Chemical Industries, >99.0%), 2,6-lutidine (Tokyo Chemical Industry, >98.0%), 2,2′-bipyridine (Wako Pure Chemical Industries, >99%), allylpalladium chloride dimer (1, Sigma-Aldrich, 98%), allylpalladium trifluoroacetate dimer (10, Sigma-Aldrich, 95%), allyl[1,3-bis(2,6-diisopropylphenyl)-2-imidazolidinylidene]chloropalladium (11, Sigma-Aldrich, 97%), allyl chloride (A1, Tokyo Chemical Industry, >98.0%), allyl trifluoroacetate (A2, Tokyo Chemical Industry, >95.0%), allyl bromide (A3, Tokyo Chemical Industry, >98.0%), allyl acetate (A4, Tokyo Chemical Industry, >97.0%), 2-chloroallyl chloride (A5, Tokyo Chemical Industry, >97.0%), methyl 2-(chloromethyl)acrylate (A6, Sigma-Aldrich, >95%), and 3-chloro-2-(methoxymethoxy)-1-propene (A7, Tokyo Chemical Industry, >96.0%) were used as received. Tris(dibenzylideneacetone)dipalladium(0)-chloroform adduct [Pd2(dba)3·CHCl3],50 Pd complexes 3 [(η3-allyl)Pd(PPh3)Cl],40 4 [(η3-allyl)Pd(pyridine)Cl],41 5,42 6 [(2-methylallyl)palladium chloride dimer],45 7 [(π-cinnamyl)palladium chloride dimer],46 8 [bischloro(3,2,10-η-pinene)palladium],47 9 [bis(μ-chloro)bis(η3-indenyl)palladium],48 A8 {3-chloro-2-[(2-methoxyethoxy)methyl]-1-propene},51 ethyl diazoacetate (EDA),52 and methyl diazoacetate (MDA)52 were prepared according to the literature. A9 [2-chloromethyl-3-(2,6-dimethylphenoxy)-1-propene]53 was prepared by a procedure similar to that in the literature. Caution! Extra care must be taken for the preparation and handling of the diazoacetates because of their potential explosiveness.

Polymerization procedure

Polymerization in various solvents. As a representative example, the procedure for the polymerization of EDA with 1 in THF (run 1 in Table 1) is described as follows. Under a nitrogen atmosphere, a THF (2.0 mL) solution of 1 (2.3 mg, 6.3 × 10−3 mmol) was placed in a Schlenk tube and was cooled to −20 °C. A dichloromethane solution of EDA (2.23 M, 0.56 mL) was added to the Schlenk tube and the resulting solution was stirred for 13 h at the same temperature. After the volatiles were removed under reduced pressure, 10 mL of 1 M HCl/methanol, 10 mL of 1 M HCl aqueous solution, and 20 mL of CHCl3 were added to the residue. The resulting mixture was then extracted with CHCl3 three times and the combined organic layer was washed with NaCl aqueous solution and water. The organic layer was dried over Na2SO4, and concentrated under reduced pressure. Purification with preparative recycling size-exclusion chromatography (SEC) gave a polymer (69.4 mg, 64% yield).
Polymerization in the presence of various additives. As a representative example, the procedure with pyridine as an additive (run 5 in Table 2) is described as follows. Under a nitrogen atmosphere, a THF (1.9 mL) solution of 1 (2.2 mg, 6.0 × 10−3 mmol) was placed in a Schlenk tube and was cooled to −20 °C. A diluted solution of pyridine [0.10 mL out of a mixed solution of THF (9.9 mL) and pyridine (0.10 mL), 1.3 × 10−2 mmol] was added to the Schlenk tube and the resulting solution was stirred at the same temperature for 30 min. The polymerization was started by the addition of a dichloromethane solution of EDA (2.34 M, 0.50 mL). After stirring for 13 h at the same temperature and removing the volatiles under reduced pressure, 10 mL of 1 M HCl/methanol, 10 mL of 1 M HCl aqueous solution, and 20 mL of CHCl3 were added to the residue. The resulting mixture was then extracted with CHCl3 three times and the combined organic layer was washed with NaCl aqueous solution and water. The organic layer was dried over Na2SO4, and concentrated under reduced pressure. Purification with preparative recycling SEC gave a polymer (67.5 mg, 67% yield).
Polymerization with Pd2(dba)3·CHCl3/allyl compound systems. As a representative example, the procedure with A1 as an allyl compound is described as follows. Under a nitrogen atmosphere, a THF (1.9 mL) solution of Pd2(dba)3·CHCl3 (6.5 mg, 6.3 × 10−3 mmol) was placed in a Schlenk tube and was cooled to −20 °C. To the solution, a diluted solution of A1 [0.10 mL out of a mixed solution of A1 (0.10 mL) and THF (9.9 mL), 1.3 × 10−2 mmol] was added at the same temperature. The resulting solution was stirred at the same temperature for 1 h. The polymerization was started by the addition of a dichloromethane solution of EDA (2.47 M, 0.50 mL). After stirring for 13 h at the same temperature and removing the volatiles under reduced pressure, 10 mL of 1 M HCl/methanol, 10 mL of 1 M HCl aqueous solution, and 20 mL of CHCl3 were added to the residue. The resulting mixture was then extracted with CHCl3 three times and the combined organic layer was washed with NaCl aqueous solution and water. The organic layer was dried over Na2SO4, and concentrated under reduced pressure. Purification with preparative recycling SEC gave a polymer (71.2 mg, 67% yield).

Measurements

The number-average molecular weight (Mn) and polydispersity ratio [weight-average molecular weight/number-average molecular weight (Mw/Mn)] were measured by means of SEC on a Jasco-ChromNAV system equipped with a differential refractometer detector using THF as the eluent at a flow rate of 1.0 mL min−1 at 40 °C, calibrated with six poly(methyl methacrylate) (PMMA) standards (Shodex M-75; Mp = 2400–212[thin space (1/6-em)]000, Mw/Mn < 1.1) and dibutyl sebacate (molecular weight = 314.5). The columns used for the SEC analyses are a combination of Styragel HR4 and HR2 (Waters; exclusion limit molecular weight = 600 and 20 kDa for polystyrene, respectively; column size = 300 mm × 7.8 mm i.d.; average particle size = 5 μm). Purification by preparative recycling SEC was performed on a JAI LC-918R equipped with a combination of JAIGEL-3H and JAIGEL-2H (Japan Analytical Industry; exclusion limit molecular weight = 70 and 5 kDa for polystyrene, respectively; column size = 600 mm × 20 mm i.d.) using CHCl3 as the eluent at a flow rate of 3.8 mL min−1 at room temperature. 1H (500 MHz) and 13C (126 MHz) NMR spectra were recorded on a Bruker Avance III HD 500 spectrometer in CDCl3 at 25 °C (Pd complexes and monomers) or at 50 °C (polymers). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analyses were performed on a JMS-S3000 (JEOL, spiral mode) using dithranol as a matrix and sodium trifluoroacetate as an ion source. The calibration was carried out using polyethylene glycol (Mn = 4000). Elemental analyses were performed on a YANAKO CHN Corder MT-5.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by a Grant-in-Aid for Young Scientists (B) (JSPS KAKENHI Grant Number 16K17916), a Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number 18H02021), a Grant-in-Aid for Scientific Research (C) (JSPS KAKENHI Grant Number 19K05586), and a Grant-in-Aid for Challenging Exploratory Research (JSPS KAKENHI Grant Number 19K22219). The authors acknowledge the Applied Protein Research Laboratory in Ehime University for assistance with NMR measurements, the Ehime Institute of Industrial Technology for assistance with MALDI-TOF-MS measurements, and the Advanced Research Support Center in Ehime University for assistance in elemental analyses.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/C9PY01654F

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