Ï-AllylPdCl-Based Initiating Systems for Polymerization of Alkyl

Article pubs.acs.org/Macromolecules

π‑AllylPdCl-Based Initiating Systems for Polymerization of Alkyl Diazoacetates: Initiation and Termination Mechanism Based on Analysis of Polymer Chain End Structures Eiji Ihara,* Masaki Akazawa, Takashi Itoh, Mototaka Fujii, Kazuki Yamashita, Kenzo Inoue, Tomomichi Itoh, and Hiroaki Shimomoto Department of Material Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan S Supporting Information *

ABSTRACT: Polymerization of ethyl and benzyl diazoacetates (EDA and BDA) initiated with π-allylPdCl-based systems [π-allylPdCl/NaBPh4, π-allylPdCl/NaBArF4 (ArF = 3,5-{CF3}2C6H3), and π-allylPdCl] is described. Initiation efficiencies of the π-allylPdCl-based systems are much higher than those of the previously reported (NHC)Pd/borate (NHC = N-heterocyclic carbene) systems, and the new systems are capable of polymerizing the alkyl diazoacetates at low temperatures (0 ∼ −20 °C), where the (NHC)Pd/borate systems cannot initiate the polymerization. MALDI−TOF−MS analyses of the polymers obtained from EDA provide information for the chain-end structures of the polymers, based on which initiation and termination mechanisms are proposed. Interestingly, EDA polymerization by the π-allylPdCl-based systems in the presence of alcohols (EtOH, nPrOH, and nBuOH) or water was found to afford RO- or HO-initiated polymers as major products, as confirmed by MALDI−TOF−MS analyses.

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INTRODUCTION Recently, polymerization of alkyl diazoacetates has attracted much attention as an efficient method for preparing C−C main chain polymers bearing an alkoxycarbonyl substituent on each main chain carbon atom.1−3 The resulting polymer, poly(alkoxycarbonylmethylene), is one example of “poly(substituted methylene)s”1−4 and the structural characteristic of dense packing of the ester substituents around the polymer main chain is expected to lead to unique physical properties, particularly in comparison to those of vinyl polymer counterpart, namely, poly(alkyl acrylate)s with various ester groups.5 As initiators for the polymerization of alkyl diazoacetates, Rhdiene complexes are quite effective, affording remarkably high molecular weight polymers in a highly stereospecific manner.6−9 On the other hand, we have found that the (NHC)Pd/borate systems [NHC = N-heterocyclic carbene (IMes or IPr), borate = NaBPh4 or NaBArF4 (ArF = 3,5{CF3}2C6H3)] are also capable of giving polymers from various alkyl diazoacetates, although the Mns are lower than those of polymers obtained by the Rh−diene complexes and the tacticity of the polymers is atactic.10,11 In order to find a new Pd-based initiating system that can control the polymerization with respect to molecular weight and tacticity, we have been examining initiating ability of various Pd complexes for the polymerization of alkyl diazoacetates. As a result, we have found that π-allylPdCl-based systems [π-allylPdCl/NaBPh4, π-allylPdCl/NaBArF4 (ArF = 3,5-{CF3}2C6H3), and π-allylPdCl] are effective for the polymerization without any coordinating © 2012 American Chemical Society

ligand such as NHC. Herein, we will describe the initiating ability of the π-allylPdCl-based systems for the polymerization of ethyl and benzyl diazoacetates (EDA and BDA) and MALDI−TOF−MS analysis of the polymers to clarify the chain-end structure, which provides important information with respect to the initiation and termination mechanisms. In addition, we will report that EDA polymerization with these initiating systems in the presence of some alcohols (ROH) and H2O predominantly affords RO- and HO-initiated polymers, respectively.

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RESULTS AND DISCUSSION Polymerization of EDA and BDA Initiated with πallylPdCl-Based Systems. In the course of our search for a new Pd-based initiating system for the polymerization of alkyl diazoacetates, we have found that a mixture of π-allylPdCl (used as a commercially available dimer [π-allylPdCl]2) and sodium tetraphenylborate (NaBPh4) in THF is effective. The initiating system was generated by reacting π-allylPdCl and NaBPh4 in a ratio of [NaBPh4]/[Pd] = ca. 1.5−312 ([Pd] = 2[{π-allylPdCl}2]) at −78 °C, and a diazoacetate monomer was added to the mixture at −78 °C. Then, the temperature of the reaction mixture was raised to a certain degree, and it was stirred for 13 h at the temperature. The polymerization results are Received: June 30, 2012 Revised: August 3, 2012 Published: August 27, 2012 6869

dx.doi.org/10.1021/ma3013527 | Macromolecules 2012, 45, 6869−6877

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Table 1. Polymerization of EDA and BDA with π-AllylPdCl-Based Systemsa run

initiating system

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBArF4 π-allylPdCl/NaBArF4 π-allylPdCl π-allylPdCl π-allylPdCl π-allylPdCl π-allylPdCl

monomer (M) EDA, EDA, EDA, EDA, EDA, BDA, BDA, BDA, BDA, EDA, BDA, EDA, EDA, EDA, BDA, BDA,

1 1 1 1 1 2 2 2 2 1 2 1 1 1 2 2

temp (°C)

[M]/[Pd]b

yield (%)

Mnc

Mw/Mnc

IE (%)

−20 −20 0 RT −20 −20 −20 −20 −20 −20 −20 −20 −20 −20 −20 −20

50 100 100 100 200 50 100 200 300 100 100 100 200 300 100 200

60 63 68 66 69 67 67 68 80 69 83 75 62 44 66 72

4500 5000 4400 4300 7600 7100 10 300 17 100 18 000 7300 11 800 7700 10 800 9100 11 900 12 900

1.57 1.71 1.73 1.62 1.60 1.51 1.50 1.60 1.61 1.58 1.73 1.64 1.62 1.63 1.64 1.79

57 109 132 132 156 70 95 118 198 81 104 84 98 125 80 166

a

In THF (2.0 mL) for 13 h; monomer = 0.5−3.0 mmol. EDA was used as a 2.1−4.2 M solution in CH2Cl2. b[Pd] = 2[π-allylPdCl]2; [NaBPh4]/ [Pd] = ca. 1.5−3.0. cMn and Mw/Mn were obtained by GPC calibration using standard PMMAs and dibutyl sebacate in THF solution.

calibration) was 4300 (run 4 in Table 1). By comparing the result with that observed with the IMesPd/NaBPh4 system under the same condition, where a poly1′ with Mn = 15100 was obtained in a 49% yield,10 it is obvious that the distinct difference between the two initiating systems is the initiation efficiency (IE) calculated based on the polymer yield and Mn, where the values are 132% and 28% for the π-allylPdCl/NaBPh4 and IMesPd/NaBPh4 systems, respectively. The IE value over 100% for the former system suggests that chain transfer occurs during the polymerization. As shown in runs 2 and 3, the polymerization of EDA proceeded at −20 and 0 °C, giving poly1′ in a similar manner as at room temperature with respect to Mn and polymer yield. However, the lower IE observed at −20 °C (109%) suggested that the chain transfer was suppressed at the temperature to some extent. The activity of the π-allylPdCl/NaBPh4 system at the low temperatures is remarkable in comparison with the (NHC)Pd/borate systems, which drastically diminish their

summarized in Table 1. When EDA 1 was employed as a standard alkyl diazoacetate monomer (Scheme 1), the polymerization with Scheme 1. Polymerization of Alkyl Diazoacetates Initiated with π-AllylPdCl-Based Systems

a feed ratio of [EDA]/[Pd] = 100 at room temperature afforded poly1′ in a 66% yield, whose SEC-estimated Mn (PMMA

Figure 1. Part of the MALDI−TOF−MS spectrum of poly1′ (Mn = 1820, Mw/Mn = 1.80) obtained by π-allylPdCl/NaBPh4 system in reflector mode (a) and the theoretical isotopic distribution of a Na-adduct of Ph-initiated polymer (degree of polymerization [DP] = 18) terminated by protonolysis (b). 6870


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activity even at 0 °C to give poly1′ only in a trace yield.10 Throughout the polymerization temperature variation (runs 2−4), the appearance of main chain CH signals in their 1H NMR spectra, which has been shown to be dependent on the main chain tacticity10,11 was almost identical as those observed in the spectra of poly1’s obtained with the IMesPd/borate systems, indicating that the π-allylPdCl-based systems gave atactic polymers.5 In runs 1 and 5, the polymerization of EDA at −20 °C was conducted with [EDA]/[Pd] feed ratios of 50 and 200, respectively. With the increase of the feed ratio (runs 1, 2, and 5), while Mn of poly1′ increased gradually, the IE drastically increased up to 156% at [EDA]/[Pd] = 200. These results suggest that occurrence of chain transfer becomes more significant with the higher [EDA]/[Pd] ratios, rendering

the synthesis of high molecular weight poly1′ difficult. It is possible that the observed high IEs do not demonstrate quantitative generation of active species from the π-allylPdCl complex, but the generation of the active species in lower efficiency followed by frequent occurrence of chain transfer, particularly in the case of higher [EDA]/[Pd] feed ratios. Runs 6−9 in Table 1 summarize the results of polymerization of BDA 2 initiated with the π-allylPdCl/NaBPh4 system at −20 °C. Compared to the IE value (156%) of the aforementioned polymerization of EDA with a feed ratio of [EDA]/[Pd] = 200, the IE (118%) of the BDA polymerization with the feed ratio was much lower, resulting in the formation of poly2′ with a relatively higher Mn of 17100 (run 8), although, with a higher feed ratio of [BDA]/[Pd] = 300, the IE became 198% and the Mn of the poly2′ did not increase further (run 9). These results indicate that the reactivity of 2 as a monomer is somewhat different from those of 1 and enables us to control the polymerization with relatively low [BDA]/[Pd] ratios. As shown in runs 10 and 11, NaBArF4 was also effective as a borate, although a small amount of polyTHF was formed, suggesting that some cationic species capable of polymerizing THF are present in this system.13 To our surprise, as shown in runs 12−16, it was revealed that π-allyPdCl alone was active for the polymerization of EDA, exhibiting initiating ability comparable to the π-allyPdCl/ borate systems. Initiation and Termination Mechanisms for EDA Polymerization with π-allylPdCl-Based Systems. In order to clarify the chain-end structures and obtain information

Scheme 2. Proposed Mechanism for π-allylPdCl/NaBPh4Inititated Polymerization of EDA

Figure 2. Part of the MALDI−TOF−MS spectrum of poly1′ (Mn = 2000, Mw/Mn = 1.54) obtained by π-allylPdCl/NaBArF4 system in reflector mode (a), theoretical isotopic distribution of a Na-adduct of HO-initiated polymer (DP = 19) terminated by protonolysis (b), and theoretical isotopic distribution of a Na-adduct of HO-initiated polymer (DP = 21) terminated by backbiting (c). 6871


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for the initiation and termination mechanisms, MALDI−TOF− MS analysis for poly1’s was carried out. For the measurement

in reflector mode with high resolution, poly1’s with a relatively low Mn were prepared by conducting the polymerization with feed ratios of [EDA]/[Pd] = 15 or 30 to afford poly1’s with Mn = 1800−2900. Figure 1a shows a part of MS spectrum of a poly1′ obtained with the π-allylPdCl/NaBPh4 system (Mn = 1820, Mw/Mn = 1.80), where we can observe a main set of signals, whose interval between peak clusters corresponds to the molecular weight of the repeating unit derived from EDA (1′, CHCO2Et, m/z = 86). We have found that the main set of signals can be assigned to poly1′ with Ph and H at α- and ω-chain ends, respectively, as exemplified by a good agreement between one of the peak clusters in Figure 1a with simulated appearance of a peak cluster for a Na+-adduct of Ph(CHCO2Et)18H in Figure 1b. Accordingly, we can propose an initiation mechanism for the polymerization, where a Pd−Ph species generated by transmetalation from the borate BPh4− to the Pd center initiates the polymerization. The propagating chain end should be terminated by protonolysis to give the ω-chain end (Scheme 2). As shown in Figure 2a, a MALDI−TOF−MS spectrum of a poly1′ (Mn = 2000, Mw/Mn = 1.54) obtained with the π-allylPdCl/NaBArF4 system exhibits two main sets of signals with an interval of the repeating unit of 1′ in similar intensities. On the basis of comparison of each set of signals with two types of simulated peak appearances in Figure 2, parts b and c, we have found that both poly1′ structures should have a OH group at their α-chain ends but different structures at their ω-chain ends. The Pd−OH initiating species resulting in the α-chain ends could be generated from H2O adventitiously existing in

Scheme 3. Proposed Mechanism for π-allylPdCl/NaBArF4Inititated Polymerization of EDA

Figure 3. Part of the MALDI−TOF−MS spectrum of poly1′ (Mn = 2900, Mw/Mn = 1.44) obtained by π-allylPdCl alone in reflector mode (a), theoretical isotopic distribution of a Na-adduct of Cl-initiated polymer (DP = 21) terminated by ring-opening of THF followed by protonolysis (b), and theoretical isotopic distribution of a Na-adduct of Cl-initiated polymer (DP = 25) terminated by protonolysis (c). 6872


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Scheme 4. Proposed Mechanism for π-allylPdCl-Inititated Polymerization of EDA

the system, probably with the NaBArF4.14 Whereas one of the polymers is terminated via protonolysis (path A in Scheme 3) on the basis of comparison with simulated signal appearance in Figure 2b, the other ω-chain end was revealed to contain a cyclic framework resulting from a backbiting reaction (path B in Scheme 3) from a comparison of one of the peak clusters with simulated appearance of the cyclic structure in MALDI−TOF−MS analysis in Figure 2c. Although the number of the ring-forming carbons (x + 3 in Scheme 3) cannot be defined, the unique backbiting reaction is reminiscent of one generally observed in the anionic polymerization of methyl methacrylate.15 In a MALDI−TOF−MS spectrum for poly1′ obtained with π-allylPdCl alone in Figure 3(a), we can observe two sets of signals (major and minor) with an interval of m/z = 86. From the comparison of the signals in Figure 3a with those obtained with simulation in Figure 3, parts b and c, both sets of signals should have a Cl-group at their α-chain ends, which demonstrate the uniform initiation from Pd−Cl from the single component initiator, π-allylPdCl. The minor set of the signals agrees well with the structure with H at its ω-chain end (Figure 3c) resulting from protonolysis as described in path A in Scheme 4. On the other hand, the major set of the signals agrees well with a rather unexpected structure, where one THF molecule and H are attached to the ω-chain end of the Cl-initiated polymer chain (Figure 3b). Accordingly, we can propose a termination mechanism for the major component, where the propagating chain end nucleophilically attacks a THF

Figure 4. Part of the MALDI−TOF−MS spectrum of poly1′ (Mn = 3370, Mw/Mn = 1.62) obtained by π-allylPdCl alone in reflector mode (a), theoretical isotopic distribution of a Na-adduct of Cl-initiated polymer (DP = 21) terminated by ring-opening of THF-d8 followed by protonolysis (b), and theoretical isotopic distribution of a Na-adduct of HO-initiated polymer (DP = 25) terminated by protonolysis (c). 6873


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mechanism, we carried out the π-allyPdCl-initiated polymerization of EDA ([EDA]/[Pd] = 30) in deutrated THF (THF-d8) and analyzed the chain end structures of the resulting polymer (Mn = 3370, Mw/Mn = 1.62) with MALDI−TOF−MS. As shown in Figure 4, compared to the spectrum in Figure 3, the major set of signals moved their positions by m/z = 8 as expected from our proposed mechanism, with appearance of one of the peak clusters agreeing with that of simulated signals derived from the original structure with additional eight H's: Cl(CHCO2Et)21CH2CH2CH2CH2OH + 8H. A minor set of signals in Figure 4 was identified to be derived from HO-initiated poly1′ instead of Cl-initiated one in Figure 3, which could be ascribed to the presence of H2O in THF-d8 used as received without purification. The formation of HO-initiated poly1′ by the polymerization in the presence of H2O will be described in the following section. As also described in the following section, Pd−OR species can initiate the polymerization of EDA. Thus, once the propagating chain end was transformed into Pd−OR species after the ring-opening of THF, propagation should follow the ring-opening, affording random copolymer of EDA and THF as a product. However, judging from MALDI−TOF−MS spectrum of the product in Figure 3, we cannot confirm the formation of such random copolymers, which should significantly disturb the regular appearance of signals with the constant interval of m/z = 86. Accordingly, we tentatively assume that the ring-opening of THF only occurs after all the monomer is completely consumed.

molecule employed as solvent, resulting in the ring-opening of the cyclic ether to give OH chain end after protonolysis (path B in Scheme 4). In order to confirm the unique termination Table 2. Polymerization of EDA with π-AllylPdCl-Based Systems in the Presence of ROH or H2O at −20 °C in THFa run

initiating systemb

ROH or H2O

yield (%)

Mnc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBPh4 π-allylPdCl/NaBArF4 π-allylPdCl/NaBArF4 π-allylPdCl/NaBArF4 π-allylPdCl/NaBArF4 π-allylPdCl/NaBArF4 π-allylPdCl π-allylPdCl π-allylPdCl π-allylPdCl π-allylPdCl

H2O EtOH nPrOH nBuOH − H2O EtOH nPrOH nBuOH − H2O EtOH nPrOH nBuOH −

73 52 78 73 59 78 72 66 64 48 72 62 58 72 41

1710 1270 1790 2110 2750 1370 1340 1930 1940 2000 1860 1920 2450 3230 4030

IE Mw/Mnc (%) 1.89 1.78 1.83 1.55 1.64 2.24 1.83 1.56 1.35 1.54 1.83 1.91 1.67 1.61 1.52

111 105 112 90 55 147 138 89 85 62 99 84 61 58 26

a

In THF (2.0 mL) for 13 h; monomer = 0.5−3.0 mmol. EDA was used as a 2.1−4.5 M solution in CH2Cl2. b[Pd] = 2[π-allylPdCl]2 ; [NaBAr4]/ [Pd] = ca. 1.5−3.0. cMn and Mw/Mn were obtained by GPC calibration using standard PMMAs and dibutyl sebacate in THF solution.

Figure 5. Part of the MALDI−TOF−MS spectrum of poly1′ (run 2 in Table 2) obtained by π-allylPdCl/NaBPh4/EtOH system in reflector mode (a), theoretical isotopic distribution of a Na-adduct of EtO-initiated polymer (DP = 18) terminated by protonolysis (b), theoretical isotopic distribution of a Na-adduct of Ph-initiated polymer (DP = 18) terminated by protonolysis (c), and theoretical isotopic distribution of a Na-adduct of HO-initiated polymer (DP = 21) terminated by protonolysis (d). 6874


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The unique variation of the termination mode of these systems indicates that the reactivity of propagating species of the πallylPdCl-based systems differs among the three systems at least with respect to the termination. The most probable difference between these systems would be the structure of the Pd center with respect to the factors such as the kind of ligand attached,16

charge of the Pd (neutral or cation), and valency of Pd (0, I, II, or higher),17 and these structural differences would cause the observed nonuniformity of the termination. In addition, the observation of various minor signals (appearing as small noises) in MALDI−TOF−MS spectra throughout this study indicates that there could exist more propagating species than we identified in each system. In order to control the polymerization, in other words to establish living polymerization of alkyl diazoacetates, further exploration to find much more improved initiating systems is required. Polymerization in the Presence of Alcohol and H2O as a Chain Transfer Agent: Initiation with Pd−OR and Pd−OH Species. Recently, de Bruin and co-workers reported that some alcohols and H2O can act as a chain transfer agent in their Rh(diene)-initiated polymerization of EDA to give poly1’s with RO- or HO- and H- groups at their α- and ω-chain ends, respectively.18 To examine whether alcohols and H2O can promote chain transfer in our π-allylPdCl-based systeminitiated polymerization of EDA, the polymerization of EDA was conducted in the presence of some alcohols (EtOH, nPrOH, and nBuOH) and H2O with a feed ratio of [Pd]/ [ROH or H2O]/[EDA] = 1:20:30, which should not afford poly1′ if the chain transfer occurs frequently or the propagating species is deactivated by protonolysis with the H-sources. As summarized in Table 2, all the polymerization with the alcohols and H2O proceeded to give low Mn polymers with Mn = 1300−3200 in relatively good yields, clearly indicating that neither termination via protonolysis nor the frequent chain transfer with these H-sources did operate in these systems.

Scheme 5. Proposed Mechanism for EDA Polymerization in the Presence of ROH

Figure 6. Part of the MALDI−TOF−MS spectrum of poly1′ (run 7 in Table 2) obtained by π-allylPdCl/NaBArF4/EtOH system in reflector mode (a), theoretical isotopic distribution of a Na-adduct of EtO-initiated polymer (DP = 19) terminated by protonolysis (b), and theoretical isotopic distribution of a Na-adduct of HO-initiated polymer (DP = 20) terminated by protonolysis (c). 6875


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Figure 7. Part of the MALDI−TOF−MS spectrum of poly1′ (run 12 in Table 2) obtained by π-allylPdCl/EtOH system in reflector mode (a), theoretical isotopic distribution of a Na-adduct of EtO-initiated polymer (DP = 11) terminated by protonolysis (b), theoretical isotopic distribution of a Na-adduct of Cl-initiated polymer (DP = 11) terminated by ring-opening of THF followed by protonolysis (c), and theoretical isotopic distribution of a Na-adduct of HO-initiated polymer (DP = 14) terminated by protonolysis (d).

As listed in Table 2, IE and Mn reveals tendencies that the IE value increases and Mn decreases in the order of none, nBuOH, nPrOH, and EtOH to H2O. These results suggest that the chain transfer with ROH and H2O in these systems actually occurs but in a rather inefficient manner and that the reaction occurs more frequently with H-sources with higher dielectric constants, probably because of higher accessibility of such compounds to the rather polar Pd-containing propagating chain end. In order to confirm the occurrence of the chain transfer, MALDI−TOF−MS measurements were performed for the samples obtained in Table 2. As representative examples, the MS spectra for poly1′ obtained with EtOH are shown in Figures 5−7. In Figure 5a, which is the spectrum for the sample obtained from the π-allylPdCl/NaBPh4/EtOH system (run 2 in Table 2), we can identify the main set of signals as that derived from EtO(CHCO2Et)nH, based on the comparison with simulated appearance of peak clusters shown in Figure 5b. Similarly, two minor sets of signals in the spectrum were assigned to those derived from Ph(CHCO2Et)nH and HO(CHCO2Et)nH. These results combined with the analysis of IE values described above indicate that during the period for initiator preparation in the presence of EtOH, a mixture of species containing Pd−Ph, Pd−OEt, and Pd−OH was generated, where the Pd−OH species was probably generated with a contaminated H2O in the EtOH. In addition, because the system contains excess of EtOH, a part of EtO-initiated poly1′ could be formed by chain transfer of propagating chains

with EtOH followed by initiation with the resulting Pd-OEt species (Scheme 5). Likewise, the MALDI−TOF−MS spectrum of a polymer obtained by π-allylPdCl/NaBArF4/EtOH system (run 7 in Table 2) shown in Figures 6 indicates that a major component in the sample is EtO(CHCO2Et)nH and a minor one is HO(CHCO2Et)nH. In the same way, π-allylPdCl/EtOH system afforded a poly1′ sample (run 12 in Table 2), whose MALDI−TOF−MS spectrum in Figure 7 demonstrate the presence of a major component of EtO(CHCO2Et)nH and minor ones of Cl(CHCO 2 Et) n CH 2 CH 2 CH 2 CH 2 OH and HO(CHCO2Et)nH. These results of chain-end structure analyses confirm the generality of the mechanism described in Scheme 5 for the polymerizations with these π-allylPdCl-based initiating systems in the presence of the H-sources. Similar results were obtained from the MALDI−TOF−MS analyses for the samples obtained with H2O, nPrOH, and nBuOH in Table 2, where we can observe difference of peak positions corresponding to the structure variation of ROH and H2O employed as a H-source (See Supporting Information). In conclusion, we have demonstrated that the π-allylPdClbased systems are effective for polymerization of alkyl diazoacetates. The new initiating system has some important characteristics compared to the (NHC)Pd/borate systems, such as high IE and high activity at low temperatures down to −20 °C. In addition, the use of relatively less expensive commercially available compound of π-allylPdCl instead of (NHC) Pd complexes could be an important advantage of these systems. 6876


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of the polymerization behavior of alkyl diazoacetates on the [NaBPh4]/[Pd] ratio. (13) Because polyTHF was not formed in control experiments where a mixture of π-allylPdCl and NaBArF4 in THF was stirred under the polymerization conditions in the absence of EDA, EDA would participate in the generation of polyTHF-forming species. (14) Our drying procedure for NaBArF4 could be insufficient: Yakelis, N. A.; Bergmann, R. G. Organometallics 2005, 24, 3579−3581. (15) Odien, G. Principles of Polymerization; John Wiley & Sons, Inc: New York, 2004; pp 416−420. (16) The results of the polymer chain-end analyses indicate that the π-allyl group in the initial π-allylPdCl does not participate in the initiation as a nucleophile, but probably plays an important role as a ligand attached to the Pd center. Recently, de Bruin and coworkers also reported the presence of a Rh complex with a π-allyl-type ligand as an active species in their Rh-mediated polymerization of alkyl diazoacetates. Walters, A. J. C.; Troeppner, O.; Ivanović-Burmazović, I.; Tejel, C.; del Río, M. P.; Reek, J. N. H.; de Bruin, B. Angew. Chem., Int. Ed. 2012, 21, 5157−5161. (17) Involvement of low valent (0 or I) Pd species was suggested for (NHC)Pd/borate systems with experimental evidences. Franssen, N. M. G.; Reek, J. N. H.; de Bruin, B. Polym. Chem. 2011, 2, 422−431. (18) Walters, A. J. C.; Jellema, E.; Finger, M.; Aamoutse, P.; Smits, J. M. M.; Reek, J. N. H.; de Bruin, B. ACS Catal. 2012, 2, 246−260.

Although the detailed chain end analyses of poly1’s revealed nonuniformity of these initiating systems, which is not beneficial for controlling the polymerization, these information would be important for further development of new initiating systems with improved initiating ability. Further studies using this initiating system for the polymerization of various diazocarbonyl compounds is underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and MALDI−TOF−MS spectra of the poly1’s obtained with the π-allylPdCl-based systems in the presence of H2O, nPrOH, and nBuOH. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone and Fax: +81-89-927-8547. E-mail: ihara@ehime-u. ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Grants-in-Aid for Scientific Research (C) (No. 22550113) from Japan Society for the Promotion of Science (JSPS). The authors thank Venture Business Laboratory in Ehime University for its assistance in MALDI−TOF−MS and NMR measurements.

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REFERENCES

(1) Jellema, E.; Jongerius, A. L.; Reek, J. N. H.; de Bruin, B. Chem. Soc. Rev. 2010, 39, 1706−1723 and references cited therein. (2) Ihara, E. Adv. Polym. Sci. 2010, 231, 191−231 and references cited therein. (3) Franssen, N. M. G.; Walters, A. J. C.; Reek, J. N. H.; de Bruin, B. Catal. Sci. Technol. 2011, 1, 153−165. (4) Luo, J.; Shea, K. J. Acc. Chem. Res. 2010, 43, 1420−1433 and references cited therein. (5) Preparation of atactic poly(dialkyl fumarate)s or poly(dialkyl maleate)s, whose structures are almost equivalent to the polymers from alkyl diazoacetates, by radical polymerization was reported. Toyota, N.; Yoshida, M.; Otsu, T. Polym. J. 1983, 15, 255−260. (6) Hetterscheid, D. G. H.; Hendriksen, C.; Dzik, W. I.; Smits, J. M. M.; van Eck, E. R. H.; Rowan, A. E.; Busico, V.; Vacatello, M.; Van Axel Castelli, V.; Segre, A.; Jellema, E.; Bloemberg, T. G.; de Bruin, B. J. Am. Chem. Soc. 2006, 128, 9746−9752. (7) Jellema, E.; Budzelaar, P. H. M.; Reek, J. N. H.; de Bruin, B. J. Am. Chem. Soc. 2007, 129, 11631−11641. (8) Jellema, E.; Jongerius, A. L.; Walters, A. J. C.; Smits, J. M. M.; Reek, J. N. H.; de Bruin, B. Organometallics 2010, 29, 2823−2826. (9) Jellema, E.; Jongerius, A. L.; van Ekenstein, G. A.; Mookhoek, S. D.; Dingemans, T. J.; Reingruber, E. M.; Chojnacka, A.; Schoenmakers, P. J.; Sprenkels, R.; van Eck, E. R. H.; Reek, J. N. H.; de Bruin, B. Macromolecules 2010, 43, 8892−8903. (10) Ihara, E.; Ishiguro, Y.; Yoshida, N.; Hiraren, T.; Itoh, T.; Inoue, K. Macromolecules 2009, 42, 8608−8610. (11) Ihara, E.; Takahashi, H.; Akazawa, M.; Itoh, T.; Inoue, K. Macromolecules 2011, 44, 3287−3292. (12) Because a globe box is not available in our laboratory, it is difficult for us to precisely adjust the [NaBPh4]/[Pd] feed ratio owing to the hygroscopic and air-sensitive nature of the borate. Thus, we tried to use at least excess amount of the borate with respect to [Pd], and the ratio varied run to run. Although we cannot rule out the possibility of the influence of the remaining NaBPh4 to the polymerization to a certain extent, we have not observed dependence 6877


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