Youko
Tamura
,
Yumiko
Ito
,
Yukari
Segawa
,
Tomoya
Higashihara
and
Mitsuru
Ueda
*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-H120, O-okayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail: ueda.m.ad@m.titech.ac.jp; Fax: +81-3-5734-2127; Tel: +81-3-5734-2127
First published on 7th June 2011
A linear polymer was successfully prepared by self-polycondensation of 1-(3-phenoxypropyl)piperidine-4-one as an AB2 monomer, which was previously reported to produce a hyperbranched polymer (HBP) with 100% degree of branching (DB). Based on results of the model reaction, the DB could be controlled by changing the acidity of polymerization media.
In this paper, we present the synthesis of a linear polymer by self-polycondensation of an AB2 monomer 1-(3-phenoxypropyl)piperidine-4-one in TFSA. Moreover, a model reaction between 1-ethylpiperidine-4-one and anisole was carried out to investigate the reactivity of the second B versus that of the first of the AB2 monomers.
Scheme 1 Reaction of 1 and 2 in MSA or TFSA. |
However, the reaction between equimolar amounts of 1 and 2 in TFSA has not been performed in detail. Thus, a model reaction between equimolar amounts of 1 and 2 was carried out in TFSA to clarify the reactivity of the second B versus that of the first of the AB2 monomers. The reaction was completed in 5 min at 0 °C and the solution was poured into anhydrous methanol (Scheme 2).
Scheme 2 Reaction between an equimolar amount of 1 and 2 in TFSA. |
The structure of the isolated compound 4 was characterized based on elemental analysis as well as FT-IR, 1H and 13C NMR spectroscopy. The IR spectrum of compound 4 showed a characteristic absorption at 1630 cm−1 which is assignable to the CC stretching. No absorption due to the CO group of 1 was observed at 1716 cm−1. Fig. 1 shows the 1H NMR spectrum of compound 4 with the assignment of the observed resonances. The peaks due to characteristic methoxy and olefin protons are observed at 3.80 and 5.97 ppm, respectively. In the 13C NMR spectrum of compound 4, twelve signals are observed as expected (Fig. 2).
Fig. 1 1NMR spectrum of compound 4. |
Fig. 2 13C NMR spectrum of compound 4. |
The structure of 4 was further confirmed by the elemental analysis, in which the measured C, H, and N compositions agreed well with the calculated values. These observations suggested the following plausible reaction mechanism for the formation of 4. The diprotonated carbonyl compound of 1 in TFSA reacts with 2 to give intermediate 6, whose reactivity is lower than that of protonated 1 (5). Because an equivalent amount of 2 to 1 is used, no compound 2 remains in the reaction solution after the first arylation. Thus, intermediate 6 remains in TFSA after consumption of 2. If intermediate 6 is more reactive than 5, diarylated compound 3 is formed even in the presence of an equivalent of 2 to 1. On the other hand, when two equivalents of 2 to 1 are used, compound 2 attacks intermediate 6 to produce diarylated compound 3 (Scheme 3).
Scheme 3 Plausible reaction mechanism. |
When the solution of intermediate 6 in TFSA was poured into anhydrous methanol, the product would be 1-ethyl-4-methoxy-4-(4-methoxyphenyl)piperidine, which is easily converted to 4 through elimination of methanol. To elucidate the above mechanism, after the reaction between equimolar amounts of 1 and 2 for 5 min, one equivalent of 2 was added to the reaction solution. The product was diarylated compound 3, supporting the above plausible mechanism.
Scheme 4 Synthesis of HBP with a DB of 100% using 7 as a monomer. |
Run | Conc./mol l−1 | Temp./°C |
---|---|---|
a Polycondensation was carried out with 8.6 × 10−2 mmol of monomer 7 in TFSA for 5 min. | ||
1 | 0.10 | 0 |
2 | 0.10 | −5 |
3 | 0.10 | −22 |
4 | 0.17 | 0 |
5 | 0.020 | 0 |
6 | 0.020 | −5 |
7 | 0.020 | −22 |
Although the reaction temperature and concentration were changed, gel formation immediately occurred and isolated polymers were insoluble in organic solvents such as N,N-dimethylformamide, NMP, trifluoroacetic acid (TFA) and MSA probably because of cross-linking reactions. The viscous oily monomer takes time to dissolve in TFSA and oligomerization occurs on the surface of 7. Thus, the carbocations formed may attack the phenyl rings of other oligomers to yield cross-linked polymers. To prevent the cross-linking reaction, monomer 7 was dissolved in NMP and the solution was added to TFSA. In this case, polymerization proceeded in a homogeneous state without gelation (Scheme 5). The results of polymerizations are summarized in Table 2. The polymer was isolated by pouring the solution into anhydrous methanol. The obtained polymer was a faint yellow solid and was soluble in TFA at room temperature. Polymer 8 was characterized by FT-IR and NMR spectroscopy. In the FT-IR spectrum, the characteristic absorption of CC and ether bonds appeared at 1639 and 1249 cm−1, respectively.
Scheme 5 Synthesis of polymer 8. |
Run | Temp./°C | Time/min | Yield (%) |
---|---|---|---|
a Polycondensation was carried out with 8.6 × 10−2 mmol of monomer 7 in TFSA (4.1 mL) in the presence of NMP. | |||
8 | −17 | 5 | 82 |
9 | 0 | 5 | 89 |
10 | 0 | 10 | 96 |
11 | 0 | 15 | 97 |
12 | 0 | 180 | 99 |
The 1H NMR spectrum of protonated 8, which is soluble in acetone-d6, is presented in Fig. 3 with the assignments of all peaks. The signals at 7.37 and 6.88 ppm assignable to aromatic protons and 6.06 to olefin protons are clearly observed. The degree of polymerization (DP) of polymer 8 was determined to be 15 by NMR spectroscopy, where the integral ratios between aromatic protoni (2 protons) and the end aromatic proton B (2 protons) in polymer 8 were used.
Fig. 3 1H NMR spectrum of protonated polymer 8. |
Furthermore, the structure of polymer 8 was investigated by 13C NMR spectroscopy. The characteristic aromatic carbon next to the ether bond and the olefin carbon of protonated polymer 8 are observed at 160.0 and 115.8, respectively. The 13C NMR spectra of protonated compound 3, compound 4, and polymer 8 are shown in Fig. 4. No signal at 44 ppm which is assigned to the quaternary carbon of the dendritic unit was observed in the spectrum of polymer 8, which means that polymer 8 does not have a branching unit.
Fig. 4 13C NMR spectra of protonated compound 3, 4 and polymer 8. |
The thermal properties of polymer 8 were evaluated by TG. The polymer exhibited a 5% weight loss at 260 °C in nitrogen.
This journal is © The Royal Society of Chemistry 2011 |