Hyperbranched pyridylphenylene polymers based on the first-generation dendrimer as a multifunctional monomer

Abstract

An A₆ + B₂ approach was applied for the first time to synthesize novel hyperbranched pyridylphenylene polymers by Diels–Alder cyclocondensation reaction. For this, the first-generation pyridylphenylene dendrimer with six ethynyl functionalities (A₆) was used as a branching core for the molecule growth. The phenyl-substituted bis(cyclopentadienone)s (B₂) of different structures were used as co-monomer in the reaction. A careful choice of reaction conditions allowed us to obtain high molecular weight polymers without undesirable gelation. The molecular weight of the polymers varied in the range of 10 800–80 100 with a polydispersity degree of 1.69 to 4.07 according to SEC analysis. The ¹H and inverse-gated decoupling ¹³C NMR combined with heteronuclear single quantum correlation and heteronuclear multiple bond correlation measurements were used to estimate the branching degree of the polymers synthesized.

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Introduction

Hyperbranched polymers (HBP) have received considerable attention from academic researchers and industry in the last twenty years. They are characterized by higher solubility compared to that of linear polymers, low viscosity, and a generous amount of functional groups.^1–5 Typically, HBP are obtained by an one-pot procedure, which limits the ability to control the molecular weight and leads to “heterogeneous” products with a broad molecular weight distribution and differences in branching compared to dendrimers, which are monodisperse macromolecules with regard to both size and structure.⁶ The main advantages of HBP over dendrimers are the facile synthesis and low cost of the final product. HBP have been applied in catalysis,^7–9 opto-electronic materials,¹⁰ drug release,^11–13 etc. There are two scenarios for the synthesis of HBP: the single-monomer methodology (SMM) and the double-monomer methodology (DMM).¹ SMM is based on the use of AB_n monomer type (n ≥ 2). According to the reaction mechanism, the SMM strategy includes at least four approaches: (1) polycondensation of AB_n monomers, (2) self-condensing vinyl polymerization (SCVP), (3) self-condensing ring-opening polymerization (SCROP), and (4) proton-transfer polymerization (PTP). A broad range of hyperbranched polymers, including hyperbranched polyphenylenes,^14–16 polyethers,¹⁷ polyesters,¹⁸ polyamides,^19,20 polycarbonates,²¹ polyurethanes²² and polycarbosilanes,²³ were synthesized by polycondensation of AB_n monomers. Various hyperbranched polyacrylates,^24–26 hyperbranched azobenzene-containing polymers,²⁷ and hyperbranched glycopolymers²⁸ have been prepared via SCVP. Hyperbranched polyamines,^29,30 polyethers^31,32 and polyesters^33,34 were synthesized through SCROP. Hyperbranched polyesters with epoxy or hydroxyl end groups³⁵ and hyperbranched polysiloxanes³⁶ were synthesized through PTP. DMM is based on the use of a pair of monomers such as A_n (n ≥ 3) and B₂. For example, hyperbranched aromatic polyamide,³⁷ pyrimidine-based hyperbranched polyimides,³⁸ hyperbranched polybenzoxazole³⁹ and hyperbranched polyacenaphthenequinones⁴⁰ were synthesized by an A₃ + B₂ approach. The A₄ + B₂ (ref. 41–43) and A₃ + B₃ (ref. 44) methods have also been used for the synthesis of hyperbranched polymers.

A wide assortment of organic reactions are known for the construction of HB polymers.^45–51 Among them, the Diels–Alder reaction has been successfully used for the synthesis of HB polyphenylenes using the AB₂ or A₃ + B₂ approaches.^52,53

The main challenge in the HBP synthesis is to obtain the perfect dendritic polymer structure by a one-pot procedure; therefore, the known methods of synthesis are constantly improving. It was proposed to combine the advantages of dendrimers and hyperbranched polymers. In this way the synthesis becomes easier than that of high-generation dendrimers, and the polymer defects become less pronounced than those of HBP synthesized via a routine monomer method. Dendrons have been used to design branched polymers, named “dendronized polymers”.⁵⁴ Usually, it is a linear polymer with dendritic units in side chains.^54,55

In this paper, we report for the first time the A₆ + B₂ approach based on a Diels–Alder polycycloaddition of the first-generation, six-functional pyridine containing polyphenylene dendrimer with the aromatic bis(cyclopentadienone)s of different structures, leading to novel hyperbranched polyphenylenes with pyridine moieties. The structure of the synthesized HB polymers was characterized by NMR spectroscopy, while molecular weight and polydispersity degree were evaluated by size exclusion chromatography (SEC). The branching degree of the polymers synthesized was unambiguously defined by a combination of 1D and 2D NMR techniques.

Experimental section

Materials

1,3,5-Triethynylbenzene (98%), Bu₄NF (1 M solution in tetrahydrofuran), N-methylpyrrolidone (99%), tetrahydrofuran (anhydrous, 99.9%), diphenyl ether (99%), and o-xylene (anhydrous, 97%) were purchased from Aldrich and used as received.

Measurements

¹H NMR and ¹³C NMR spectra were recorded on the Avance-IIIHD-500 MHz NMR spectrometer operating at 500.13 MHz for ¹H and at 125.76 MHz for ¹³C. Chemical shifts are given in parts per million (ppm), using the solvent signal as a reference. CD₂Cl₂ was used as solvent for all standard 1D and 2D NMR measurements [δ(¹H) = 5.35 ppm; δ(¹³C) = 53.4 ppm]. For 2D spectra, heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) were recorded by using the standard pulse sequences of the Bruker software. The inverse-gated decoupling ¹³C NMR spectra were obtained at 25 °C, 30° pulse angel, inverse-gated decoupling with 5.0 s delay, and 5000 scans.

Size-exclusion chromatography (SEC) analyses were carried out using a Shimadzu LC-20AD chromatograph equipped with TSKgel G5000H_HR 7.8 mm × 30 cm (Tosoh Bioscience) and with TSKgel HHR-L. Detection was achieved with refractive index detector. SEC was performed in tetrahydrofuran at a flow rate of 1.0 mL min⁻¹ with polystyrene as a standard.

The intrinsic viscosity [η] measurements were performed with an Ubbelohde viscometer at 25 °C in N-methylpyrrolidone (N-MP). The samples were dissolved at room temperature.

Mass spectral analysis was carried out on the Bruker Biflex III MALDI-TOF instrument. MALDI-TOF mass spectra were measured by using a 337 nm nitrogen laser and tetracyanoquinodimethane (TCNQ) as matrix.

Synthetic procedures

Synthesis of monomers. 3,3′-(1,4-Phenylene)bis(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (1), 3,3′-(oxydi-1,4-phenylene)bis(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (2), and 3,3′-(carbonyldi-1,4-phenylene)-bis(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (3) (Scheme 1) were synthesized according to the reported procedure.⁵⁶

Scheme 1 Synthesis of hyperbranched pyridylphenylene polymers.

First-generation dendrimer G1-6N-(ethyn)₆ (4) (Scheme 1) was synthesized according to the procedure described by us elsewhere.⁵⁷

G1-6N-(ethyn)₆ ¹H NMR spectrum (500 MHz, CD₂Cl₂): 8.64 (d, 3H, PyH), 8.02 (d, 3H, PyH), 7.81 (s, 3H, ArH), 7.45–6.55 (m, 45H, ArH), 3.01, 2.97 (2s, 6H, acetylenes). MALDI-TOF, m/z: 1370 (M⁺, calcd 1369.66). Elem. anal. found, %: C 88.65; H 4.35; N 6.05. Calcd for C₁₀₂H₆₀N₆, %: C 89.45; H 4.42; N 6.14.

Synthesis of hyperbranched pyridylphenylene polymers. All procedures were performed using a Schlenk flask under argon atmosphere in diphenyl ether at 160 °C using total monomer concentrations of 0.01–0.05 mol L⁻¹, at various molar ratios of monomers. The amounts of reagents employed are given in Table 1. In the typical procedure, 0.050 g (0.073 mmol) of 3,3′-(1,4-phenylene)-bis-(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (B₂) and 0.1 g (0.073 mmol) of first-generation dendrimer G1-6N-ethyn (A₆) were dissolved in 14.6 mL of diphenyl ether at 160 °C (Table 1, entry 1, a). All reactions were carried out for 6.5 h, and then the mixture was allowed to cool. The cold reaction mixture was diluted with chloroform and added dropwise into ethanol. A white precipitate was filtered, washed with ethanol and reprecipitated twice from chloroform into ethanol. Polymers were dried in vacuum at 120 °C.

Table 1 The synthesis parameters for polymer preparations

N	A6 : B2 (molar ratio)	A6, g (mmol)	B2, g (mmol)	B2	C^a, mol L^−1
a Total concentration of monomers A6 and B2.
1	1 : 1	0.100 (0.073)	0.050 (0.073)	1	(a) 0.010
					(b) 0.020
					(c) 0.033
					(d) 0.050
2	1 : 1.5	0.100 (0.073)	0.075 (0.109)		(a) 0.010
					(b) 0.020
					(c) 0.033
					(d) 0.050
3	1 : 2	0.100 (0.073)	0.101 (0.146)		(a) 0.050
					(b) 0.033
4	1 : 3	0.100 (0.073)	0.151 (0.219)		(a) 0.010
					(b) 0.033
					(c) 0.050
5	1 : 1	0.100 (0.073)	0.057 (0.073)	2	(a) 0.033
					(b) 0.050
6	1 : 1.5	0.100 (0.073)	0.085 (0.109)		(a) 0.010
					(b) 0.020
					(c) 0.050
7	1 : 2	0.100 (0.073)	0.114 (0.146)		(a) 0.010
					(b) 0.020
8	1 : 1	0.100 (0.073)	0.058 (0.073)	3	(a) 0.010
					(b) 0.033
					(c) 0.050
9	1 : 1.5	0.100 (0.073)	0.087 (0.109)		(a) 0.010
					(b) 0.020
					(c) 0.033
					(d) 0.050
10	1 : 2	0.100 (0.073)	0.116 (0.146)		(a) 0.010
					(b) 0.020
11	1 : 3	0.100 (0.073)	0.174 (0.219)		(a) 0.010
					(b) 0.033
					(c) 0.050

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Results and discussion

Polymer synthesis

Hyperbranched aromatic polymers were synthesized by a Diels–Alder reaction using an A₆ + B₂ approach. For this, the pyridylphenylene dendrimer of the first generation with six terminal acetylene functionalities—monomer A₆—was used as a branching point for the molecule growth (Scheme 1, compound 4). Bis(cyclopentadienone)s (B₂) of different structures (Scheme 1, compounds 1, 2, and 3) were used as a second monomer in the reaction. Dendrimer G1-6N-ethyn (4) was synthesized according to the protocol described by us previously.⁵⁷ The aromatic bis(cyclopentadienone)s were obtained by the Knoevenagel condensation of bis(α-diketones) with two-fold molar amounts of diphenylacetone according to the procedure described in ref. 56. In this way, 3,3′-(1,4-phenylene)bis(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (1), 3,3′-(oxydi-1,4-phenylene)-bis(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (2) and 3,3′-(carbonyldi-1,4-phenylene)-bis(2,4,5-triphenylcyclo-penta-2,4-dien-1-one) (3) were synthesized.

The polycyclocondensation of multifunctional monomers may lead to rapid undesirable gelation, which can be controlled by reaction parameters such as temperature, reaction time, concentration of monomers, etc. To establish the dependence of the reaction parameters on the polymer structure, the Diels–Alder polycyclocondensation was carried out using a pair of monomers: B₂, 3,3′-(1,4-phenylene)-bis(2,4,5-triphenyl-cyclopenta-2,4-dien-1-one) (1) and A₆, G1-6N-ethyn (4). Since we observed gelation at 180 °C within one hour, the synthesis of the polypyridylphenylenes was performed at 160 °C for 6.5 h. The molar ratios of the A₆ : B₂ monomers employed were 1 : 1, 1 : 1.5, 1 : 2, and 1 : 3. When the A₆ : B₂ molar ratio was 1 : 3, complete gelation occurred at all monomer concentrations (0.010–0.050 mol L⁻¹) (Table 2, polymers 1, 2, and 3). However, when the reaction time was decreased to 3 h, a soluble polymer with a molecular weight of 23 500 (according to SEC) was obtained, at the total monomer concentration of 0.033 mol L⁻¹ (Table 2, polymer 4). The same trend was observed for the A₆ : B₂ molar ratio of 1 : 2. Gelation occurred at the concentrations of 0.050 and 0.033 mol L⁻¹ (Table 2, polymers 5 and 6), while a soluble polymer of molecular weight 17 900 was prepared when the reaction time was decreased to 3 h, at the total monomer concentration of 0.033 mol L⁻¹ (Table 2, polymer 7).

Table 2 The synthesis conditions and characterization data of different hyperbranched pyridylphenylene polymers

Polymer^a	A6 : B2 (molar ratio)	B2	C^b, mol L^−1	[η]^c, dL g^−1	Mw × 10^−3, g mol^−1	Mw/Mn	Yield %
a Syntheses were carried out in diphenyl ether at 160 °C for 6.5 h.b Total concentration of monomers A6 and B2.c Intrinsic viscosity of the polymer solution in NMP at 25 °C.d Insoluble polymer.e Synthesis was carried out in diphenyl ether at 160 °C for 3 h.
1^d	1 : 3	1	0.050	—	—	—	—
2^d	1 : 3		0.033	—	—	—	—
3^d	1 : 3		0.010	—	—	—	—
4^e	1 : 3		0.033	0.32	23.5	3.10	75
5^d	1 : 2		0.050	—	—	—	—
6^d	1 : 2		0.033	—	—	—	—
7^e	1 : 2		0.033	0.30	17.9	2.15	77
8^d	1 : 1.5		0.050	—	—	—	—
9^d	1 : 1.5		0.033	—	—	—	—
10	1 : 1.5		0.020	0.46	40.3	4.07	78
11	1 : 1.5		0.010	0.10	13.4	2.44	75
12^d	1 : 1		0.050	—	—	—	—
13	1 : 1		0.033	0.72	33.7	3.27	69
14	1 : 1		0.020	0.49	22.1	2.11	68
15	1 : 1		0.010	0.13	10.8	2.15	66
16^d	1 : 2	2	0.02	—	—	—	—
17	1 : 2		0.01	0.11	29.1	1.84	78
18^d	1 : 1.5		0.02	—	—	—	—
19	1 : 1.5		0.01	0.12	17.1	1.69	79
20	1 : 1		0.033	0.29	40.3	2.94	63
21^d	1 : 2	3	0.02	—	—	—	—
22	1 : 2		0.010	0.19	35.1	2.69	69
23	1 : 1.5		0.020	—	—	—	—
24	1 : 1.5		0.010	0.17	18.4	1.85	70
25	1 : 1		0.033	0.37	80.1	3.90	66

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At the A₆ : B₂ = 1 : 1.5 molar ratio, the soluble polymer was obtained at C_total = 0.02 mol L⁻¹ after the 6.5 h reaction, with a molecular weight of 40 300 (Table 2, polymer 10). At higher concentrations, gelation was observed for the polymers (Table 2, polymers 8 and 9). A further decrease of the concentration led to polymer molecular weight drop at the same monomer ratio (Table 2, polymer 11).

At the equimolar monomer ratio, the soluble polymers were prepared at the total monomer concentration of 0.033 mol L⁻¹ and below (Table 2, polymers 13, 14, and 15). Here, the drop of the monomer concentrations led to lower molecular weights of the polymers as well.

Based on the above observations, we determined that the monomer concentration dramatically affects the soluble polymer formation. Moreover, the effect is more pronounced for the high B₂ : A₆ monomer ratio. In some cases, the soluble polymers were obtained only when the reaction time was reduced (Table 2, polymer 4 and 7).

Using the above findings, we synthesized the polymers based on monomers A₆ and B₂ (compounds (2) and (3), Scheme 1) having bridging groups. At the monomer ratio 1 : 1, polymers with higher molecular weights were obtained at C_total = 0.033 mol L⁻¹ (Table 2, polymers 20 and 25) as compared to those based on monomer 1 (polymer 13). This effect can be ascribed to a more flexible structure of B₂ monomers, allowing the favourable polymer growth due to lower steric constraints. At the same time, at a higher B₂ content (A₆ : B₂ = 1 : 2 or 1 : 1.5), soluble polymers were obtained only at C_total = 0.01 mol L⁻¹ (Table 2, polymers 17, 19, 22, and 24) for both B₂ monomers.

According to SEC, the highest molecular weight polymer (80 100) was obtained by reacting 3,3′-(carbonyl-di-1,4-phenylene)-bis(2,4,5-triphenylcyclopenta-2,4-dien-1-one) (3) with G1-6N-ethyn (4) (Table 2, polymer 25), probably due to the higher reactivity of 3 compared with 1 and 2.

All polymers showed good solubility in common organic solvents such as chloroform, dichloromethane, tetrahydrofuran, toluene, benzene, and N-methyl-2-pyrrolidone.

The intrinsic viscosity ([η]) of the synthesized polymers measured in N-MP at 25 °C was rather low (Table 2), which is characteristic of hyperbranched polymers. It is well known that the intrinsic viscosities of the hyperbranched polymers are much lower compared to that of linear polymer of the same molecular weight. For linear polymers in a good solvent, [η] increases monotonically with a molecular weight following the Mark–Houwink equation with α = 0.7.⁵⁸ On the other hand, dendrimers and hyperbranched polymers demonstrate unusual viscosity behaviour. For example, [η] of some dendrimers increases initially and then decreases after a maximum is reached at a certain generation.⁵⁹ The intrinsic viscosity of star and hyperbranched polymers generally has less dependence on the molecular weight than that of corresponding linear polymers.^60,61 A small dependence of [η] on the molecular weight was also observed for the synthesized polymers (Table 2). The [η] values of these polymers are quite low, and the increase of the molecular weight of the polymers obtained at the same molar concentrations but at different molar ratios of monomers (compare pairs 4 and 7, 10 and 14, 17 and 19, 22 and 24, Table 2) does not lead to a significant change in [η], indicating the formation of compact macromolecules.⁶²

It is worth noting that the average yields of the polymers are in the range of 63–79% (Table 2). We assume that these comparatively low yields can be because monomer A₆ does not fully participate in the reaction with B₂ due to lower solubility or slow solubilization of the former. Indeed, during the work-up procedure, A₆ is isolated in practically all cases. This effect is especially pronounced for polymers obtained at the 1 : 1 molar ratio. Instead of the expected predominantly linear polymers, branched polymers are formed, apparently as a result of disturbance of the initial monomer loading ratio.

More accurate assessment of the polymer branching was then obtained from NMR data.

NMR analyses of polymers

The degree of branching (DB) is the most important characteristic of hyperbranched polymers. NMR spectroscopy has proven to be the most powerful tool for the direct assessment of DB.⁶³ Typically, the DB of polymers is determined by comparing the NMR spectra of suitable model compounds and a polymer.^64,65 For this, model compounds mimicking branched, linear, and terminal polymer fragments should be synthesized. In order to determine the DB of hyperbranched polypyridylphenylenes, it was necessary to synthesize partially substituted monomer A₆, i.e., models containing one to five terminal acetylene groups (Fig. S1, ESI†). While mono- or disubstituted monomers A₆ (Fig. S1, a and b,† respectively) were obtained with a high yield and purity, the further substitution (Fig. S1, c–e†) led to the mixture of differently substituted compounds that could not be separated by column chromatography. Therefore, in this study we used NMR spectroscopy to estimate the relative amount of free (unreacted) functional groups in the repeating unit of the polymers synthesized, which is directly related to the branching degree of the polymer.

As proof of concept, we demonstrated the effect of the molar ratio of monomers on the branching of the synthesized polymers, for the pair of monomers A₆ and B₂ (compound 1) (Table 2). For this study, polymers 4, 7, 10 and 13, obtained at the molar ratios of monomers A₆ : B₂ = 1 : 3, 1 : 2, 1 : 1.5 and 1 : 1, respectively, were chosen.

All ¹H NMR spectra of polymers 4, 7, 10, and 13 (Fig. 1a and S2a–c in ESI†) show three groups of signals. The signals at 8.61 (or 8.64) and 8.07 (or 8.01) ppm belong to α-protons H(1) and H(1′) of nonequivalent pyridine moieties of the polymer; the signals at 3.08 (or 3.02), 3.04 (or 2.99), and 3.00 (or 2.95) ppm correspond to the H(2) protons of the terminal acetylene groups; while signals in the range of 7.7–6.25 ppm are assigned to other aromatic protons of the polymer. According to the integral ratio of the signals of the acetylene protons and α-protons of pyridine fragments, their ratio in polymer 13 (A₆ : B₂ = 1 : 1) is 3 : 6, in polymer 10 (A₆ : B₂ = 1 : 1.5) it is 2.34 : 6, in polymer 7 (A₆ : B₂ = 1 : 2) it is 2 : 6, and in polymer 4 (A₆ : B₂ = 1 : 3) it is 1.23 : 6. Therefore, an increase of the monomer B₂ content led to the decrease of the number of unreacted acetylene groups in polymers. Because every repeating unit of polypyridylphenylene contains six pyridyl moieties that come from the branching monomer A₆, the ratio of the pyridyl fragments and unreacted acetylene groups (which did not participate in the growth of macromolecules) is directly related to the branching of the polymers. Ideally, if the polymer growth occurs in six directions (this might be accomplished at A₆ : B₂ = 1 : 3), the structure of the repeating unit of the polymer would be similar to the structure shown in Fig. 2. In this case, no signal of acetylene protons should be observed in the ¹H NMR spectrum, and the branching degree should be 100%.

Fig. 1 ¹H NMR spectrum (a) and the proposed structure of polymer 4 (b). For polymers 7, 10, and 13, see ESI.†

Fig. 2 Proposed ideal structure of the repeating unit of the polymers.

However, as discussed above, in the most branched polymer 4, the ratio of the acetylene and pyridine fragments is 1.23 : 6, revealing that macromolecule growth occurred mostly in five directions (Fig. 1). Accordingly, for polymer 7, macromolecule growth occurred mostly in four directions; for polymer 10, in three or four directions; and for polymer 13, in three directions, respectively. Here, it should be noted that the NMR signal ratio is an average value, and a mixture of different possible repeating units may coexist in polymers (see Fig. S3–S6,† ESI for illustration).

For more accurate signal assignment, we chose polymer 25 with highest molecular weight and combined 1D and 2D NMR techniques. All signal assignments in the inverse-gated decoupling ¹³C NMR spectrum of the polymer (Fig. 3a) are made with the help of HSQC and HMBC spectra (Fig. S7–S10, ESI†). The spectrum contains the signal at 196.4 ppm, which corresponds to the C(3) carbon of the carbonyl group in the benzophenone unit of the polymer. The signal at 159.4 ppm is assigned to quaternary carbon atoms C(4) and C(5) in the pyridine fragments of the polymer. The signals at 149.6 and 148.9 ppm refer to the carbon atoms C(1, 1′) located at the α-position of the pyridine fragments. The signals at 122.1 and 121.5 ppm are ascribed to the carbon atoms C(6) and C(7) in pyridine, while the signals at 120.2 and 119.7 ppm refer to the carbon atoms C(8) in phenyl rings bound to the acetylene group of the polymer. The signals at 83.9 and 77.5 ppm refer to the quaternary carbon atoms C(9) and the carbon atoms C(2) of the terminal acetylene groups, respectively.

Fig. 3 ¹³C NMR spectrum (a) and the proposed structure of polymer 25 (b).

Because no signal from the carbonyl group of the cyclopentadienone unit is detected at about 200 ppm (signal at 196.4 ppm belongs to the bridging carbonyl group of B₂), and considering the functionality of the monomers and the monomer loading ratio, we concluded that there are no unreacted functional groups from the B₂ monomer in polymer 25. To allow for accurate integration of the ¹³C signals, the ¹³C spectrum was recorded with proton decoupling only during the acquisition period to avoid non-uniform enhancement of carbon signals from protons (nuclear Overhauser effect). Thus, the integration of the aromatic and acetylene carbon signals in the spectrum in Fig. 3a allows us to estimate the ratio of the terminal acetylene and pyridine fragments in the polymer as 3 : 6. The same ratio was obtained from the ¹H NMR data (see Fig. S11, ESI†).

Special attention is paid to the splitting in a “triplet” of the signal at 3.00 ppm. The HSQC spectrum (Fig. S8, ESI†) demonstrates that the “triplet” of acetylene protons gives intensive correlation signals with at least four signals of carbon at 77.5 ppm (position 2 in the polymer structure, Fig. S8, ESI†). Moreover, poor correlation signals are observed with acetylene carbons at 83.9 ppm (position 9 in the polymer structure, Fig. S8,† ESI) due to geminal spin–spin interactions ¹H–C≡¹³C. In the HMBC spectra, there are also correlations of acetylene protons with at least four ipso¹³C nuclei of phenyl fragments, connected with the acetylene group, at 120.2 ppm (position 8, Fig. S10,† ESI). Thus, in the polymer there are a few acetylene groups in which indicator nuclei have different magnetic shielding. It should be noted that in the polymer with a symmetric repeating unit structure, the magnetic shielding is similar. Possible conformation impact and/or deviation from the plane of conjugation of aromatic groups due to steric reasons should not affect shielding of acetylene H and C atoms that are spatially separated from the aromatic rings. Obviously, there are structurally different repeating units in the polymer (Fig. S3,† ESI) for which the magnetic shielding of indicator nuclei of acetylenes differs, which leads to the splitting in a “triplet” of the signal at 3.00 ppm.

Thus, the detailed NMR study allows us to propose the structure of the polymers synthesized and estimate their branching degree.

Conclusions

In this work, we proposed an efficient and facile A₆ + B₂ approach to synthesizing hyperbranched pyridylphenylene polymers using Diels–Alder polycycloaddition of the first-generation, six-functional pyridine-phenylene dendrimer (A₆) and the aromatic bis(cyclopentadienone)s (B₂). A thorough screening of the reaction conditions allowed us to find optimal conditions for the formation of high molecular weight polymers as confirmed by SEC. The low intrinsic viscosity of polymers combined with high molecular weights was another proof of polymer branching, whereas the detailed NMR study of the polymers clearly demonstrated that the growth of the polymer macromolecule occurs spatially in several directions, depending on the A₆ : B₂ molar ratio, leading to hyperbranched polypyridylphenylenes.

The approach developed paves the way for the one-pot synthesis of well-defined polymers bearing perfect dendritic fragments in the polymer backbone.

Acknowledgements

Financial support from Russian Foundation for Basic Research, (grant No. 14-03-00876_a) and the European Community's 7th Framework Programme for Research and Technological Development under grant agreement no. 604296 (BIOGO for Production) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16847c

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