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Metal-Free Multicomponent Tandem Polymerizations of Alkynes, Amines, and Formaldehyde toward Structureand Sequence-Controlled Luminescent Polyheterocycles Bo Wei, Weizhang Li, Zujin Zhao, Anjun Qin, Rongrong Hu, and Ben Zhong Tang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12767 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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Journal of the American Chemical Society

Metal-Free Multicomponent Tandem Polymerizations of Alkynes, Amines, and Formaldehyde toward Structure- and SequenceControlled Luminescent Polyheterocycles Bo Wei,† Weizhang Li,† Zujin Zhao,† Anjun Qin,† Rongrong Hu,*,† and Ben Zhong Tang*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. ‡

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China. ABSTRACT: Sequence-controlled polymers, including biopolymers such as DNA, RNA and proteins, have attracted much attention recently because of their sequence-dependent functionalities. The development of efficient synthetic approach for non-natural sequence-controlled polymers are hence of great importance. Multicomponent polymerizations (MCPs) as a powerful and popular synthetic approach for functional polymers with great structural diversity have been demonstrated to be a promising tool for the synthesis of sequence-controlled polymers. In this work, we developed a facile metal-free one-pot multicomponent tandem polymerization (MCTP) of activated internal alkynes, aromatic diamines and formaldehyde to successfully synthesize structural-regulated and sequence-controlled polyheterocycles with high molecular weights (up to 69 800 g/mol) in high yields (up to 99%). Through such MCTP, polymers with the in situ generated multisubstituted tetrahydropyrimidines or dihydropyrrolones in the backbone and inherent luminescence can be easily obtained with high atom economy and environmental benefit, which is inaccessible by other synthetic approaches.

INTRODUCTION Sequence-controlled polymers, whose monomer units of different chemical nature are arranged in an ordered fashion, are of great interests to polymer scientists.1-10 Biopolymers such as DNA, RNA, and proteins with ordered sequences of building blocks are well-known sequencecontrolled natural polymers, with their biological functions highly dependent on the structures, in particular, the strict sequence of subunits.11-14 To realize biomimic synthesis, while the current polymerization techniques for the precise control of molecular weights and their distributions are well-developed, the construction of nonnatural sequence-defined polymers remains to be a great challenge for polymer chemists. The current method for sequenced-controlled synthetic polymers are mainly based on solid-phase synthesis. It is hence urgently demanded to develop new polymerization methodologies for the efficient and convenient preparation of functional polymers with defined sequence of their subunits. Recently, the preparation of sequence-controlled polymers have been realized by RAFT emulsion polymerization,15 ring opening metathesis polymerization,16,17 radical chain polymerization,18 DNA-templated polymerization,19,20 and assembly-line synthesis,21 etc. Multicomponent polymerizations (MCP) as a rapidly developing field in polymer chemistry enjoys a series of

advantages such as product structural diversity, high efficiency, mild reaction condition, high atom economy, operational simplicity, and most importantly, well-defined polymer structure with multiple building blocks arranged in a strictly ordered fashion.22,23 It hence provides a great opportunity to employ MCPs in the preparation of sequence-controlled polymers and further modulation of their properties.24-29 The most widely explored MCPs are the isocyanidebased MCPs such as Passerini three-component polymerization and Ugi four-component polymerizations which involve toxic and smelly monomers and produce polyesters or polyamides that are synthetically accessible by other methods.30-33 Alkyne-based MCPs have attracted much attention recently because of the rich chemical property of alkynes which leads to great structural diversity and the unsaturated product structure which may endow polymer with potential optoelectronic properties.34,35 For example, metal-catalyzed A3-polycouplings of alkynes, aldehydes, and amines are reported to produce poly(dipropargylamine)s;36,37 Cu(I)-catalyzed MCPs of alkynes, sulfonyl azides, and amines/alcohols are reported to synthesis poly(N-sulfonylamidines) and poly(Nsulfonylimidates).38-40 Recently, we have developed a series of multicomponent tandem polymerizations (MCTPs) to combine Sonogashira coupling reaction of alkyne and carbonyl chloride, as well as a sequential addition or

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Figure 1. (A) Syntheses of compounds 4a-b and 5a-b and (B) proposed mechanisms for the MCRs.

cyclization reaction with a freshly added third component to produce conjugated polymers such as polythiophenes and polypyrazoles in a one-pot procedure.41-43 In such MCTPs, the intermediate formed in the first step directly undergoes the following reactions, avoids isolation/purification and guarantees the complete conversion of each step, which greatly simplifies the synthetic operation and improves the efficiency. These MCTPs do not just link the functional units together in polymer chains, they can also build new functional units embedded in the polymer main chain at the same time, producing heteroatom or heterocyclic-containing conjugated polymers with high yield, high molecular weight, high regio- and stereoselectivity. Currently, transition metal catalysts are required in most alkyne-based polymerizations, however, even trace amount of metallic residues from the catalyst may affect the optoelectronic properties of the product and bring biological toxicity.44 It is hence desirable to develop met-

al-free polymerization of alkynes to avoid such problems. There are two strategies to realize metal-free alkynebased MCPs. One is to use specific monomers with unique chemical reactivity. For example, a catalyst-free MCP of elemental sulfur, alkynes, and aliphatic amines are reported to directly convert cheap elemental sulfur to soluble polythioamides with well-defined structures, high molecular weights (Mws) and high yields.45 Another strategy is to use activated alkyne monomers with high reactivity for a large number of metal-free multicomponent reactions (MCRs) to conduct polymerization.46-48 For example, ester group-activated internal alkyne, dimethyl acetylenedicarboxylate 1a, is reported to react with 2 equivalents of aniline 2 and 2 equivalents of formaldehyde 3 at room temperature through a five molecule-reaction to form multisubstituted tetrahydropyrimidine 4a (reaction A).49 1a can also react with 2 equivalents of aniline 2 and 1 equivalent of formaldehyde 3 at elevated temperature through a four molecule-reaction to form multisub-


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stituted dihydropyrrolone 5a (reaction B) (Figure 1A).50 The proposed mechanism is shown in Figure 1B: in the presence of acetic acid, 1a first reacts with aniline 2 through a hydroamination reaction to afford intermediate A, and another half of 2 reacts with formaldehyde 3 t0 form Schiff base B at the same time. Intermediates A and B then react together through an aza-ene type reaction to furnish intermediate C.51,52 In the presence of excess amount of 3, C undergoes a nucleophilic addition with 3 to afford intermediate D, followed by a cyclization reaction to produce 4a; if without excess amount of 3 exists in the system, C will directly undergoes an amidation cyclization at elevated temperature to generate fivemember ring-containing intermediate E, which is then converted to 5a through an imine-enamine tautomerization. In this work, inspired by the highly efficient metal-free MCRs of acetylenedicarboxylates, convenient metal-free one-pot three/four-component tandem polymerizations of acetylenedicarboxylates, aromatic diamines, and formaldehyde are developed at room temperature to pr0duce polyheterocycles with well-defined structures, high Mws, and high yields. Structural modulation regard-

ing to the ratio of tetrahydropyrimidines and dihydropyrrolones in the polymer backbone is achieved by optimization of polymerization temperature and monomer loading ratio. Furthermore, sequence regularity of the monomer building blocks are realized through adjustment of the timing and order of monomer loading, furnishing sequence-controlled polymer products. Last but not least, although all the polymers contain no traditional luminophore, interesting aggregation-induced emission behavior is observed from them, indicating new emission mechanism from such polyheterocycles.

RESULTS AND DISCUSSION Metal-free MCTP of Acetylenedicarboxylates, Aromatic Diamine, and Formaldehyde. To explore the metal-free MCTP, 4,4'-methylenedianiline 6 was selected as a bifunctional monomer to react with 1a and 3. The polymerization was conducted in methanol at room temperature in a tandem manner: 1a and 6 were first reacted for 30 min, 3 and acetic acid were then added to react for another 16 h to afford polymer P1a (Figure 2A). After a series of optimization on polymerization condition such as monomer concentration, loading ratio, reaction time,

Figure 2. Synthesis and Characterization of P1a-b. (A) The synthetic routes of P1a-b. IR spectra of (B) 1b, (C) 6, (D) 4b, and (E) P1b. 1H NMR spectra of (F) 1b, (G) 6, (H) 4b, and (I) P1b in CDCl3.



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the amount of acetic acid, as well as solvent, only polymeric precipitates with low Mw were obtained, owing to the poor solubility of P1a in the reaction solvents such as methanol, ethanol, acetic acid, etc (Table S1). However, considering the solubility of acetylenedicarboxylate 1a, 6, and the aqueous solution of formaldehyde, these hydrophilic solvents which are miscible with water are required. To improve the solubility of the polymer product in methanol and further increase its Mw, monomer 1b with hydrophilic 2-(2-methoxyethoxy)ethyl groups was designed and synthesized (Figure S1). Based on the aforementioned procedure of MCTP, product P1b with better hydrophilicity and good solubility in methanol can be expected. The monomer concentrations of 1b, 6, and 3 were first optimized while the monomer loading ratio was fixed as [1b]:[6]:[3] = 1:1:3 (Table S2). When the concentration of 1b gradually increases from 0.08 M to 0.30 M, the Mw of P1b generally increases from 30 600 g/mol to 69 800 g/mol, while keeping high yields of up to 99%. Compared with the reported polymers prepared from multicomponent polymerizations, the molecular weights of these polymers are among the highest. To characterize the chemical structure of the polymers, model compounds 4b and 5b were synthesized (Figure 1A and Figure S2-S3). The IR, 1H and 13C NMR spectra of monomers 1b and 6, model compound 4b, and polymer P1b were compared and analyzed. The absorption peak at 1726 cm-1 in the IR spectra of 1b associated with the stretching vibration of carbonyl groups was splitted into two peaks at ~1694 and 1741 cm-1 in the spectra of 4b and P1b after the tetrahydropyrimidine ring forms, owing to the different chemical environments of the two types of carbonyl groups in 4b and P1b. Meanwhile, the absorption bands of 6 at 3423 and 3322 cm-1 associated with the N-H stretching vibrations disappeared in the spectra of 4b and P1b, suggesting the complete consumption of amine in the model reaction and polymerization (Figure 2B-E). In the 1H NMR spectra, the five peaks of 1b resonanced at δ 4.37, 3.73, 3.64, 3.54, and 3.38, corresponding to the CH2 or CH3 groups, are generally splitted into two peaks for each in the spectrum of 4b, owing to the different chemical environments of the two 2-(2methoxyethoxy)ethyl groups in 4b. Meanwhile, the repre-

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sentative new peaks emerged at δ 4.27 and 4.91 in the spectrum of 4b, attributed to the CH2 protons on the newly formed tetrahydropyrimidine ring, noted as He and Hd, respectively (Figure 2F-H). Similarly, the CH2 peaks at δ 4.20 and 4.83, and the splitted peaks from the two 2-(2methoxyethoxy)ethyl groups all emerged in the spectrum of P1b, suggesting the formation of tetrahydropyrimidine rings (Figure 2I). Moreover, the NH2 resonance of 6 at δ 3.55 has disappeared in the spectrum of P1b, proving the total consumption of the monomer. Interestingly, the CH2 resonances of 6 at δ 3.78 have splitted into three peaks at δ 3.87, 3.83 and 3.79 in the spectrum of P1b, suggesting three different chemical environments of the methylenedianiline moieties in the polymer chain. There are two different nitrogen atoms exist on the tetrahydropyrimidine ring: Na between two CH2 groups and Nb between a C=C bond and a CH2 group. During the MCTP, the two NH2 groups of 6 react randomly with 3 or 1b, which eventually converted to Na and Nb, respectively. As a result, the methylenedianiline moieties from 6 may possess three different forms in the polymer chain: those with two Na atoms, with two Nb atoms, or with one Na atom and one Nb atom, and the corresponding middle CH2 protons are noted as Ha, Hb and Hc, respectively (Figure 2A). Luckily, Ha, Hb and Hc can be distinguished from 1H NMR spectra and their ratio is calculated to be 1.1: 1.2: 1.0. In addition, the C≡C resonance of 1b at δ 75.04 has disappeared, while the carbonyl peak of 1b at δ 151.84 has splitted and shifted to δ 165.40/163.85 and δ 165.48/163.93 in the 13C NMR spectra of 4b and P1b, respectively (Figure S4). Meanwhile, two new peaks emerged at δ 100.60 and 47.54 in the spectrum of 4b, and at δ 100.12 and 47.74 in the spectrum of P1b, representing the carbons on the tetrahydropyrimidine ring. Similar with the 1H NMR spectra, the CH2 resonance of 6 at δ 40.20 has splitted into three peaks at δ 40.25, 40.56, and 40.86, corresponding to the carbon atoms bonded with Ha, Hc, and Hb, respectively. The structural analysis results suggested that P1b with 100% tetrahydropyrimidine structure in the main chain was obtained from the MCTP at room temperature with the monomer ratio of [3]/[1b] = 3:1. To explore the monomer applicability, we further tried this MCTP with five additional bis-aromatic amines 7-11 with

Figure 3. The MCTP with different aromatic diamine monomers.


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different electronic property and steric hindrance (Figure 3). Their polymerization results are summarized in Table S3, which indicate that this MCTP applies to all these aromatic diamines. The electron-rich aromatic diamine monomers 7-8 are more suitable than electron-deficient aromatic diamines 9 or diamine monomer 11 with high steric hindrance, owing to their reactivity difference in the hydroamination and Schiff base formation reactions.

Structural Modulation of Tetrahydropyrimidines and Dihydropyrrolones. To modulate the polymer backbone structure according to the mechanism, the MCTP of 1b, 6, and 3 was then conducted at elevated temperature with decreased loading ratio of formaldehyde to form dihydropyrrolones (Figure 4A). When the polymerization temperature was raised to 70 oC and the [3]/[1b] ratio was decreased to 3:2, polymer P2a with 90% yield and a Mw of 14 000 g/mol was obtained. When the [3]/[1b] ratio was further decreased from 1:1 to 1:2, continuous reduction of Mws and yields were observed from P2b to P2e (Table S4). Such results suggested that the MCTP based on reaction B is less efficient than the above-

mentioned MCTP based on reaction A. The structures of P2a-b were characterized by IR, 1H NMR, and 13C NMR spectra through comparison with the spectra of model compounds 4b and 5b, as well as P1b. In the IR spectra, a N-H stretching vibration peak of 5b emerges at 3288 cm-1, representing the remaining N-H group of 5b, which can not be observed in the spectra of P1b, but becomes obvious in the spectra of P2a-b. In the meantime, the two carbonyl peaks in the spectrum of 5b at 1639 and 1702 cm-1 are different from those two peaks in the spectrum of 4b at 1694 and 1741 cm-1. From the spectra of P1b to that of P2a-b, the two peaks at 1694 and 1741 cm1 have gradually changed to 1639 and 1700 cm-1, respectively, suggesting that the content of dihydropyrrolone in the polymer backbone is increased in P2a-b (Figure 4B-F). The ratio of dihydropyrrolone structure and tetrahydropyrimidine structure in the polymer backbone can be calculated from the 1H NMR spectra. The representative CH2 resonance of 4b at δ 4.91 (Hd) and the CH2 resonance of 5b at δ 4.56 (Hf) both emerged in the spectra of P2a-b (Figure 4G-K). The integration ratio of Hd peak and Hf

Figure 4. Synthesis and Characterization of P2a-b. (A) The synthetic routes of P2a-b. IR spectra of (B) 4b, (C) 5b, (D) P1b, (E) P2a, and (F) P2b. 1H NMR spectra of (G) 4b, (H) 5b, (I) P1b, (J) P2a, and (K) P2b in CDCl3.



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peak can be calculated to be 100:0 (P1b), 50:50 (P2a), and 18:82 (P2b), suggesting that the dihydropyrrolone content ratio in the polymer chain can be tuned from 0% to 82%. This ratio can be further increased to 92%, with the sacrifice of the yield and Mw in P2d-e (Table S4). Meanwhile, the unique N-H resonance of 5b at δ 8.03 (Hg) and the aromatic proton resonance at δ 7.78 both emerge in the spectra of P2a-b, proving the formation of such dihydropyrrolone structures. Similarly, from the 13C NMR spectra of P1b to that of P2a-b, the two carbonyl peaks of 4b at δ 165.40 and 163.85, as well as the two characteristic peaks of 4b at δ 100.60 and 47.54, are gradually changed to the carbonyl peaks of 5b at δ 164.46 and 163.88, as well as the characteristic peaks of 5b at δ 103.22 and 48.41, respectively (Figure S5). In the analysis of IR, 1H and 13C NMR spectra of P1b, P2a and P2b, the intensities of the characteristic peaks of 5b generally increase from P1b to P2a, then to P2b, accompanying with the decrease in the intensities of the representative peaks of 4b, suggesting the increasement of dihydropyrrolone content from P1b to P2a-b. Synthesis of Sequence-controlled Poly(tetrahydropyrimidines)s. As discussed in the mechanism, the NH2 groups of diamine monomer 6 serve

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two different roles in the MCTP and eventually converted to Na and Nb in P1b at room temperature, respectively. The random distribution of Na and Nb leads to the abovementioned three different chemical environments of the monomer moieties in P1b. To avoid such uncertainty and synthesize polymers with well-defined structure, diamine monomer 6 was added in two separate steps and each portion serves one specific role (Figure 5A). Firstly, 1 equivalent of 6 was reacted with 2 equivalent of 1b at room temperature for 30 min to afford double addition product; another equivalent of 6 and 3 were then added to react for 16 hours to afford P3a. Through such strategy, the NH2 groups on the first portion of 6 all converted to Nb, and the NH2 groups on the second half of 6 all converted to Na. Hence the structure of P3a is well-defined with Ha-containing diamine moieties and Hb-containing diamine moieties take a strict alternative order in the polymer chain. Similarly, P3b with well-defined structure was obtained when diamine monomer 7 without CH2 protons was used instead of 6 in this MCTP (Figure 5B). If two different diamine monomers are used in each step, the four-component tandem polymerization can afford sequence-controlled polymers P3c-d (Figure 5C-D). For example, one equivalent of 7 was first reacted with

Figure 5. Synthesis of sequence-controlled polymers P3a-d. The synthetic routes of (A) P3a, (B) P3b, (C) P3c, and (D) P3d.


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two equivalent of 1b, and then one equivalent of 6 and 3 were added to complete the MCTP and furnish P3c. In the polymer chain of P3c, moieties from 6 and 7 take an alternative order and the CH2 protons from 6 are all Hatype. Alternatively, when the feed sequence of 7 and 6 was exchanged, P3d can be obtained and the CH2 protons from 6 are all Hb-type. The polymerization yields of P3a-d are almost quantitative, and the Mws of P3c-d are generally smaller compared with that of P3a-b. Such sequence-controlled polymer structure can be proved by spectroscopic analysis. For example, P3a-d share similar IR spectra profile with P1b, which possess the representative two carbonyl peaks at ~1741 and 1691 cm-1, suggesting similar structure with P1b (Figure S6). The 1H NMR spectra of P3a and P1b are also very much alike, except that in the region of δ 3.79-3.87, instead of observing three peaks as in the spectrum of P1b, only two peaks emerged at δ 3.79 and 3.87 with equal integration area in the spectrum of P3a, suggesting equal amount of Ha protons and Hb protons in P3a. In the same region, no peak is observed in the spectrum of P3b because no CH2 proton exists in monomer 7; only Ha peak at δ 3.81 is observed in the spectrum of P3c, and only Hb peak at δ 3.87 emerged in the spectrum of P3d, proving the expected structure of P3a-d (Figure 6A-D). Similarly, in the 13C NMR spectra, besides the common representative peaks of P3a-d located at about δ 165.55, 164.00, 100.00, and 48.00, P3a possesses two peaks at δ 40.29 and 40.90, representing the two carbons bonded with Ha and Hb, respectively; P3b has no peak in such region; P3c has a single peak at δ 40.29 and P3d has a single peak at δ 40.90, which also prove the expected well-defined and sequencecontrolled structure of P3a-d (Figure 6E-H).

Careful analysis of the 1H NMR spectra of P3c-d can prove the sequence-controlled structures. Take P3c for example, the 1H NMR peak for the CH2 proton from monomer 6 only emerges at  3.81, indicating that nitrogen atoms from monomer 6 are all Na-type; The integration ratio of Ha peak at  3.81 and Hd peak at  4.82 are strictly 1:2 in the 1H NMR spectrum of P3c, which are the same as in structure I (Figure 7A); The number of Na- and Nb-type nitrogen atoms are the same in the polymer structure, which equals to half of the number of Hd atoms according to the polymer structure; Hence the number of Ha atom equals to the number of Na atom or Nb atom, indicating that Na are all from monomer 6 and the nitrogen atoms from monomer 7 are all Nb-type. In another word, moieties from monomer 7 only exist as structure II and structures III-IV with Na from monomer 7 do not exist in the polymer product, otherwise the integration ratio of Ha and Hd should be less than 1:2. The only possible combination of structure I and II are the proposed alternative structure of P3c shown in Figure 5c. Similarly for P3d, the 1H NMR peak for the CH2 proton from monomer 6 only emerges at  3.87, and the integration ratio of Hb peak at  3.87 and Hd peak at  4.81 are strictly 1:2, proving the well-defined polymer structure with predesigned subunit sequence of P3d. We have also designed and synthesized two low molecular weight oligomers 13 and 14 with precise structure and sequence and fully characterized the structure to further compare with P3c-d and prove the sequence of the polymers (Figure 7B). Diamine 6 and mono-amine 12 were added into the reaction system with different order to produce compounds 13-14 quantitatively, respectively, suggesting high specificity of these reactions. The structures of 13-14 are fully characterized by 1H NMR, 13C NMR, IR, as well as highresolution MS spectra, proving the expected structure shown in Figure 7B (Figure S7-8). The representative 1H and 13C NMR peaks of 13 and 14 can be correlated with the characteristic peaks in that of P3d and P3c, respectively, proving the sequence-controlled polymer structures (Figure S9-11).

E

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Figure 6. NMR spectra of sequence-controlled polymers P3a-d. 1H NMR spectra of (A) P3a, (B) P3b, (C) P3c, and (D) P3d in CDCl3. 13C NMR spectra of (E) P3a, (F) P3b, (G) P3c, and (H) P3d in CDCl3.



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Figure 7. (A) Structures of I, II, III, and IV. (B) Synthesis of model oligomers 13-14. A random copolymer P3ab was also prepared for comparison by adding monomer 6 and 7 with 1:1 ratio at the first step. The 1H NMR spectrum of P3ab was compared with that of P3a-d as shown in Figure S12. The peaks in spectra of P3ab is much broader compared with the other spectra. Most importantly, the characteristic peaks of CH2 from monomer 6 emerge as broad multiple peaks, mainly composed of peaks for Ha, Hb, and Hc, suggesting a random structure of P3ab, in comparison with the regular structure of P3c and P3d with strict sequence of the subunits. The thermal properties of these polymers are studied. Thermogravimetric analysis suggests that the polymers generally possess good thermal stability, with their decomposition temperature under nitrogen at a 5% weight loss ranging from 234258 oC (Figure S13). Differential scanning calorimetry measurement suggests that P1b, P2ab, and P3ad with hydrophilic 2-(2-methoxyethoxy)ethyl groups possess glass transition temperature ranging from 30-54 oC (Figure S14-15).

The Luminescence Behaviors. There is no classical luminogen exists in the model compounds and no report has indicated the emission from the solution or amorphous state of their derivatives, although it is reported that the crystal of 4a can emit light, owing to the crystal confinement of the structure which may form possible “heteroatom cluster” to serve as luminogen.53 In this work, luminescence is generally observed from the model compounds and polymers, and their luminescence behaviors are systematically investigated.

The photophysical properties of the model compounds and polymers are summarized in Figure 8 and Table S5. The absorption maxima of these compounds were generally located at 300-316 nm with similar absorption profile. In dilute THF solutions, no emission can be observed from them, however, the solid state and the PMMA-doped thin film state of these model compounds and polymers exhibit different emission behaviors (Figure 8B and Figure S16). They generally possess wide emission peaks with the emission maxima located at 493-513 nm in the solid state and at 484-514 nm in the thin film state. The fluorescence images of the polymers were taken under UV irradiation and yellow emission can be generally observed by naked eyes from their solid powders (Figure S17). The dihydropyrrolone-containing model compounds 5a-b generally possess blue-shifted emission maxima and decreased emission efficiencies compared with the tetrahydropyrimidine-containing small molecules and polymers. Although the emission efficiency of the polymers lies in the range of 1.6-5.3%, aggregation-induced emission (AIE) characteristics of the model compounds and polymers are suggested, considering that their solutions are non-emissive, especially for model compound 4a whose solid state emission efficiency is 40%. The PL spectra of P1a-b in THF/hexane solutions with different hexane contents were then investigated to study the AIE behavior of the polymers as examples (Figure 8C-


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D). In dilute THF solutions, P1a-b are nonemissive. Gradual addition of hexane into the THF solution results in aggregation of the polymer chains because hexane is a nonsolvent for P1a-b, and new emission peaks emerged at 507 nm (P1a) and 512 nm (P1b), respectively, which were further enhanced upon addition of hexane while keeping the spectral profile unchanged, demonstrating typical AIE phenomenon. In the aggregated state, the large amount of carbonyl groups as well as heteroatoms in the polymer chains are clustered together to form “heteroatom clusters”, which might serve as the luminogen and are responsible for such fluorescence enhancement.54

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650

Figure 8. Photophysical Properties of the model compounds and polymers. (A) Absorption spectra of THF solutions and (B) PL spectra of solid state of the model compounds and polymers (4b is liquid at room temperature). Concentrations of 4a-b, 5a-b and P1a-b: 1 × 10−5 M; concentrations of P2a-b and P3a-d: 5.4 mg/L. Excitation wavelength: 340 nm. PL spectra of (C) P1a and (D) P1b in THF/hexane mixtures with different hexane contents. Concentrations: 1 × 10−5 M. Excitation wavelength: 350 nm.

EXPERIMENTAL SECTION General procedure for the synthesis of P1a-b. The synthetic procedure of P1b is given below as an example. 4,4'-Methylenedianiline 6 (119 mg, 0.6 mmol), bis[2-(2methoxyethyoxy)ethyl] but-2-ynedioate 1b (191 mg, 0.6 mmol) and 1 mL of methanol were added into a 10 mL polymerization tube equipped with a magnetic stir bar. The mixture was stirred for 30 min at room temperature in air. Then aqueous solution of formaldehyde 3 (37 wt%, 135 μL, 1.8 mmol), acetic acid (205 μL, 3.6 mmol) and 1 mL of methanol were added and the mixture was reacted at room temperature for 16 h. After the reaction, the polymerization mixture was diluted with 5 mL of dichloromethane and the solution was added dropwise to 100 mL of n-hexane through a cotton filter to precipitate

the polymer. The precipitate was allowed to stand overnight. The product was filtered and washed with nhexane (3 × 20 mL), and then dried under vacuum at 40 o C to a constant weight. General procedure for the synthesis of P2a-b. The synthetic procedure of P2a is given below as an example. 4,4'-Methylenedianiline 6 (119 mg, 0.6 mmol), bis[2-(2methoxyethyoxy)ethyl] but-2-ynedioate 1b (191 mg, 0.6 mmol) and 1 mL of methanol were added into a 10 mL polymerization tube equipped with a magnetic stir bar. The mixture was stirred at room temperature for 30 min in air. Then aqueous solution of formaldehyde 3 (37 wt%, 68 μL, 0.9 mmol), acetic acid (205 μL, 3.6 mmol) and 1 mL of methanol were added and the mixture was allowed to react at 70 oC for 16 h. After the reaction, the polymerization mixture was diluted with 5 mL of dichloromethane and the solution was added dropwise to 100 mL of nhexane through a cotton filter to precipitate the polymer. The precipitate was allowed to stand overnight. The product was filtered and washed with n-hexane (3 × 20 mL), and then dried under vacuum at 40 oC to a constant weight. General procedure for the synthesis of P3a-d. The synthetic procedure of P3a is given below as an example. 4,4'-Methylenedianiline 6 (59 mg, 0.3 mmol), bis[2-(2methoxyethyoxy)ethyl] but-2-ynedioate 1b (191 mg, 0.6 mmol) and 1 mL of methanol were added into a 10 mL polymerization tube equipped with a magnetic stir bar. The mixture was stirred at room temperature for 30 min. Then 4,4'-methylenedianiline 6 (59 mg, 0.3 mmol), aqueous solution of formaldehyde 3 (37 wt%, 135 μL, 1.8 mmol), acetic acid (205 μL, 3.6 mmol) and 1 mL of methanol were added and the mixture was allowed to react at room temperature for 16 h. After the reaction, the polymerization mixture was diluted with 5 mL of dichloromethane and the solution was added dropwise to 100 mL of n-hexane through a cotton filter to precipitate the polymer. The precipitate was allowed to stand overnight. The product was filtered and washed with nhexane (3 × 20 mL), and then dried under vacuum at 40 o C to a constant weight.

CONCLUSIONS In summary, we present a metal-free one-pot multicomponent tandem polymerization of ester group-activated alkynes, aromatic diamines, and formaldehyde with high efficiency and convenience, which enables facile preparation of polyheterocycles with well-defined structures, regulated sequence, high molecular weights, and high yields. Most of the polymerizations only link the monomer building blocks together, few of them can build new functional groups in situ. In these MCTPs, functional heterocycles are constructed directly from the polymerization of commercially available, inexpensive, simple monomers, and the resultant polymer structures are inaccessible by other synthetic approaches. Moreover, disubstituted internal alkynes were used as monomers in these MCTPs rather than traditional terminal alkynes, which greatly enriched the monomer variety, product diversity



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and functionalities. Interesting and unique inherent fluorescence behavior was observed from the model compounds and the polyheterocycles even without typical luminophore existed in the structure, indicating new luminescence mechanism and luminophores. Such findings may be correlated with and reveal the underlying reason for the commonly observed but mysterious autofluorescence phenomenon in biological systems. These MCTPs provide a great opportunity for the facile construction of synthetic sequence-controlled polymers, which can pave the way for future investigation of sequence-dependent functionalities of polymer materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials/Instrumentation, synthetic procedures and characterization data; effect of monomer concentration, loading ratio, and diamine monomers on the polymerization; HRMS spectra of 1b, 4b, 5b, and 13-14; 13C NMR spectra of the model compounds and polymers; IR spectra of P3a-d; 1H and 13C NMR, and IR spectra of 13-14; 1H NMR spectra of P3ab; TGA and DSC spectra of the polymers; PL spectra of the PMMAdoped films, fluorescence photos of the solid state, and the photophysical properties of the model compounds and polymers.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the National Science Foundation of China (21404041, 21490573 and 21490574), the Natural Science Foundation of Guangdong Province (2016A030306045 and 2016A030312002), the National Basic Research Program of China (973 Program; 2013CB834701), the Innovation and Technology Commission of Hong Kong (ITCCNERC14SC01), and the Guangdong Innovative Research Team Program (201101C0105067115).

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