Green Monomer of CO2 and Alkyne-based Four-component Tandem Polymerization toward Regio- and Stereoregular Poly(aminoacrylate)s

Green monomers, such as carbon dioxide (CO2), are closely related to our daily life and highly desirable to be transferred to functional polymers with diverse structures and versatile properties because they are abundant, cheap, nontoxic, renewable, and sustainable. However, the polymerizations based on these green monomers are to be further developed. In this work, a facile CO2 and alkyne-based one-pot, two-step, four-component tandem polymerization was successfully established. The polymerization of CO2, diynes, alkyl dihalides, and primary/secondary amines can proceed under mild reaction conditions and regio- and stereoregular poly(aminoacrylate)s with good solubility and thermal stability were obtained in high yields (up to 95%). Notably, distinctly different stereoregularity of resultant poly(aminoacrylate)s was realized via using primary or secondary amines. Using the former would readily generate polymers with 100% Z-isomers, whereas the latter furnished products with over 95% E-isomers. Through different monomer combination, the polymers with tunable structures and properties were obtained. Moreover, the tetraphenylethene units containing poly(aminoacrylate)s, showing the unique aggregation-induced emission characteristics, could function as a fluorescent probe for sensitive explosive detection. Thus, this work not only develops a facile CO2 and alkyne-based multicomponent tandem polymerization but also provides a valuable strategy to fine-tune the polymer structures and properties, which could be potentially applied in diverse areas.


INTRODUCTION
Multicomponent reactions (MCRs), in which three or more starting materials react together to form a product, are significant for living organisms. Various MCRs take place all the time in our bodies for the synthesis of DNA, RNA, proteins and so on. Nowadays, a great number of MCRs have been developed such as Mannich, Passerini, and Ugi reactions, which play important roles in organic synthesis. [1−4] Recently, multicomponent polymerizations (MCPs) as an emerging direction of synthetic polymer chemistry have been well-studied and inherit the advantages of multicomponent reactions such as structural diversity, high efficiency, mild reaction condition, high atom economy, and simple operation. [5−17] Very recently, efficient catalyst-free MCPs of sulfur, amines, and alkynes/isocyanides/acids have been developed and sul-fur-containing polymers were prepared. [18−20] Compared with the sulfur monomer, one of the major byproducts from the petroleum industry, green monomers, such as oxygen (O 2 ), carbon dioxide (CO 2 ), and water (H 2 O), are more closely related to our daily life and highly desirable to be used for MCPs because all of them are abundant, cheap, nontoxic, renewable, and sustainable. A few MCPs based on green monomers have been established. For example, a facile polymerization of H 2 O, isocyanides, and bromoalkynes has been reported and functional polyamides were produced. [21] Afterwards, an efficient three-component polymerization involving benzoxazines, isocyanides, and H 2 O was also developed to generate polyamides. [22] Similarly, O 2 was also firstly used in the alkyne-based polymerization, readily furnishing poly(tetrasubstituted furan)s with versatile properties and potential applications in optoelectronic and biological fields. [23] Recently, several direct polymerizations of CO 2 , diynes/diamines, and dihalides under very mild reaction conditions and normal CO 2 pressure have also been developed, and functional polymers were yielded. [24−28] Besides MCRs, the tandem reactions, in which multiple steps could be combined into one synthetic operation, have also drawn much attention because they can simplify experimental procedures, save time, and enable us to synthesize structurally complex compounds from simple and readily available substrates compared with traditional "step-by-step" operations. [29−32] Thus, combining tandem reactions and MCPs to develop efficient multicomponent tandem polymerizations (MCTPs) based on green monomers is very attractive. A prominent advantage of MCTPs is structural control and diversity of resultant polymers. There are two main approaches to control the structure and expand the diversity of polymers via MCTPs. One is to change the type of third/fourth monomers to control the stereoselectivity of polymers. The other is to alter the monomer combinations to control the backbones and side chains of polymers.
Attracted by the advantages of MCTPs, in this work, we succeeded in developing a facile and efficient one-pot, two-step, four-component tandem polymerization of CO 2 , diynes, alkyl dihalides, and primary/secondary amines (Scheme 1). The MCTP could propagate smoothly under mild conditions and normal CO 2 pressure, and soluble, thermally stable, regio-and stereoregular poly(aminoacrylate)s could be furnished in high yields. It is worth noting that using primary or secondary amine can cause distinctly different stereoregularities of resultant poly(aminoacrylate)s. Moreover, through different combinations of monomers, the polymers with tunable structures and properties could be obtained. In addition, the tetraphenylethene (TPE) units containing poly(aminoacrylate)s show unique aggregation-induced emission (AIE) characteristics and could be applied in sensitive explosive detection.

Instruments
1 H-and 13 C-NMR spectra were measured on a Bruker Avance 500 MHz NMR spectrometer using deuterated dichloromethane as solvent and tetramethylsilane (TMS, δ=0) as internal reference. FTIR spectra were recorded on a Bruker Vector 33 FTIR spectrometer. High resolution mass spectrometry measurements were performed on a GCT premier CAB 048 mass spectrometer. The number-(M n ) and weight-average (M w ) molecular weights and polydispersity indices (Ð = M w /M n ) of polymers were estimated by a Waters advanced polymer chromatography (APC) system with a photo-diode array (PDA) detector, and THF was used as an eluent at a flow rate of 0.5 mL/min. A set of monodispersed linear polystyrenes covering the M w range of 10 3 −10 7 g/mol were utilized as standards for molecular weight calibration. Thermogravimetric analysis was carried out on a Shimadzu TGA-50 analyzer under a nitrogen atmosphere at a heating rate of 20 °C/min. UV-Vis absorption spectra were recorded on a Shimadzu UV-2600 spectrophotometer. Fluorescence spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer.

Polymerization
All the polymerization reactions were carried out under CO 2 atmosphere with normal pressure using the standard Schlenk technique. A typical polymerization procedure of 1a, 2, CO 2 , and 3a is given below as an example.
Into a 10 mL dried Schlenk tube equipped with a magne-

Model Reaction
Into a 10 mL dried Schlenk tube equipped with a magnetic stirrer were placed 5 (269.4 mg, 1.0 mmol), 1-bromooctane 6 (231.7 mg, 1.2 mmol), Ag 2 WO 4 (23.2 mg, 0.05 mmol), and Cs 2 CO 3 (977.5 mg, 3.0 mmol) under CO 2 (balloon). Dried DMAc (3 mL) was injected into the tube by a syringe. The resultant mixture was stirred at 80 °C under CO 2 atmosphere for 12 h. Afterward, 3a (163.8 μL, 1.5 mmol) was injected. The mixture was stirred for additional 4 h in air. Then the reaction mixture was cooled to room temperature and extracted with DCM (60 mL×3). The organic layer was washed with water (100 mL×3) and dried over Na 2 SO 4 . After purification by silica gel column chromatography using petroleum ether (PE)/EA mixture as the eluent, a yellow solid of compound

Multicomponent Tandem Polymerization
Encouraged by our previous work that the poly(alkynoate)s obtained from MCP of CO 2 , diynes, and alkyl dihalides could be postfunctionalized by benzylamine, [25] we would like to combine them to develop a CO 2 and alkyne-based one-pot, two-step, four-component tandem polymerization using the monomers 1a, 1b, and 2−4. The polymerization of triphenylamine (TPA)-containing diyne 1a, 1,8-dibromooctane 2, primary amine 3a, and CO 2 was first investigated as an example. This polymerization was conducted in two steps. First, 1a, 2, and CO 2 were polymerized to afford poly(alkynoate) intermediate, and then, the intermediate was directly reacted with the fourth component 3a via amino-yne click reaction without separation and purification. [33] When the reaction time of the second step (t 2 ) reached 4 h, the poly(aminoacrylate) with a weight-average molecular weight (M w ) of 1.05×10 4 g/mol was obtained in a high yield (86%).
To test the generality of this four-component tandem polymerization, we polymerized other monomers. As listed in Table 1, all the polymerizations propagated smoothly, and soluble poly(aminoacrylate)s were produced in high yields (up to 95%). These results demonstrate that this polymerization has wide monomer scope, which will greatly enrich the structures and functionality of the obtained poly(aminoacrylate)s.
The high efficiency and the multicomponent reaction nature of this polymerization enable us to use versatile monomer combination such as "A 2 +B 2 +CO 2 +C 1 ", "A 2 +B 1 +CO 2 +C 2 ", and "A 1 +B 2 +CO 2 +C 2 " to construct polymers with different mainchains and properties, where, A, B, and C represent alkyne, bromoalkyl, and amino monomers, respectively. As shown in Scheme 2, the reaction of diyne 1a, 1-bromooctane 6, and CO 2 in the first step could produce a new alkynoate monomeric intermediate with 100% conversion, in which the diamine 7 was added readily furnished poly(aminoacrylate) P1a/6/7/CO 2 with alkyl side chains and M w of 5000 g/mol in a yield of 56%. Similarly, the polymerization of monoyne 5, dibromooctane 2, CO 2 , and diamine 7 could be used to generate the poly(aminoacrylate) P5/2/7/CO 2 with different main-chains and M w of 7200 g/mol in a yield of 75%. The yields and M w of P1a/6/7/CO 2 and P5/2/7/CO 2 are both lower than those of P1a/2/3a/CO 2 , probably due to the poor solubility of diamine 7 in DMAc when the monomer concentration reached 0.2 mol/L, which further resulted in incomplete conversion of diamine 7 and stoichiometric imbalance of monomers during the polymerization (Table 2).
Scheme 2 CO 2 and alkyne-based four-component tandem polymerization with different monomer combination.

Structural Characterization
To facilitate the characterization of polymer structures, a model compound 8 was synthesized through a similar one-pot, twostep, four-component tandem reaction in 90% yield (Scheme 3). All the monomers, model compound, and polymers were fully characterized by standard spectroscopic techniques, and satisfactory analysis data corresponding to their expected molecular structures were obtained. The FTIR spectra of monomer 1a, model compound 8, and polymer P1a/2/3a/CO 2 are provided in Fig. 1 as an example. The absorption bands of 1a associated with ≡C-H and C≡C stretching vibrations could be observed at 3290 and 2099 cm −1 , respectively. These bands, however, could not be observed in the spectra of P1a/2/3a/CO 2 and 8. Meanwhile, a new band associated with C＝O stretching vibrations appeared at 1649 cm −1 , revealing the occurrence of the polymerization. Similar results were obtained in the FTIR spectra of other polymers (Figs. S1−S8 in the electronic supplementary information, ESI).
To gain more structural details, 1 H-NMR spectra of the monomers, 8, and P1a/2/3a/CO 2 in DCM-d 2 were compared and analyzed (Fig. 2). The ethynyl protons of 1a, CH 2 protons adjunct to the Br atom of 2, and NH 2 protons of 3a resonated at 3.08, 3.40, and 1.84 ppm, respectively, which could not be found in the spectra of 8 and P1a/2/3a/CO 2 . Meanwhile, four new signals at 8.84, 4.66, 4.33, and 4.03 ppm, assigned to the resonances of the NH proton, ethenyl proton, CH 2 protons next to the N and O atoms, respectively, appeared in the spectra of 8 and P1a/2/3a/CO 2 . Only one resonance signal of ethenyl proton could be observed and the N-H resonance was located at downfield in the 1 H-NMR spectra of 8 and P1a/2/3a/CO 2 , indicative of the formation of an intramolecular hydrogen bond between the N-H and C＝O, which has been confirmed by X-ray single crystal analysis and density functional theory (DFT) calculations in our previous reports. [34,35] Thus, the newly formed vinyl group was confirmed to be a Z-isomer in 8 and P1a/2/3a/CO 2 , suggesting that this polymerization is highly regio-and stereoselective (Scheme 1). The stereoregularity of P1a/2/3a/CO 2 is better than those of poly(aminoacrylate)s obtained from primary amine and ester-activated terminal alkyne monomers (Z/E = 61/39). [33,36] However, when the secondary amines were used as the comonomers instead of primary ones, the stereoregularity of resultant poly(aminoacrylate)s was totally different. As shown in Fig. S9 (in ESI), the E-and Z-vinyl protons could be readily distinguished at 4.91 and 4.70 ppm in the spec-   trum of P1a/2/4b/CO 2 , respectively. From the peak integrals at these signals, the ratio of E-and Z-isomers in P1a/2/4b/CO 2 was calculated to be 97/3, indicative of a high stereoregularity of resultant poly(aminoacrylate). Notably, this regioregularity is consistent with previous report because the E-isomers are more stable than the Z-ones with an energy difference of 22.6 kJ/mol via DFT calculation. [33,36] The 13 C-NMR spectra further confirmed the success of this four-component tandem polymerization. As shown in Fig. 3

Solubility and Thermal Stability
Due to the flexible structures of the resultant polymers, they all enjoyed good solubility in commonly used organic solvents, such as DCM, chloroform, THF, and DMAc, and could be fabricated into high-quality films by spin-coating process. The polymers were also thermally stable as evaluated by thermogravimetric analysis (TGA).
As shown in Fig. 4, the temperatures of 5% weight loss (T d ) are in the range of 240−324 °C under nitrogen. It is noteworthy that the poly(aminoacrylate)s with different mainchains showed different thermal stability as concluded from the T d values of P1a/2/3a/CO 2 , P1a/6/7/CO 2 , and P5/2/7/CO 2 . When the backbones of polymers become rigid, their T d values increase correspondingly. Hence, tunable T d values can be achieved via different monomer combination.

Photoluminescence Properties
The incorporation of tetraphenylethene (TPE), a typical moiety featuring the aggregation-induced emission (AIE) characteristics, [37−41] into the polymer skeletons of P1b/2/3a/CO 2 and P1b/2/4b/CO 2 made them AIE-active, too. Their photoluminescence (PL) spectra obtained in THF/water solutions with different water fractions (f w ) were then investigated to study their AIE behaviors (Fig. 5a, Figs. S25 and S26 in ESI). The PL curves of their THF solutions are almost a flat line parallel to the abscissa, indicating that the polymers were virtually nonemissive in the solutions. Gradual addition of water as the poor solvent into the THF solution led to aggregation of the polymer chains, which induced the emission intensified slowly. From the photographs of P1b/2/3a/CO 2 taken in THF and a THF/water mixture with a f w of 90% (insets of Fig. 5b), we can find that the former emits faintly, while the latter fluoresces intensely. The highest PL intensities of P1b/2/3a/CO 2 and P1b/2/4b/CO 2 were recorded for the THF/water mixtures with f w of 90%, which are 41-and 7-folds higher than those in THF solutions, respectively (Fig. 5b). Such PL behaviors further confirm that P1b/2/3a/CO 2 and P1b/2/4b/CO 2 are AIE-active. Notably, the PL enhancement of P1b/2/3a/CO 2 is higher than that of P1b/2/4b/CO 2 because when P1b/2/3a/CO 2 is dissolved in good solvents, the large free volumes enable the phenyl rings of TPE units to rotate  freely, making the polymer non-emissive. In comparison, the P1b/2/4b/CO 2 has a more crowded structure, which could partly restrict the rotation of phenyl rings of TPE units, so it has a weak fluorescence when dissolved in good solvents.

Explosive Detection
Thanks to its excellent fluorescent behaviors in the aggregate state, P1b/2/3a/CO 2 was used as a chemosensor for explosive detection, which is important for antiterrorism and homeland security. [42,43] Herein, the commercially available picric acid (PA) was used as a model explosive, and the nanoaggregates of P1b/2/3a/CO 2 in THF/water mixtures with a f w of 90% were utilized as the probe. With gradual addition of PA into the system, the PL intensity of the aggregates of P1b/2/3a/CO 2 progressively decreased (Fig. 6a). The quenching constant of P1b/2/3a/CO 2 was deduced from the Stern-Volmer plots to be 1.45×10 4 L/mol (Fig. 6b) in the PA concentration range of 0−140 μmol/L and the limit of detection (LOD) was thus calculated to be 5.5×10 −6 mol/L according to the equation of LOD = 3SB/m. [33] In this equation, SB represents the standard deviation of the blank measurements and m is the slope of intensity versus sample concentration. This LOD value is comparable with those in the previous reports. [44−46] CONCLUSIONS In summary, we successfully developed a facile and efficient CO 2 and alkyne-based one-pot, two-step, four-component tandem polymerization under mild reaction conditions. Regioregular and tunable stereoregular poly(aminoacrylate)s could be generated in high yields. Through adjusting the monomer combination, such as "A 2 +B 2 +CO 2 +C 1 ", "A 2 +B 1 +CO 2 +C 2 ", and "A 1 +B 2 +CO 2 +C 2 ", the polymers with tunable structures and properties could be obtained. The resultant polymers possessed excellent solubility in organic solvents and thermal stabilities. The TPE units containing poly(aminoacrylate) showed the unique AIE characteristics, and their aggregates could function as the fluorescent probe for sensitive explosive detection. Thus, this work not only develops a facile CO 2 and alkyne-based multi-component tandem polymerization but also provides a valuable strategy to construct polymers with diverse structures and versatile properties, which could potentially find wide applications in diverse areas.

Electronic Supplementary Information
Electronic supplementary information (ESI) is available free of