Synthesis and characterization of benzodioxinone mono-telechelics and their use in block copolymerization

Poly(methyl methacrylate) (PMMA) and poly(ethylene glycol) methyl ether (mPEG)-based monotelechelics were quantitatively prepared by copper (I)-catalyzed azide/alkyne cycloaddition (CuAAC) click reactions using azido-terminated polymers and alkyne functional benzodioxinones. The monotelechelic containing dimethyl moities (2,2-dimethyl-5-(prop-2-yn-1-yloxy)-4H-benzo[d][1,3]dioxin-4-one) was heat-sensitive, whereas the monotelechelic containing diphenyl moieties (2,2-diphenyl-5-(prop-2-yn-1-yloxy)-4H-benzo[d][1,3]dioxin-4-one) was UV light sensitive. Based on the FT-IR, 1H-NMR, and GPC investigations, the CuAAC click reactions enable the quantitative syntheses of monotelechelics under mild conditions. Moreover, the photosensitive mPEG-based monotelechelic was further utilized for the block copolymer synthesis upon UV-light irradiation. The photoinduced acylation of mPEG monotelechelic consist of (2,2-diphenyl-5-(prop-2-yn-1-yloxy)-4H-benzo[d][1,3]dioxin-4-one) in the presence of hydroxy-terminated poly(epsilon caprolactone) enabled the successful block copolymer formation.


Synthesis of azido-functionalized poly(ethylene glycol) methyl ether (mPEG-N 3 )
2.2.1. Mesillation of mPEG mPEG5000 (2 g, 0.4 × 10 −3 mol -OH group) was dissolved at 30 mL dry dichloromethane in 50 mL round bottom flask. Subsequently, triethylamine (0.56 mL, 4 × 10 −3 mol) was added to the solution as a catalyst. After that, methylsulfonyl chloride was added drop by drop to the solution at inert atmosphere at 0°C. The final solution was stirred vigorously until its temperature gradually rises to room temperature for 24 h. The reaction solution was precipitated in cold diethyl ether (100 mL) and filtered via suction filtration. The filtrate was dried in a vacuum oven at room temperature for 24 h. The yield was found as 1.76 g and calculated as 88%, gravimetrically.

Azidation of mPEG-MSCl
Obtained mesylated mPEG 5000 (1g, 0.2 × 10 −3 mol) was dissolved in DMF (15 mL) followed by excess sodium azide (0.13 g, 2 × 10 −3 mol) addition. The solution was heated up to 45°C and stirred for 3 days. Dry dichloromethane (30 mL) was added to the solution and washed with distilled water 3 times. The organic phase was separated by 250 mL separation funnel and dried over Na 2 SO 4 , and the mixture was filtered. The obtained solution was concentrated to 30 mL by rotary evaporator and precipitated with diethyl ether (300 mL), filtered, and dried in vacuum oven for 24h. The yield (0.92 g, 92%, gravimetrically) and molecular characteristics of PMMA-N 3 were found as M n,GPC = 5100 g/mol and Ɖ = 1.41.

Synthesis of azido-functionalized poly (methyl methacrylate) (PMMA-N 3 )
Firstly, a bromo-functionalized poly(methyl methacrylate) (PMMA-Br) was synthesized by ATRP according to the published procedure [25]. The yield and molecular characteristics of PMMA-Br were found as M n,GPC = 3700 g/mol and Ɖ = 1.33. And then, the obtained PMMA-Br (2 g, 5.12 × 10 −3 mol) and sodium azide (0.167 g, 21.6 × 10 −3 mol) were dissolved in 40 mL DMF in a 100 mL round bottom flask. The reaction mixture was deaerated by nitrogen and stirred via a magnetic stirrer at 60 °C for overnight. After the given time, the mixture was cooled to room temperature and precipitated in 400 mL technical grade methanol. The precipitate was filtrated via a suction filtration setup and dried in a vacuum oven. The yield (1.88 g, 94%, gravimetrically) and molecular characteristics of PMMA-N 3 were found as M n,GPC = 3900 g/mol and Ɖ = 1.37.

Synthesis of hydroxyl-functionalized poly(epsilon caprolactone) (PCL-OH)
The PCL-OH was synthesized by ring-opening polymerization followed by the published procedure [26]. The eCL (5.0 mL, 45.12 mmol) was added as a monomer in Schlenk tubes equipped with a magnetic stirrer and then a solution of tin(II) 2-ethyl-hexanoate in toluene (1.0 mL, 2.256 × 10 −2 mmol), a solution of n-propanol in toluene (1.0 mL, 0.45 mmol) and toluene (3.0 mL) were added under nitrogen. The ring-opening polymerization of eCL was carried out at 110 °C for 24 h. After the given time, the mixture was poured into a 10-fold excess of cold heptane. The resulting hydroxyl-functionalized PCL was filtrated and dried at room temperature in a vacuum oven and obtained as a white powder. The yield (3.35 g and 65%, gravimetrically) and molecular characteristics of PCL-OH were found respectively as M n,GPC = 7400 g/mol and Ɖ = 1.34.

Synthesis of mPEG-based Benzodioxinone mono-telechelics via CuAAC click reaction
The mPEG-N 3 (0.51 g, 0.1 × 10 −3 mol) and Alkyne-DPh-Bd (0.0356 g, 0.1 × 10 −3 mol) was dissolved in 10 mL dissolved. Then, Cu 2 SO 4 .5H 2 O (0.025 g, 0.1 × 10 −3 mol) and sodium ascorbate (0.02 g, 0.1 × 10 −3 mol) were added to the solution and vigorously stirred for overnight at room temperature. After the given time, the mixture was precipitated in hexane, filtered, and dried in a vacuum oven at room temperature. The click reaction of PEG-N 3 and Alkyne-DMe-Bd was carried out with the same procedure (Scheme 2).

Synthesis of mPEG-b-PCL copolymer via photoinduced ketene chemistry
The mPEG-DPh-Bd (0.102 g, 0.02 × 10 −3 mol) and PCL-OH (0.148 g, 0.02 × 10 −3 mol) were dissolved in a small amount of CH 2 Cl 2 (1 mL) in a quartz tube. Subsequently, the reaction mixture was degassed with nitrogen and was placed in a UV photoreactor to illuminate UV light-emitting 370 nm for 48 h (Scheme 4). Finally, the product was precipitated in cold methanol and dried in a vacuum at room temperature after filtration. The yield (0.203 g, 81%, gravimetrically) and molecular characteristics of PCL-OH were found as M n,GPC = 10800 g/mol and Ɖ = 1.44.

Characterization
Fourier transform infrared (FT-IR) and 1 H-NMR spectra of the intermediates and final polymers were recorded on a Perkin-Elmer (Perkin Elmer Italia S.p.A., Milano, Italy) FT-IR Spectrum One B and Varian Unity Inova 500 MHz spectrometers, respectively. The 1 H-NMR measurements were performed in CDCl 3 with Si(CH 3 ) 4   room temperature. Molecular characteristics (molecular weights and molecular weight distributions of resulting polymers were determined by size exclusion chromatography (SEC) by a Viscotek GPCmax Autosampler consisting of a Viscotek differential refractive index detector, a pump, three ViscoGEL GPC columns (G4000H HR , G3000H HR and G2000H HR ) with a tetrahydrofuran flow rate of 1.0 mL/min at 30 °C. The refractive index detector was calibrated by a series of polystyrene standards, which had a narrow polydispersity set consisting of 7 individual standards. All SEC data were analyzed using Viscotek (Malvern Panalytical Ltd., Malvern, UK) OmniSEC Omni-01 software. (Malvern Panalytical Ltd., Malvern, UK)

Results and discussion
Ketenes are very reactive molecules towards unsaturated and nucleophilic compounds. The reactions of ketenes with amines, alcohols, and carboxylic acids are simple routes for the synthesis of corresponding amides, esters, and anhydrides. However, all ketene derivatives are unstable compounds that cannot be isolated due to their instabilities. Therefore, the desired ketenes can be in-situ generated by either thermolysis or photolysis of precursors and readily reacted with antagonist nucleophilic compounds. Benzodioxinones are important precursors to form reactive ketenes upon heat or UV illuminations. Recently, benzodioxinone chemistry has been utilized in homopolymerization, block, and graft copolymerization and cross-linking reactions for the construction of various macromolecular architectures including homopolymer block copolymers, graft copolymers and polymer networks. Here, thermally (Alkyne-DMe-Bd) and photochemically (Alkyne-DPh-Bd) active alkyne functionalized benzodioxinones were firstly synthesized by etherification reactions of propargyl bromide with either 5-hydroxy-2,2-dimethyl-4h-benzo[d] [1,3]dioxin-4-one or 5-hydroxy-2,2-diphenyl-4h-benzo[d] [1,3]dioxin-4one. Then, chemical structures of Alkyne-DMe-Bd and Alkyne-DPh-Bd were confirmed by both FT-IR and 1 H-NMR spectroscopies. The aromatic and aliphatic C-H bands as well as the C=O band and C-O-C bands at 2950, 2800, 1730, and 1100 cm -1 in FT-IR spectroscopy confirmed the chemical structures of Alkyne-DMe-Bd and Alkyne-DPh-Bd. In addition, the characteristic aromatic bands of benzodioxinone and alkyne groups in both Alkyne-DMe-Bd and Alkyne-DPh-Bd molecules were clearly detected at 6.7-6.5, 4.9 and 2.5 ppm, respectively. Furthermore, the phenyl groups of Alkyne-DPh-Bd coming from benzophenone and methyl groups of Alkyne-DMe-Bd coming from acetone were also revealed at 7.60-7.2 and 1.7 ppm, respectively (see experimental part).
After successful synthesis of initial Alkyne-DPh-Bd and Alkyne-DMe-Bd, their installations on the azido groups of mPEG-N 3 were done by CuAAC click reactions at room temperature using Cu 2 SO 4 /sodium ascorbate catalyst system. For both cases, quantitative yields were determined gravimetrically (92 and 85% for mPEG-DPh-Bd and mPEG-DMe-Bd, respectively). On the other hand, the CuAAC click reactions were monitored by FT-IR spectroscopy through the disappearance of the characteristic azido band at 2111 cm -1 . The complete disappearance of azido peak in the FT-IR spectra of mPEG-DPh-Bd and mPEG-DMe-Bd in Figure 1 proved the successful CuAAC click reactions.
In addition to FT-IR investigations, the 1 H-NMR spectroscopy gives more detailed information about the chemical structures of mPEG-DPh-Bd and mPEG-DMe-Bd monotelechelics ( Figure 2). In both cases, the characteristic peaks (e+f) of mPEG were clearly detected around 3.31-3.92 ppm, while the aromatic and aliphatic peaks of benzodioxinones also appeared at 6.51-7.63, 1.73 and 4.76 ppm. Furthermore, the new peak corresponding to triazole rings at 7.83 ppm confirmed the successful installation of benzodioxinones onto the terminal positions of mPEG. On the other hand, the molecular weights of the resulting polymers were slightly higher than mPEG-N 3 due to the contributions of Alkyne-DPh-Bd and Alkyne-DMe-Bd molecules. On the contrary, the polydispersity indices of the mPEG-DPh-Bd and mPEG-DMe-Bd monotelechelics were negligibly decreased from 1.41 to 1.39 and 1.36. This could be due to the elimination of possible unfunctionalized mPEG chains after the solution/precipitation procedure.
Finally, the peak areas of the aliphatic protons of mPEG (e and f) and aromatic protons of benzodioxinones (b) in 1 H-NMR spectra of mPEG-DPh-Bd and mPEG-DMe-Bd monotelechelics were utilized to calculate their molecular weights. The M n,NMR values of mPEG-DPh-Bd and mPEG-DMe-Bd were found as 5200 and 5100 g/mol, respectively.
The similar CuAAC click chemistry was also applied for the functionalization of PMMA-N 3 with Alkyne-DPh-Bd and Alkyne-DMe-Bd. The chemical structures of PMMA-DPh-Bd and PMMA-DMe-Bd were confirmed by FT-IR and 1 H-NMR spectroscopy analysis. The complete disappearance of azido band at 2100 cm -1 of PMMA-N 3 approved the successful syntheses of PMMA-DPh-Bd and PMMA-DMe-Bd in Figure 3. Moreover, the characteristic C=O, C-O-C, and aromatic bands of both PMMA and benzodioxinones units also determined FT-IR spectra of PMMA-DPh-Bd and PMMA-DMe-Bd.
The 1 H-NMR analyses of PMMA-DPh-Bd and PMMA-DMe-Bd were accomplished, and the characteristic bands of both PMMA and benzodioxinones components were assigned in Figure 4. The aromatic and aliphatic groups of benzodioxinones were confirmed by detecting a, b, and c peaks at 7.61, 6.62 and 4.83 ppm, respectively. In addition, the aliphatic protons coming from ATRP initiator (g+hi+j) and PMMA (e+f) also appeared at 0.78-2.05, 3.69 and 4.07 ppm.
The peak areas of the aliphatic protons of PMMA (f) and aromatic protons of benzodioxinones (b) in 1 H-NMR spectra of PMMA-DPh-Bd and PMMA-DMe-Bd monotelechelics were also utilized to calculate their molecular weights. The M n,NMR values of PMMA-DPh-Bd and PMMA-DMe-Bd were found as 4250 and 4050 g/mol, respectively.
A model study was also completed to investigate the potential use of photochemically active monotelechelic in the synthesis of block copolymers. The UV exposure of reactive benzodioxinone terminated mPEG-DPh-Bd and antagonist PCL-OH in a UV photoreactor with light-emitting 370 nm for 48 h led to the quantitative block copolymer formation (mPEG-b-PCL). Based on the FT-IR spectrum of mPEG-b-PCL, the characteristic ester bands of PCL and ether bands of mPEG were clearly detected in Figure 5. This finding confirmed that the obtained mPEG-b-PCL consists of both PCL and mPEG blocks.  On the other hand, the distinguished -CH 2 -bands of PCL and -CH 2 -bands of mPEG also appeared at 3.65 and 4.11 ppm in the 1 H-NMR spectrum of mPEG-b-PCL. Moreover, the characteristic triazole (j) and benzodioxinone (f+g+h) bands appeared between 6.5 and 8.0 ppm ( Figure 6). The peak areas of the CH 2 protons of mPEG (m) and O-CH 2 -protons of PCL (e) were utilized to calculate the composition of block copolymers. The composition of mPEG for block copolymer was calculated as 40.1%.
In a further investigation, the photoinduced ketene chemistry between mPEG-DPh-Bd and PCL-OH was followed by GPC analyses, and their traces were shown in Figure 7. The GPC traces showed us that the mPEG-b-PCL displayed a  unimodal and symmetric eluent peak with a slightly broader molecular weight distribution (Ð = 1.44) than mPEG-N 3 (Ð = 1.41) and mPEG-DPh-Bd (Ð = 1.39). Furthermore, the GPC chromatogram also revealed a significant increase between molecular weights of precursors and block copolymers as they were clearly shifted from 5.300 (mPEG-DPh-Bd) and 7.400 (PCL-OH) to 10.800 (mPEG-b-PCL) g/mol. The combined 1 H-NMR and GPC results clearly confirm that the successful formation of block copolymer without detectable unreactive free precursors. The molecular weights and molecular weight distributions of all precursors, monotelechelics, and block copolymer were summarized in Table 1.

Conclusion
In summary, heat-and light-sensitive monotelechelics were successfully synthesized by CuAAC click chemistry using mPEG-N 3 and PMMA-N 3 with Alkyne-DPh-Bd and Alkyne-DMe-Bd compounds under mild conditions. The chemical structures of precursors and monotelechelics were clearly confirmed by spectral analyses using FT-IR and 1 H-NMR spectroscopies. According the 1 H-NMR analysis, the desired monotelechelics were quantitatively obtained over 91% CuAAC  click efficiency. The photosensitive mPEG-DPh-Bd monotelechelic was further utilized for block copolymer synthesis with antagonist PCL-OH under mild conditions. The 1 H-NMR spectroscopy and GPC analysis revealed that successful block copolymer formation was achieved upon UV irradiation for 48 h. Consequently, these reactive monotelechelics will utilize a vital role for the synthesis of various macromolecular architectures such as block and graft copolymers either heat or UV-light exposure.