Design and Synthesis of a Cyclic Double-Grafted Polymer Using Active Ester Chemistry and Click Chemistry via A “Grafting onto” Method

Combing active ester chemistry and click chemistry, a cyclic double-grafted polymer was successfully demonstrated via a “grafting onto” method. Using active ester chemistry as post-functionalized modification approach, cyclic backbone (c-P2) was synthesized by reacting propargyl amine with cyclic precursor (poly(pentafluorophenyl 4-vinylbenzoate), c-PPF4VB6.5k). Hydroxyl-containing polymer double-chain (l-PS-PhOH) was prepared by reacting azide-functionalized polystyrene (l-PSN3) with 3,5-bis(propynyloxy)phenyl methanol, and further modified by azide group to generate azide-containing polymer double-chain (l-PS-PhN3). The cyclic backbone (c-P2) was then coupled with azide-containing polymer double-chain (l-PS-PhN3) via CuAAC reaction to construct a novel cyclic double-grafted polymer (c-P2-g-Ph-PS). This research realized diversity and complexity of side chains on cyclic-grafted polymers, and this cyclic double-grafted polymer (c-P2-g-Ph-PS) still exhibited narrow molecular weight distribution (Mw/Mn < 1.10).


Introduction
Grafted polymers, in conjunction with conventional linear polymers, have fascinating topological macromolecular structure with various side chains along their main backbones. They exhibit unique and remarkable properties by controlling chemical components of main backbones or side chains, the length of side chains, and grafting density [1,2]. Grafted polymers have been thoroughly probed as the precursors of drug-delivery materials [3], biosensors [4], nanowires [5] and nanotubes [6]. According to the structure of polymeric main backbones, grafted polymers can be roughly divided into linear-, star-, cyclic-, dendritic-, hyperbranched-grafted polymers and so on. With continuous development of new synthesis technologies, novel grafted polymers have exhibited more and more different properties and exploration for new strategies to construct novel grafting polymers have never stopped [7][8][9][10][11][12][13][14][15].
In this work, combing active ester chemistry and click chemistry, we constructed a cyclic doublegrafted polymer successfully via the "grafting onto" approach as shown in Scheme 1. Using active ester chemistry, a cyclic backbone (c-P2) was synthesized by post-functionalized modification by reacting propargyl amine with cyclic precursor (poly(pentafluorophenyl 4-vinylbenzoate), c-PPF4VB6.5k) (Scheme S1). Additionally, hydroxyl-containing polymer double-chain (l-PS-PhOH) was prepared by reacting azide-functionalized polystyrene (l-PSN3) with 3,5-bis(propynyloxy)phenyl methanol, and further modified by azide group to generate azide-containing polymer double-chain (l-PS-PhN3). The cyclic backbone (c-P2) was then coupled with prepared polymer double-chain (l-PS-PhN3) using CuAAC reaction to successfully construct a novel cyclic double-grafted polymer (c-P2g-Ph-PS). This research realized diversity and complexity of side chains on cyclic-grafted polymers, and this cyclic double-grafted polymer (c-P2-g-Ph-PS) still exhibited narrow molecular weight distribution. Scheme 1. Synthetic routes of a cyclic double-grafted polymer.

Characterizations
All the 1 H NMR and 13 C NMR spectra were measured on a Bruker (300 MHz) Nuclear Magnetic Resonance spectrometer (Bruker, USA). All the average molecular weights (M n ) and molecular weight distributions (M w /M n ) were measured by TOSOH HLC-8320 size exclusion chromatography (SEC, Tosoh Corporation, Japan). The recycling preparative HPLC Mode LC-9260NEXT (often called as Prep-SEC, Tosoh Corporation, Japan) was utilized to purify crude polymers. A Bruker TENSOR-27 FT-IR spectrometer was utilized to measure FT-IR spectra (Bruker, USA). Matrix assisted laser desorption ionization/time of flight mass spectra (MALDI TOF MS) (Bruker, USA) were gained by using an UltrafleXtreme MALDI TOF mass spectrometer. The UV-light resource was considered using one low-pressure lamp purchased from Beijing China Education Au-light Co. Ltd (CEL-LPH120-254, 120 W, Beijing, China). All the parameters and measure conditions of these spectrometers are shown in detail in the supporting information.

Synthesis of Azide-Containing Polymer Double-Chain (l-PS-PhN 3 )
Hydroxyl-containing polymer double-chain (l-PS-PhOH, 100 mg, 0.02 mmol) was dissolved in DMF (1 mL) and put in an ampoule (5 mL) containing a magnetic stirrer in nitrogen atmosphere. The ampoule was wrapped in aluminum foil to avoid light. DPPA (110.08 mg, 0.4 mmol) and DBU (60.90 mg, 0.4 mmol) were added into the ampoule under nitrogen atmosphere. The ampoule was placed in an oil-bath at 80 • C for 24 h. The mixed solution was purified by passing through a short Al 2 O 3 column, precipitated in anhydrous methanol and dried under vacuum. (l-PS-PhN 3 , 93.4 mg, yield: 93.4%, M n,SEC = 5000 g/mol, M w /M n = 1.04).

Synthesis of l-PS-PhOH and l-PS-PhN 3
Hydroxyl-containing polymeric double-chain (l-PS-PhOH) was synthesized by reacting 3,5-bis(propargyloxy)benzyl alcohol (Scheme 2) and azide-functionalized polystyrene (l-PS-N 3 ) by virtue of Copper-catalyzed azide/alkyne cycloaddition (CuAAC) reaction. The synthesis and characterization of l-PS-N 3 (M n,SEC = 2500 g/mol, M w /M n = 1.05) was shown in our previous publication [50]. The usage of slightly excessive l-PS-N 3 was necessary in the process of preparing l-PS-PhOH, the gained crude l-PS-PhOH needs to be easily purified by Prep-SEC.
Hydroxyl-containing polymer double-chain (l-PS-PhOH) was verified by SEC, NMR, MALDI TOF MS and FT-IR spectroscopy. As shown in Figure 1, corresponding to l-PS-N3, 1 H NMR spectra of l-PS-PhOH showed that the characteristic signal of the methine hydrogen (-CH(Ph)-, f) shifted from 3.8-4.1 ppm to 4.9-5.2 ppm completely. A new peak was clearly observed at 4.6 ppm, which was assigned to the benzylic hydrogen (-CH2-, i). In addition, the (f+h)/i/b integration ratio is close to 6/2/4, which means the successful formation of l-PS-PhOH. The number average molecular weight of l-PS-PhOH (Mn,SEC = 5000 g/mol, Figure 2) was twice than that of l-PS-N3 (Mn,SEC = 2500 g/mol, Figure  2) and the molecular weight distribution remained at 1.04, which also indicated the successful preparation of l-PS-PhOH. MALDI TOF MS ( Hydroxyl-containing polymer double-chain (l-PS-PhOH) was verified by SEC, NMR, MALDI TOF MS and FT-IR spectroscopy. As shown in Figure 1, corresponding to l-PS-N 3 , 1 H NMR spectra of l-PS-PhOH showed that the characteristic signal of the methine hydrogen (-CH(Ph)-, f) shifted from 3.8-4.1 ppm to 4.9-5.2 ppm completely. A new peak was clearly observed at 4.6 ppm, which was assigned to the benzylic hydrogen (-CH 2 -, i). In addition, the (f+h)/i/b integration ratio is close to 6/2/4, which means the successful formation of l-PS-PhOH. The number average molecular weight of l-PS-PhOH (M n,SEC = 5000 g/mol, Figure 2) was twice than that of l-PS-N 3 (M n,SEC = 2500 g/mol, Figure 2) and the molecular weight distribution remained at 1.04, which also indicated the successful preparation of l-PS-PhOH.

Synthesis of l-PS-PhN3 and c-P2-g-Ph-PS
Azide-containing polymer double-chain (l-PS-PhN3) was synthesized by hydroxyl-containing polymer double-chain (l-PS-PhOH) under the system of DPPA/DBU mixtures. Azide-containing polymer double-chain (l-PS-PhN3) was verified by SEC, NMR, MALDI TOF MS and FT-IR spectroscopy. Comparing to the spectrum of l-PS-PhOH (Figure 1), 1 H NMR spectrum of l-PS-PhN3 ( Figure 5) demonstrated that the benzylic hydrogen (-CH2-, i) shifted from 4.6 to 4.2 ppm completely, which indicated the complete formation of azide-containing polymeric double-chain (l-PS-PhN3). After azidation, the (f+h)/i/b integration ratio still kept at 6/2/4, which also indicated the successful preparation of l-PS-PhN3. From SEC curves (Figures 2 and 6), there are no obvious changes before and after azidation. The average molecular weight of l-PS-PhN3 is 4900 g/mol and the molecular weight distribution remained at 1.03. In FT-IR spectrum (Figure 4

Synthesis of l-PS-PhN3 and c-P2-g-Ph-PS
Azide-containing polymer double-chain (l-PS-PhN3) was synthesized by hydroxyl-containing polymer double-chain (l-PS-PhOH) under the system of DPPA/DBU mixtures. Azide-containing polymer double-chain (l-PS-PhN3) was verified by SEC, NMR, MALDI TOF MS and FT-IR spectroscopy. Comparing to the spectrum of l-PS-PhOH (Figure 1), 1 H NMR spectrum of l-PS-PhN3 ( Figure 5) demonstrated that the benzylic hydrogen (-CH2-, i) shifted from 4.6 to 4.2 ppm completely, which indicated the complete formation of azide-containing polymeric double-chain (l-PS-PhN3). After azidation, the (f+h)/i/b integration ratio still kept at 6/2/4, which also indicated the successful preparation of l-PS-PhN3. From SEC curves (Figures 2 and 6), there are no obvious changes before and after azidation. The average molecular weight of l-PS-PhN3 is 4900 g/mol and the molecular weight distribution remained at 1.03. In FT-IR spectrum (Figure 4), the vibrational absorption peak from azide group of l-PS-PhN3 appeared at 2094 cm −1 . MALDI TOF MS provided persuasive evidence for successful formation of l-PS-PhN3.   The active ester chemistry, such as the nucleophilic substitution of activated ester bearing pentafluorophenyl groups with diverse amines, is one kind of high-effective chemical reaction, which is often utilized as one post-modification technology for constructing functional polymers that cannot be obtained by conventional polymerization technologies [53]. Here, we chose propargylamine as the amine to react with cyclic poly(pentafluorophenyl 4-vinylbenzoate) (c-PPF4VB 6.5k ) for synthetizing functional cyclic polymer (c-P2). All the synthesis and characterizations of c-PPF4VB 6.5k and c-P2 are shown in supporting information in detail (Figures S2-S8).
Furthermore, functional cyclic polymer (c-P2) was used as cyclic polymeric backbone to react with polymer double-chain (l-PS-PhN 3 ) via CuAAC reaction for constructing cyclic double-grafted polymer (c-P2-g-Ph-PS). The crude cyclic double-grafted polymer (c-P2-g-Ph-PS) was purified by Prep-SEC and further characterized by NMR and SEC. As shown in Figure 5, 1 H NMR spectrum of c-P2-g-Ph-PS exhibited that the characteristic signals from the methine hydrogen (-CH(Ph)-, f) adjacent to 1,2,3-triazole, the methylene hydrogen (-CH 2 -, h and p) adjacent to 1,2,3-triazole and the benzylic hydrogen (-CH 2 -, i) were assigned in the 4.2-5.5 ppm region. It is hard to calculate grafting density of cyclic double-grafted polymer (c-P2-g-Ph-PS) by the integration ratio from 1 H NMR spectrum, but the difference between l-PS-PhN 3 and c-P2-g-Ph-PS indicated the successful preparation of cyclic double-grafted polymer. Additionally, according to the SEC curves of c-P2, l-PS-PhN 3 and c-P2-g-Ph-PS ( Figure 6), the shifts toward high molecular weight field can be observed clearly, demonstrating the successful formation of cyclic double-grafted polymer. The molecular weight of cyclic double-grafted polymer (c-P2-g-Ph-PS) was 30,700 g/mol and the molecular weight still stayed at 1.04. The active ester chemistry, such as the nucleophilic substitution of activated ester bearing pentafluorophenyl groups with diverse amines, is one kind of high-effective chemical reaction, which is often utilized as one post-modification technology for constructing functional polymers that cannot be obtained by conventional polymerization technologies [53]. Here, we chose propargylamine as the amine to react with cyclic poly(pentafluorophenyl 4-vinylbenzoate) (c-PPF4VB6.5k) for synthetizing functional cyclic polymer (c-P2). All the synthesis and characterizations of c-PPF4VB6.5k and c-P2 are shown in supporting information in detail (Figures S2-8).
Furthermore, functional cyclic polymer (c-P2) was used as cyclic polymeric backbone to react with polymer double-chain (l-PS-PhN3) via CuAAC reaction for constructing cyclic double-grafted polymer (c-P2-g-Ph-PS). The crude cyclic double-grafted polymer (c-P2-g-Ph-PS) was purified by Prep-SEC and further characterized by NMR and SEC. As shown in Figure 5, 1 H NMR spectrum of c-P2-g-Ph-PS exhibited that the characteristic signals from the methine hydrogen (-CH(Ph)-, f) adjacent to 1,2,3-triazole, the methylene hydrogen (-CH2-, h and p) adjacent to 1,2,3-triazole and the benzylic hydrogen (-CH2-, i) were assigned in the 4.2-5.5 ppm region. It is hard to calculate grafting density of cyclic double-grafted polymer (c-P2-g-Ph-PS) by the integration ratio from 1 H NMR spectrum, but the difference between l-PS-PhN3 and c-P2-g-Ph-PS indicated the successful preparation of cyclic double-grafted polymer. Additionally, according to the SEC curves of c-P2, l-PS-PhN3 and c-P2-g-Ph-PS (Figure 6), the shifts toward high molecular weight field can be observed clearly, demonstrating the successful formation of cyclic double-grafted polymer. The molecular weight of cyclic double-grafted polymer (c-P2-g-Ph-PS) was 30,700 g/mol and the molecular weight still stayed at 1.04.

Conclusions
A novel cyclic topological architecture, a cyclic double-grafted polymer, was successfully constructed using active ester chemistry and click chemistry via a "grafting onto" method. Cyclic backbone (c-P2) was synthesized by reacting propargyl amine with cyclic precursor (c-PPF4VB6.5k) using active ester chemistry as a post-modification approach. Hydroxyl-containing polymer doublechain (l-PS-PhOH) was prepared by reacting azide-functionalized polymer chain (l-PSN3) with 3,5bis(propynyloxy)phenyl methanol, and further azide-modified to generate azide-containing polymer double-chain (l-PS-PhN3) and well characterized by SEC, NMR and MALDI TOF MS. Finally, this cyclic backbone (c-P2) was coupled with azide-containing polymer double-chain (l-PS-PhN3) using CuAAC reaction to successfully construct a novel cyclic double-grafted polymer (c-P2-g-Ph-PS). Notably, this cyclic double-grafted polymer (c-P2-g-Ph-PS) still exhibited a narrow molecular weight distribution. On the basis of our previous work, this research realized diversity and complexity of side chains from cyclic-grafted polymers, which could eventually enrich the topological architecture and provide a new platform for constructing amphiphilic cyclic-brush polymers with amphiphilic polymeric double-chains along the cyclic backbone.

Conclusions
A novel cyclic topological architecture, a cyclic double-grafted polymer, was successfully constructed using active ester chemistry and click chemistry via a "grafting onto" method. Cyclic backbone (c-P2) was synthesized by reacting propargyl amine with cyclic precursor (c-PPF4VB 6.5k ) using active ester chemistry as a post-modification approach. Hydroxyl-containing polymer double-chain (l-PS-PhOH) was prepared by reacting azide-functionalized polymer chain (l-PSN 3 ) with 3,5-bis(propynyloxy)phenyl methanol, and further azide-modified to generate azide-containing polymer double-chain (l-PS-PhN 3 ) and well characterized by SEC, NMR and MALDI TOF MS. Finally, this cyclic backbone (c-P2) was coupled with azide-containing polymer double-chain (l-PS-PhN 3 ) using CuAAC reaction to successfully construct a novel cyclic double-grafted polymer (c-P2-g-Ph-PS). Notably, this cyclic double-grafted polymer (c-P2-g-Ph-PS) still exhibited a narrow molecular weight distribution. On the basis of our previous work, this research realized diversity and complexity of side chains from cyclic-grafted polymers, which could eventually enrich the topological architecture and provide a new platform for constructing amphiphilic cyclic-brush polymers with amphiphilic polymeric double-chains along the cyclic backbone.