Synthesis and characterization of poly(β-alanine-b-vinyl benzyl-g-vinyl chloride) block-graft copolymers by using reversible addition-fragmentation chain transfer polymerization and 'click' chemistry

Synthesis and characterization of poly(β-alanine-b-vinyl benzyl-g-vinyl chloride) [PBA-b-(PVB-g-PVC)] block-graft copolymer was done by reversible addition-fragmentation chain transfer polymerization and 'click' chemistry. For this, poly-β-alanine with a vinyl end group was synthesized by hydrogen transfer polymerization of acrylamide. Bromine-terminated poly-β-alanine (PBA-Br) was obtained by treating the synthesized poly-β-alanine with hydrogen bromide. Poly-β-alanine ethyl xanthate (macro-RAFT agent) was synthesized using PBA-Br and potassium ethyl xanthate. Poly(β-alanine-b-vinyl benzyl chloride) [poly(BA-b-VBC)] block copolymer was obtained by reversible addition-fragmentation chain transfer polymerization of macro-RAFT agent with vinyl benzyl chloride. Azido-terminated poly(β-alanine-b-vinyl benzyl chloride) [poly(BA-b-VBC)-N3] was synthesized by the reaction of poly(BA-b-VBC) with sodium azide. Alkyne-terminated polyvinyl chloride (PVC-propargyl) was obtained by treating polyvinyl chloride with propargyl alcohol. Finally, the synthesis of PBA-b-(PVB-g-PVC) block-graft copolymer was carried out by the 'click' chemistry method of poly(BA-b-VBC)-N3 and PVC-propargyl. The products were characterized by various spectroscopic and thermal methods.


Introduction
Macromolecular research places a high priority on the modification of the polymers that are produced as a result of successful polymerization processes. In order to meet the needs of new applications, it is now especially crucial to replace existing polymers as well as create new types of plastic materials [1,2]. One technique for enhancing the characteristics of polymers is copolymerization. In the same polymer molecule, the combination of distinct monomers, each of which possesses distinct physical and chemical properties, results in the synthesis of new materials at various rates [3][4][5][6][7][8][9][10][11][12][13][14]. There are many papers on the synthesis of polymer brushes in the literature [15][16][17][18].
Because there are many monomers that can be used with controlled/living radical-polymerization (C/LRP) techniques and because they are more tolerant of experimental conditions than living ionic polymerization, they have recently become widely used in the synthesis of complex macromolecules [19]. For the macromolecular synthesis of the vast majority of well-defined polymers, reversible addition-fragmentation chain transfer polymerization technique (RAFT) is one of the most widely used C/LRP methods. Because of the method's compatibility with a variety of monomers and reaction circumstances, it is regarded as being flexible [20][21][22][23][24][25][26][27]. Additionally, research on the bioactivity properties of nylon-3 (poly-β-alanine) copolymers and derivatives has grown. Numerous poly-β-alanine copolymers and derivatives have been reported in this research to have antibacterial, antifungal, and cell adhesion properties [28,29]. A well-known technique for creating poly-β-alanine is hydrogen transfer polymerization (HTP). The promising process of HTP can introduce functional groups into the primary chain of macromolecules [30][31][32]. However, it is crucial to add a functional group to the monomer's amide nitrogen. These days, 'click' chemistry technique known as copper-catalyzed azide-alkynic cycloactivity (CuAAC) reaction has arisen as a special way to make graft and block copolymers. The production of new nano-structured polymers has recently benefited from the use of 'click' procedures, which have also cemented their positions in a wide range of scientific disciplines. Simple product isolations, strong functional group toleration, greater regional selectivity, the absence of by-products, and light and straightforward reaction conditions, in that order, are indicators of the primary properties of 'click' chemistry .
This paper demonstrates the production of the poly(βalanine-b-vinyl benzyl-g-vinyl chloride) [PBA-b-(PVBg-PVC)] block-graft copolymer using 'click' chemistry, RAFT, and HTP. First, acrylamide underwent HTP to create poly-β-alanine with a vinyl end group. The synthesized poly-β-alanine was treated with hydrogen bromide to produce bromine-terminated poly-β-alanine (PBA-Br). Second, PBA-Br and potassium ethyl xanthate were used to create poly-β-alanine ethyl xanthate, a macro-RAFT agent. Then, by polymerizing the macro-RAFT agent with vinyl benzyl chloride by RAFT, poly(β-alanine-b-vinyl benzyl chloride) [poly(BA-b-VBC)] block copolymer was created. By reacting poly(BA-b-VBC) block copolymer with sodium azide, poly(β-alanine-b-vinyl benzyl chloride) with an azido-terminus [poly(BA-b-VBC)-N 3 ] was created. Thirdly, as demonstrated in the literature, PVC-propargyl was created via the interaction of propargyl alcohol with PVC. Last but not least, 'click' chemistry was employed to create PBA-b-(PVB-g-PVC) block-graft copolymer using poly(BA-b-VBC)-N 3 and PVC-propargyl. This work serves as an illustration of a particular sort of combination reaction, ranging from HTP and RAFT to 'click' chemistry. For instance, HTP to RAFT [54] and RAFT to 'click' chemistry [50] are two examples. These are straightforward and effective techniques for making block-graft copolymers. This viewpoint can be easily illustrated by the current investigation. Through the synergistic combination of polyβ-alanine, vinyl benzyl chloride, and polyvinyl chloride, this study offers unique, well-characterized compounds with potential industrial applications. The copolymer's extensive characterization was completed by 1 H-NMR, GPC, FT-IR, SEM, TGA and DSC.

Synthesis of poly-β-alanine with a vinyl end group by hydrogen transfer polymerization
Poly-β-alanine was synthesized by hydrogen transfer polymerization as the literature [28,29]. 7.10 g (0.1 mol) acrylamide (≥ 98.0% Sigma-Aldrich), 0.288 g (0.003 mol) sodium tertiary butoxide (97% Aldrich) (initiator), and 0.01 g hydroquinone (≥ 99% Merck) (radical polymerization inhibitor) were placed in a 25 mL flask. After tightly closed with a rubber septum, argon gas was passed through it. It was stirred by heating in an oil bath at 85 °C. A few minutes after the liquefaction of acrylamide was complete, viscous and then solidification took place. The cooled reaction mixture was extracted with excess methanol (≥ 99.7% Sigma-Aldrich). The insoluble part in methanol was separated by filtration and dried in a vacuum oven.

Synthesis of bromine-terminated poly-β-alanine (PBA-Br)
10% (wt/wt) solution of poly-β-alanine was prepared using formic acid (98-100% Sigma-Aldrich) as solvent. By adding a few drops of 33% wt. hydrogen bromide in acetic acid (33% wt. in acetic acid, Sigma-Aldrich) to the solution in a 50 ml glass flask, continuous mixing was carried out for 12 h under argon gas at room temperature. Excess hydrogen bromide was removed by washing with distilled water. PBA-Br was precipitated in anhydrous acetone, then filtered and left to dry in a vacuum oven at room temperature.

Synthesis of poly-β-alanine ethyl xanthate (Macro-RAFT agent)
1.500 g of PBA-Br, 4.506 g of potassium ethyl xanthate (96% Aldrich), and 15 mL of formic acid were placed in a 50 ml flask, respectively, and mixed under argon gas at 35 °C for 72 h. At the end of the period, the content of the flask was filtered and the unreacted potassium ethyl xanthate was removed. The resulting filtrate was then precipitated in excess petroleum ether (Riedel-de Haen). After decanting, the product was dried in an oven at room temperature.

Synthesis of poly(β-alanine-b-vinyl benzyl chloride) [poly(BA-b-VBC)] block copolymer by RAFT polymerization
In accordance with the RAFT mechanism, 1.540 g of vinyl benzyl chloride (≥ 90% Aldrich), a trace amount of 2,2'-azobisisobutyronitrile (99% Aldrich), 0.153 g of macro-RAFT agent, 2 mL of formic acid, 1 mL of N,N-dimethylformamide (99.9% Isolab Chemicals) were placed in a flask to obtain a homogeneous solution. Then argon gas was passed through the solution. After the flask was closed, it was dipped in an oil bath at 90 °C for about 2 h and then at 70 °C for 13 h to ensure the polymerization. At the end of the polymerization, the content of the flask was poured into excess petroleum ether, and poly(BA-b-VBC) block copolymer was precipitated. After decanting, the block copolymer was allowed to dry in an oven at 30 °C for three days.

Synthesis of azido-terminated poly(β-alanine-b-vinyl benzyl chloride) [poly(BA-b-VBC)-N 3 ]
0.5 g of poly(BA-b-VBC) block copolymer and 1.057 g of sodium azide (Sigma-Aldrich) were added to 5 mL of formic acid/N,N-dimethylformamide solvent mixture at a ratio of 1/1 in the glass balloon. The balloon was treated at 70 °C for 20 h in an oil bath on a magnetic stirrer. At the end of this period, the contents of the balloon were filtered with filter paper, precipitated in excess petroleum ether, and then decanted. The product was dried in an oven at 25 °C for three days.

Synthesis of alkyne-terminated polyvinyl chloride (PVC-propargyl)
PVC-propargyl was synthesized as the literature [55]. After 10 ml dichloromethane (≥ 99.8% Merck) was placed in a 25 ml flask, 0.504 g polyvinyl chloride (Sigma-Aldrich, approximately Mn = 22,000 g/mol,) and 3-4 drops of triethyl amine (≥ 99% Sigma-Aldrich) were added in the flask, respectively. After dissolving for two hours at room temperature, 2.012 g propargyl alcohol (99% Aldrich) was slowly added with the help of a drip funnel and stirred in an ice bath under argon gas for 40 min. After the flask content was mixed in an oil bath at 50 °C for 30 h, the product was precipitated in methanol and taken to the refrigerator. After decanting, the product was dried in an oven at 40 °C for two days.

Synthesis of poly(β-alanine-b-vinyl benzyl-g-vinyl chloride) [PBA-b-(PVB-g-PVC)] block-graft copolymer by 'click' chemistry
As shown in Table 1, PVC-propargyl, poly(BA-b-VBC)-N 3 , N,N-dimethylformamide, and N,N,N',N',N'' pentamethyldiethylenetriamine (99% Aldrich) were put into a glass tube. After dissolution, copper bromide (98% Sigma-Aldrich) was added. The tube content was passed through argon gas. The tube was kept in an oil bath at 35 °C for 72 h. After this time, the tube contents were precipitated in excess methanol. Then, after decantation, PBA-b-(PVB-g-PVC) block-graft copolymer was dried in an oven at 40 °C for two days.

Synthesis of poly-β-alanine with a vinyl end group and its bromination
Poly-β-alanine with a vinyl end group was synthesized via hydrogen transfer radical polymerization method using sodium tertiary butoxide as an alkaline catalyst and acrylamide as a monomer [28,29]. The synthesized polyβ-alanine was brominated by using hydrogen bromide. The reaction schemes for poly-β-alanine with a vinyl end group and for PBA-Br were shown in Scheme 1. The PBA-Br was obtained in a yield of 89.12% wt. According to the FT-IR (Alpha-P Bruker, ATR) spectrum of poly-β-alanine in Fig. 1; The peaks of the 3283 cm −1 -NH group stretching, 3080-2939 cm −1 aliphatic -CH 2 , 1631 cm −1 -C = O, and 1535 cm −1 -NH bending were seen. According to the FT-IR spectrum of PBA-Br in Fig. 1; The peaks of the 3286 cm −1 -NH stretching, 3070-2900 cm −1 aliphatic -CH 2 , 1632 cm −1 -C = O, 1535 cm −1 -NH bending, and 788 cm −1 -Br were seen. The peak of 788 cm −1 -Br in the FT-IR spectrum was evidence of the bromination of the poly-β-alanine. According to the 1 H-NMR (Bruker Ultra Shield Plus, ultra-long hold time 400) spectrum of the PBA-Br shown in Fig. 2; The peaks of equal intensity at 2.59 and 3.55 ppm indicate methylene protons adjacent to the carbonyl and amide groups, respectively. The single peak seen at 7.77 ppm was the peak of the amine group protons. Since PBA-Br could dissolve in formic acid, the 1 H-NMR analysis was carried out in the formic acid/ CDCl 3 solvent mixture. Therefore, formic acid peaks were observed above 8.0 ppm in the spectrum.

Synthesis of macro-RAFT agent
Macro-RAFT agent was synthesized from the reaction of PBA-Br and potassium ethyl xanthate. The synthesis route of the macro-RAFT agent was given in Scheme 1. The yield of the macro-RAFT agent was 98% wt. According to the FT-IR spectrum of the macro-RAFT agent in Fig. 3; The peaks of the 3284 cm −1 -NH, 3070-2873 cm −1 aliphatic -CH 2 , 1633 cm −1 -C = O, and 1046 cm −1 -SC peaks were observed.

Synthesis of poly(BA-b-VBC) block copolymer
Poly(BA-b-VBC) was synthesized from the reaction of macro-RAFT agent and vinyl benzyl chloride via RAFT  were observed. In the FT-IR spectrum, the -N 3 group peak seen at 2096 cm −1 was proof that the azide reaction was occurred.

Synthesis of PVC-propargyl
PVC-propargyl was synthesized using polyvinyl chloride and propargyl alcohol [55]. PVC-propargyl was obtained in 76.85% wt. yield. The synthesis outline of PVC-propargyl was given in Scheme 3. According to the FT-IR spectrum of PVC-propargyl in Fig. 5; The peaks of 2966-2908 cm −1 aliphatic -CH 2 and -CH, 1984 cm −1 propargyl group, and 1095 cm −1 -OC were observed. In the 1 H-NMR spectrum of PVC-propargyl in Fig. 6, the -OCH 2 and -OCH protons at 4.48 ppm, and propargyl group protons at 2.30 ppm were seen.

Synthesis of PBA-b-(PVB-g-PVC) block-graft copolymer by 'click' chemistry
In the synthesis of PBA-b-(PVB-g-PVC) block-graft copolymer, the yield was found to be 77.99 and 76.40% wt. The synthesis pathway of PBA-b-(PVB-g-PVC) blockgraft copolymer was given in Scheme 3. According to the FT-IR spectrum of PBA-b-(PVB-g-PVC) blockgraft copolymer in Fig. 5; The peaks of 3030 cm −1 -NH, 2916-2716 cm −1 aliphatic -CH 3 , -CH 2 , -CH, 1669 cm −1 -C = O, 1464 cm −1 aromatic -C = C, and 1090 cm −1 -OC were observed. In the 1 H-NMR spectrum of PBA-b-(PVB-g-PVC) block-graft copolymer in Fig. 7; the -CH 2 protons of polyvinyl benzyl group at 1.16 ppm, the -CH protons of polyvinyl benzyl group at 1.45 ppm, the -CH 2 protons of polyvinyl chloride residue at 1.14 ppm, the  Fig. 9. The weight loss of PBA-b-(PVB-g-PVC) blockgraft copolymer was determined using TGA instrument (SETARAM LABSyS EVO, N 2 atmosphere, rate of 10 °C/min). The percent weight loss of PBA-b-(PVB-g-PVC) block-graft copolymer was observed to be about 75% wt. as shown in Fig. 10. The block-graft copolymer has two weight loss steps in the range of about 250 °C and 550 °C that can be attributed to the decomposition temperatures (Td) of the PVC and poly(BA-b-VBC) blocks, respectively. The surface morphologies of PBAb-(PVB-g-PVC) block-graft copolymers were determined with SEM instrument (ZEISS EVO LS10, coated with a layer of gold on the copolymer surface) as shown in Fig. 11. In the SEM images, the fractured structures were observed on the surface of the block-graft copolymer. Also, the channels were available in the images.   Table 1)  Table 1)  Table 1)  Table 1)

Conclusions
PBA-b-(PVB-g-PVC) block-graft copolymer was synthesized by using reversible addition-fragmentation chain transfer polymerization and 'click' chemistry. The products were characterized by various spectroscopic and thermal methods such as 1 H-NMR, FT-IR, GPC, DSC, TGA, and SEM. These characteristic methods proved that PBA-b-(PVB-g-PVC) block-graft copolymer was synthesized.