Synthesis of Methyldopa–Tin Complexes and Their Applicability as Photostabilizers for the Protection of Polyvinyl Chloride against Photolysis

Polyvinyl chloride (PVC) is a ubiquitous thermoplastic that is produced on an enormous industrial scale to meet growing global demand. PVC has many favorable properties and is used in various applications. However, photodecomposition occurs when harsh conditions, such as high temperatures in the presence of oxygen and moisture, are encountered. Thus, PVC is blended with additives to increase its resistance to deterioration caused by exposure to ultraviolet light. In the current research, five methyldopa–tin complexes were synthesized and characterized. The methyldopa–tin complexes were mixed with PVC at a concentration of 0.5% by weight, and thin films were produced. The capability of the complexes to protect PVC from irradiation was shown by a reduction in the formation of small residues containing alcohols, ketones, and alkenes, as well as in weight loss and in the molecular weight of irradiated polymeric blends. In addition, the use of the new additives significantly reduced the roughness factor of the irradiated films. The additives containing aromatic substituents (phenyl rings) were more effective compared to those comprising aliphatic substituents (butyl and methyl groups). Methyldopa–tin complexes have the ability to absorb radiation, coordinate with polymeric chains, and act as radical, peroxide, and hydrogen chloride scavengers.


Introduction
The demand for plastics is projected to keep increasing due to their versatile properties [1]. Plastics are strong, light, and inexpensive and can be produced in different forms to suit specific applications [2][3][4]. Plastics are, therefore, efficient replacements for some common construction materials, such as steel, wood, and glass. However, a drawback is that plastics suffer from photodegradation when exposed to ultraviolet (UV) light at high temperatures in environments rich in oxygen [5]. Photodegradation leads to deterioration in their mechanical and physical properties resulting in color changes, deformation, and the appearance of cracks [6]. It is, therefore, necessary to enhance the long-term resistance of plastics to photodegradation in hot and humid conditions during manufacturing [7].
Polyvinyl chloride (PVC) is a thermoplastic polymer that ranks in the top three, along with polyethylene and polypropylene, in terms of commercial production [8,9]. PVC is inexpensive, possesses excellent mechanical properties, is available in different forms, and is simple to process and shape. It has many industrial applications, including water piping, insulation, flooring, roofing, packaging, medical instruments, cars, and furniture [10,11]. The downside, however, is that PVC waste is hazardous to humans,

Methods
The microanalytical analysis was performed using a Shimadzu AA-6880 spectrophotometer (Tokyo, Japan). A Shimadzu FT-IR8400S (Tokyo, Japan) was used to record the infrared (IR) spectra (400-4000 cm −1 ). The nuclear magnetic resonance (NMR) spectra were recorded in deuterated dimethyl sulfoxide (DMSO-d 6 ) using a Bruker Advance DRX 400 MHz spectrometer (Zürich, Switzerland). An Ostwald U-Tube Viscometer (Ambala, India) was used to measure the viscosity of solutions. A Bruker XFlash 6-10 (Tokyo, Japan) was used to record energy-dispersive X-ray (EDX) spectra. Before the EDX spectra were recorded, the films were coated with a thin layer (ca. 15 mm) of gold (Au). The optical images were recorded using a Meiji Techno microscope (Tokyo, Japan). The atomic force microscopy (AFM) images were captured on a Veeco instrument (Plainview, NY, USA). The scanning electron microscopy (SEM) images were recorded using an Inspect S50 microscope (FEI Company, Czechia, Czech Republic; 15 kV).

Synthesis of 1 and 2
A mixture of methyldopa (0.21 g, 1 mmol) and Ph 3 SnCl (0.39 g, 1 mmol) or Bu 3 SnCl (0.33 g, 1 mmol) in MeOH (30 mL) was refluxed for 6 h (Scheme 1). The white solid obtained was filtered, washed with MeOH, and dried in a vacuum oven at 45 • C for 6 h to give 1 or 2 in a high yield (Table 1).
India) was used to measure the viscosity of solutions. A Bruker XFlash 6-10 (Tokyo, Japan) was used to record energy-dispersive X-ray (EDX) spectra. Before the EDX spectra were recorded, the films were coated with a thin layer (ca. 15 mm) of gold (Au). The optical images were recorded using a Meiji Techno microscope (Tokyo, Japan). The atomic force microscopy (AFM) images were captured on a Veeco instrument (Plainview, NY, USA). The scanning electron microscopy (SEM) images were recorded using an Inspect S50 microscope (FEI Company, Czechia, Czech Republic; 15 kV).

Synthesis of 1 and 2
A mixture of methyldopa (0.21 g, 1 mmol) and Ph3SnCl (0.39 g, 1 mmol) or Bu3SnCl (0.33 g, 1 mmol) in MeOH (30 mL) was refluxed for 6 h (Scheme 1). The white solid obtained was filtered, washed with MeOH, and dried in a vacuum oven at 45 °C for 6 h to give 1 or 2 in a high yield (Table 1). Scheme 1. Synthesis of 1 and 2.

Preparation of PVC Films
Tetrahydrofuran (THF; 100 mL) was added to a mixture of the appropriate complex (25 mg) and PVC (5.0 g). The mixture was stirred for 2 h at 25 °C to ensure the formation of a homogenous blend. The solution was poured into a glass plate containing holes Scheme 1. Synthesis of 1 and 2.
India) was used to measure the viscosity of solutions. A Bruker XFlash 6-10 (Tokyo, Japan) was used to record energy-dispersive X-ray (EDX) spectra. Before the EDX spectra were recorded, the films were coated with a thin layer (ca. 15 mm) of gold (Au). The optical images were recorded using a Meiji Techno microscope (Tokyo, Japan). The atomic force microscopy (AFM) images were captured on a Veeco instrument (Plainview, NY, USA). The scanning electron microscopy (SEM) images were recorded using an Inspect S50 microscope (FEI Company, Czechia, Czech Republic; 15 kV).

Synthesis of 1 and 2
A mixture of methyldopa (0.21 g, 1 mmol) and Ph3SnCl (0.39 g, 1 mmol) or Bu3SnCl (0.33 g, 1 mmol) in MeOH (30 mL) was refluxed for 6 h (Scheme 1). The white solid obtained was filtered, washed with MeOH, and dried in a vacuum oven at 45 °C for 6 h to give 1 or 2 in a high yield (Table 1). Scheme 1. Synthesis of 1 and 2.

Preparation of PVC Films
Tetrahydrofuran (THF; 100 mL) was added to a mixture of the appropriate complex (25 mg) and PVC (5.0 g). The mixture was stirred for 2 h at 25 °C to ensure the formation of a homogenous blend. The solution was poured into a glass plate containing holes Scheme 2. Synthesis of 3-5.

Preparation of PVC Films
Tetrahydrofuran (THF; 100 mL) was added to a mixture of the appropriate complex (25 mg) and PVC (5.0 g). The mixture was stirred for 2 h at 25 • C to ensure the formation of a homogenous blend. The solution was poured into a glass plate containing holes (thickness = 40 µm). The plate was left in the air at 25 • C for 24 h to dry. A vacuum oven (40 • C; 8 h) was used to ensure the dryness of the films.

Assessment of IR Spectra
The PVC photodecomposition process generates active species (e.g., free radicals), which initiates the elimination of volatiles and bond cleavage [43]. As a result, fragments containing hydroxyl (OH; e.g., alcohols), carbonyl (C=O; e.g., ketones), and carbon−carbon double bonds (C=C; e.g., alkenes) are produced [44,45]. The IR absorption bands corresponding to the OH, C=O, and C=C groups appear at about 3497, 1714, and 1612 cm −1 , respectively. The intensities of the peaks corresponding to these functional groups can be monitored using IR spectroscopy during irradiation. A standard peak that appeared at 1328 cm −1 (C-H bond) was chosen for comparison [46]. Equation (1) was used to calculate the indices (I s ) of the hydroxyl, carbonyl, and polyene groups (I OH , I C=O , and I C=C , respectively) as a function of the absorbances of the functional groups (A s ) and the standard peak (A r ).

Assessment of Weight Loss
PVC irradiation causes weight loss due to the formation of small fragments. Equation (2) can be used to calculate the percentage of PVC weight loss at a time (t) of irradiation [47]. The weight loss was calculated using the weights of the non-irradiated and irradiated PVC films (W 0 and W t , respectively) [47].

Assessment of Average Molecular Weight (M V )
PVC irradiation decreases the Mv as a result of the elimination of small residues, including volatiles. Following irradiation, the blends were dissolved in a solvent (THF), and their intrinsic viscosities [η] were measured. Filtration was necessary to remove any insoluble polymeric residues as a result of cross-linking of PVC. The decreases in Mv are directly proportional to the [η]. The Mark-Houwink relationship (Equation (3)) was used to calculate the Mv of the irradiated blends [48]. The viscosity of the PVC films was determined in THF at 25 • C.

Synthesis of Complexes 1-5
The reaction of methyldopa and trisubstituted tin chlorides in a 1:1 molar proportion gave complexes 1 and 2 (Scheme 1) 83% and 74% yields, respectively ( Table 1). The reaction of methyldopa with disubstituted tin dichlorides in a 2:1 molar proportion gave complexes 3-5 (Scheme 2) in 78-84% yields ( Table 1). The procedures were simple, effective, general, repeatable, and high yielding. In addition, the introduction of substituents (aliphatic and aromatic) in different numbers (di and tri) to the skeleton of the complexes was achievable.
The IR spectra of methyldopa-tin complexes 1-5 showed several absorption bands corresponding to the OH (3217-3279 cm −1 ), Sn-C (511-530 cm −1 ), and Sn-O (443-457 cm −1 ) bonds ( Table 2). The presence of two absorption bands (symmetric and anti-symmetric) for the NH 2 (3473-3409 cm −1 ) was observed in most cases ( Table 2). The vibration signal of the asymmetric carbonyl group has shifted to a higher frequency due to coordination between the tin atom and the carboxylate group (COO − ) oxygen. The difference (∆v) between the asymmetric (asym) and symmetric (sym) vibration frequencies of the COO − group was calculated to be between 237 and 285 cm −1 . A value of ∆v in this range indicates an intermediate state between monodentate and bidentate (anisobidentate) asymmetry [49,50]. The 1 H NMR spectra confirmed the presence of protons from both methyldopa and substituted tin chloride (Table 3). It was noted that the peaks corresponding to the OH and NH 2 protons were broad, possibly due to the partial proton exchange with the deuterated solvent (DMSO-d 6 ). The structures of 1-5 were established further by the 13 C NMR spectral data ( Table 4). The carbon atoms of the carbonyl groups were detected at very low fields in the region of 172.9-174.2 ppm.

The EDX Spectroscopy of PVC Films
Homogenous, colorless thin films were produced by mixing the appropriate methyldopatin complex (0.5% by weight) with PVC. EDX spectroscopy was used to evaluate the elemental composition of the PVC blends [51][52][53]. The EDX spectra of the non-irradiated films exhibited highly abundant absorption bands corresponding to the Cl atoms of PVC, as well as those for N and O from methyldopa and Sn, but in lower proportions ( Figure 1). The peak assignment agrees with those previously reported for PVC blends with other additives containing tin and related elements [31]. 14.

The EDX Spectroscopy of PVC Films
Homogenous, colorless thin films were produced by mixing the appropriate methyldopa-tin complex (0.5% by weight) with PVC. EDX spectroscopy was used to evaluate the elemental composition of the PVC blends [51][52][53]. The EDX spectra of the non-irradiated films exhibited highly abundant absorption bands corresponding to the Cl atoms of PVC, as well as those for N and O from methyldopa and Sn, but in lower proportions ( Figure 1). The peak assignment agrees with those previously reported for PVC blends with other additives containing tin and related elements [31]. The analysis of the EDX mapping indicated that metal complexes were completely dispersed and compatible with the polymeric matrix. The elements from metal complexes (Sn, N, and O) were distributed homogenously in a similar percentage. Following irradiation, the chlorine content dropped due to the elimination of volatiles, including HCl. The blends containing methyldopa-tin complex showed a lesser decrease in the chlorine content compared to the blank film. Clearly, additives 1-5 can be used to reduce PVC photodecomposition. The irradiated PVC film containing complex 1 shows the highest chlorine content.
It was noted that the color of the PVC films became darker as the irradiation time increased (Figure 2). The analysis of the EDX mapping indicated that metal complexes were completely dispersed and compatible with the polymeric matrix. The elements from metal complexes (Sn, N, and O) were distributed homogenously in a similar percentage. Following irradiation, the chlorine content dropped due to the elimination of volatiles, including HCl. The blends containing methyldopa-tin complex showed a lesser decrease in the chlorine content compared to the blank film. Clearly, additives 1-5 can be used to reduce PVC photodecomposition. The irradiated PVC film containing complex 1 shows the highest chlorine content.
It was noted that the color of the PVC films became darker as the irradiation time increased (Figure 2).

IR Spectral Study of PVC Films
Photooxidation of PVC causes the degradation of polymeric chains and produces fragments containing alcohols (OH), ketones (C=O), and polyenes (C=C) [54,55]. The IR spectra of the PVC films were recorded at intervals of 50 h during irradiation. The intensity of the absorption peaks corresponding to the OH (3497 cm −1 ), C=O (1714 cm −1 ), and C=C (1612 cm −1 ) increased due to irradiation ( Figure 3). Therefore, the intensities of these peaks were monitored over time and compared to the C-H bond (1328 cm −1 ), which act as a reference. Irradiation has little effect on the intensity of the C-H bonds since they are highly stable.

IR Spectral Study of PVC Films
Photooxidation of PVC causes the degradation of polymeric chains and produces fragments containing alcohols (OH), ketones (C=O), and polyenes (C=C) [54,55]. The IR spectra of the PVC films were recorded at intervals of 50 h during irradiation. The intensity of the absorption peaks corresponding to the OH (3497 cm −1 ), C=O (1714 cm −1 ), and C=C (1612 cm −1 ) increased due to irradiation ( Figure 3). Therefore, the intensities of these peaks were monitored over time and compared to the C-H bond (1328 cm −1 ), which act as a reference. Irradiation has little effect on the intensity of the C-H bonds since they are highly stable. The indices of the OH (IOH), C=O (IC=O), and C=C (IC=C) groups of the irradiated films were calculated using Equation (1) and presented graphically in Figures 4, 5, and 6, respectively. The IOH, IC=O, and IC=C increased significantly as the irradiation process progressed. The increases were very sharp at the beginning of the irradiation and then continued to increase steadily. The films containing 1-5 showed lower increases in the indices of functional groups compared to pure PVC film. It was clear that methyldopa-tin complexes act as photostabilizers to protect PVC. The additives with the highest aromaticity  The indices of the OH (I OH ), C=O (I C=O ), and C=C (I C=C ) groups of the irradiated films were calculated using Equation (1) and presented graphically in Figures 4-6, respectively. The I OH , I C=O , and I C=C increased significantly as the irradiation process progressed. The increases were very sharp at the beginning of the irradiation and then continued to increase steadily. The films containing 1-5 showed lower increases in the indices of functional groups compared to pure PVC film. It was clear that methyldopa-tin complexes act as photostabilizers to protect PVC. The additives with the highest aromaticity content (i.e., 1 and 3) showed the greatest stabilizing effect. The complexes were stabilized in the following order: triphenyl (1), diphenyl (3), tributyl (2), dibutyl (4), and dimethyl (5). The complexes interact with the active species (e.g., radicals), causing PVC photodegradation and the formation of active intermediates. The stability of the intermediates is highly dependent on the number and type of aromatic moieties present. Aromatic rings tend to reduce the energy of intermediates through resonance. At the end of the irradiation process, the I OH

Weight Loss Investigation of PVC Films
Photodegradation of PVC leads to the elimination of volatiles and causes a reduction in weight [56]. The weight loss percentages of the PVC films due to irradiation were calculated using Equation (2) and are presented graphically in Figure 7. Generally, weight loss increases as radiation time progresses, and it was sharpest in the first 100 h. The greatest weight loss was seen for the pure PVC film, and the use of methyldopa-tin complexes as photostabilizers led to a significant reduction in weight loss in comparison. At the end of irradiation, the weight loss (%) was 1.45% (blank PVC film), 0.53% (PVC + 1), 0.86% (PVC + 2), 0.66% (PVC + 3), 0.96% (PVC + 4), and 1.10% (PVC + 5). Again, highly aromatic complexes (i.e., 1 and 3) led to the highest stabilizing effect compared to the others.

Weight Loss Investigation of PVC Films
Photodegradation of PVC leads to the elimination of volatiles and causes a reduction in weight [56]. The weight loss percentages of the PVC films due to irradiation were calculated using Equation (2) and are presented graphically in Figure 7. Generally, weight loss increases as radiation time progresses, and it was sharpest in the first 100 h. The greatest weight loss was seen for the pure PVC film, and the use of methyldopa-tin complexes as photostabilizers led to a significant reduction in weight loss in comparison. At the end of irradiation, the weight loss (%) was 1.45% (blank PVC film), 0.53% (PVC + 1), 0.86% (PVC + 2), 0.66% (PVC + 3), 0.96% (PVC + 4), and 1.10% (PVC + 5). Again, highly aromatic complexes (i.e., 1 and 3) led to the highest stabilizing effect compared to the others.

Weight Loss Investigation of PVC Films
Photodegradation of PVC leads to the elimination of volatiles and causes a reduction in weight [56]. The weight loss percentages of the PVC films due to irradiation were calculated using Equation (2) and are presented graphically in Figure 7. Generally, weight loss increases as radiation time progresses, and it was sharpest in the first 100 h. The greatest weight loss was seen for the pure PVC film, and the use of methyldopa-tin complexes as photostabilizers led to a significant reduction in weight loss in comparison. At the end of irradiation, the weight loss (%) was 1.45% (blank PVC film), 0.53% (PVC + 1), 0.86% (PVC + 2), 0.66% (PVC + 3), 0.96% (PVC + 4), and 1.10% (PVC + 5). Again, highly aromatic complexes (i.e., 1 and 3) led to the highest stabilizing effect compared to the others.

Viscosity Average Molecular Weight (M V ) Study of PVC Films
Both chain scission and cross-linking are common processes resulting from PVC photodegradation [54,57]. These processes can lead to a reduction in Mv due to the elimination of small fragments. The Mv for the irradiated PVC was calculated using Equation (3) for different irradiation times and is presented graphically in Figure 8. The viscosity of solutions of PVC dropped as irradiation time increased. The drop in Mv is generally massive and is initially particularly noticeable for pure PVC film. It was clear that the additives hindered the decrease in Mv. For pure PVC, the reduction in Mv was 54% after 50 h, 71% after 100 h, 88% after 200 h, and 96% after 300 h of irradiation. In contrast, the irradiated PVC film containing 1 led to a decrease in Mv by 12%, 21%, 41%, and 53% after 50, 100, 200, and 300 h of irradiation, respectively.

Viscosity Average Molecular Weight (MV) Study of PVC Films
Both chain scission and cross-linking are common processes resulting from PVC photodegradation [54,57]. These processes can lead to a reduction in Mv due to the elimination of small fragments. The Mv for the irradiated PVC was calculated using Equation (3) for different irradiation times and is presented graphically in Figure 8. The viscosity of solutions of PVC dropped as irradiation time increased. The drop in Mv is generally massive and is initially particularly noticeable for pure PVC film. It was clear that the additives hindered the decrease in Mv. For pure PVC, the reduction in Mv was 54% after 50 h, 71% after 100 h, 88% after 200 h, and 96% after 300 h of irradiation. In contrast, the irradiated PVC film containing 1 led to a decrease in Mv by 12%, 21%, 41%, and 53% after 50, 100, 200, and 300 h of irradiation, respectively.

Surface Assessment of the Irradiated PVC Films
Various types of microscopy techniques, including optical, SEM, and AFM, have been utilized to assess the damage that occurs on the surface of irradiated PVC films [58][59][60][61]. These tools are good for monitoring the changes caused in the surface of polymers due to irradiation. In addition, the AFM provides clear non-distorted two-and three-dimensional images to provide a clear picture of the roughness factor of the irradiated surface. Generally, non-irradiated PVC films have a high degree of homogeneity, regularity, and smoothness [31]. Optical microscopy (Figure 9), SEM (Figure 10), and AFM ( Figure  11) images revealed more damage (e.g., cracks, spots, darkness, irregularities, and roughness) on the surface of irradiated blank PVC in comparison to those containing 1-5. For example, the SEM images indicated the random distribution and incorporation of metal complexes within the PVC blend. The elimination of HCl and volatiles, bond cleavages, and chain scission are the main causes of surface damage. The results provided additional evidence for the efficacy of methyldopa-tin complexes as PVC photostabilizers. The microscopic images of the surface of the irradiated PVC film containing complex 1 showed the least damage in terms of heterogeneity and roughness. Thus, for example, the roughness factor (Rq) for the irradiated films was 458.6 (blank PVC film), 28

Surface Assessment of the Irradiated PVC Films
Various types of microscopy techniques, including optical, SEM, and AFM, have been utilized to assess the damage that occurs on the surface of irradiated PVC films [58][59][60][61]. These tools are good for monitoring the changes caused in the surface of polymers due to irradiation. In addition, the AFM provides clear non-distorted two-and three-dimensional images to provide a clear picture of the roughness factor of the irradiated surface. Generally, non-irradiated PVC films have a high degree of homogeneity, regularity, and smoothness [31]. Optical microscopy (Figure 9), SEM ( Figure 10), and AFM ( Figure 11) images revealed more damage (e.g., cracks, spots, darkness, irregularities, and roughness) on the surface of irradiated blank PVC in comparison to those containing 1-5. For example, the SEM images indicated the random distribution and incorporation of metal complexes within the PVC blend. The elimination of HCl and volatiles, bond cleavages, and chain scission are the main causes of surface damage. The results provided additional evidence for the efficacy of methyldopa-tin complexes as PVC photostabilizers. The microscopic images of the surface of the irradiated PVC film containing complex 1 showed the least damage in terms of heterogeneity and roughness. Thus, for example, the roughness factor (Rq) for the irradiated films was 458.6 (blank PVC film), 28.3 (PVC + 1), 52.8 (PVC + 2), 40.2 (PVC + 3), 68.3 (PVC + 4), and 70.1 (PVC + 5). It was clear that complex 1 led to a reduction in Rq by 16.2-fold, which is remarkable.

Suggested Mechanisms for Photostabilization
Methyldopa-tin complexes act as good PVC photostabilizers and reduce the damage due to photodegradation. The additives act as absorbers of harmful UV light, thereby protecting the polymer [73]. In addition, these stabilizers deactivate reactive species produced during photodegradation [74]. For example, the tin atom (a Lewis acid) in complex 1 can act as an acidic center and is, therefore, capable of scavenging the HCl (i.e., a secondary photostabilizer) released during photodegradation (Scheme 3). Moreover, the additives (e.g., complex 1) act as decomposers for the hydroperoxides (PO2H; Scheme 3) produced by the photodegradation of PVC [75]. The reactive species (e.g., peroxide radicals, POO • ) produced during PVC photodegradation can form stable intermediates with methyldopa-tin complexes (e.g., complex 1) Figure 11. AFM images of the irradiated PVC films.

Suggested Mechanisms for Photostabilization
Methyldopa-tin complexes act as good PVC photostabilizers and reduce the damage due to photodegradation. The additives act as absorbers of harmful UV light, thereby protecting the polymer [73]. In addition, these stabilizers deactivate reactive species produced during photodegradation [74]. For example, the tin atom (a Lewis acid) in complex 1 can act as an acidic center and is, therefore, capable of scavenging the HCl (i.e., a secondary photostabilizer) released during photodegradation (Scheme 3). Moreover, the additives (e.g., complex 1) act as decomposers for the hydroperoxides (PO2H; Scheme 3) produced by the photodegradation of PVC [75]. The reactive species (e.g., peroxide radicals, POO • ) produced during PVC photodegradation can form stable intermediates with methyldopa-tin complexes (e.g., complex 1) Scheme 3. Complex 1 acts as an HCl and PO 2 H decomposer.
The reactive species (e.g., peroxide radicals, POO • ) produced during PVC photodegradation can form stable intermediates with methyldopa-tin complexes (e.g., complex 1) as a result of the resonance within the aryl ring [76]. Thus complex 1 acts as a radical scavenger and leads to a significant stabilization of the PVC (Scheme 4). as a result of the resonance within the aryl ring [76]. Thus complex 1 acts as a radical scavenger and leads to a significant stabilization of the PVC (Scheme 4).

Scheme 4.
The function of complex 1 as a radical scavenger.
Finally, the interaction between the polarized atoms within the PVC (i.e., C-Cl bonds) and the heteroatoms (oxygen and nitrogen) within the organic motif in complexes could stabilize the polymer ( Figure 12). However, in large molecules, such as blended PVC, steric hindrance may interfere with the effectiveness of the process.

Conclusions
A simple procedure has been used to obtain, in high yield, five methyldopa-tin complexes containing aromatic and aliphatic substituents. Polyvinyl chloride thin films can be damaged by ultraviolet irradiation, and the complexes have been assessed as additives for stabilization against irradiation. The use of a small concentration of methyldopa-tin complexes as additives significantly improved the photostability of polyvinyl chloride. The methyldopa-tin complexes coordinate well with the polymeric chains and act by absorbing the radiation. In addition, they act as radical, peroxide, and hydrogen chloride scavengers. The most effective additives were the ones containing aromatic residues. The methyldopa-tin complexes clearly have the potential for application as stabilizers for polyvinyl chloride, but additional research is required to evaluate any health risks associated with these additives.   Finally, the interaction between the polarized atoms within the PVC (i.e., C-Cl bonds) and the heteroatoms (oxygen and nitrogen) within the organic motif in complexes could stabilize the polymer ( Figure 12). However, in large molecules, such as blended PVC, steric hindrance may interfere with the effectiveness of the process. as a result of the resonance within the aryl ring [76]. Thus complex 1 acts as a radical scavenger and leads to a significant stabilization of the PVC (Scheme 4).

Scheme 4.
The function of complex 1 as a radical scavenger.
Finally, the interaction between the polarized atoms within the PVC (i.e., C-Cl bonds) and the heteroatoms (oxygen and nitrogen) within the organic motif in complexes could stabilize the polymer ( Figure 12). However, in large molecules, such as blended PVC, steric hindrance may interfere with the effectiveness of the process.

Conclusions
A simple procedure has been used to obtain, in high yield, five methyldopa-tin complexes containing aromatic and aliphatic substituents. Polyvinyl chloride thin films can be damaged by ultraviolet irradiation, and the complexes have been assessed as additives for stabilization against irradiation. The use of a small concentration of methyldopa-tin complexes as additives significantly improved the photostability of polyvinyl chloride. The methyldopa-tin complexes coordinate well with the polymeric chains and act by absorbing the radiation. In addition, they act as radical, peroxide, and hydrogen chloride scavengers. The most effective additives were the ones containing aromatic residues. The methyldopa-tin complexes clearly have the potential for application as stabilizers for polyvinyl chloride, but additional research is required to evaluate any health risks associated with these additives.

Conclusions
A simple procedure has been used to obtain, in high yield, five methyldopa-tin complexes containing aromatic and aliphatic substituents. Polyvinyl chloride thin films can be damaged by ultraviolet irradiation, and the complexes have been assessed as additives for stabilization against irradiation. The use of a small concentration of methyldopa-tin complexes as additives significantly improved the photostability of polyvinyl chloride. The methyldopa-tin complexes coordinate well with the polymeric chains and act by absorbing the radiation. In addition, they act as radical, peroxide, and hydrogen chloride scavengers. The most effective additives were the ones containing aromatic residues. The methyldopa-tin complexes clearly have the potential for application as stabilizers for polyvinyl chloride, but additional research is required to evaluate any health risks associated with these additives.