FTIR, Weight, and Surface Morphology of Poly(vinyl chloride) Doped with Tin Complexes Containing Aromatic and Heterocyclic Moieties

Poly(vinyl chloride) (PVC) is an important synthetic plastic that is produced in large quantities (millions of tons) annually. Additives to PVC are necessary to allow its use in many applications, particularly in harsh conditions. In regard to this, investigation of the synthesis of trimethoprim–tin complexes and their use as PVC additives is reported. Trimethoprim–tin complexes were obtained from the reaction of trimethoprim and tin chlorides using simple procedures. Trimethoprim–tin complexes (0.5% by weight) were added to PVC to produce homogenous mixtures and thin films were made. The effect of ultraviolet irradiation on the surface and properties of the PVC films was investigated. The level of both photodecomposition and photo-oxidation of PVC films containing trimethoprim–tin complexes was observed to be lower than for the blank film. The effectiveness of tin complexes as PVC photostabilizers reflects the aromaticity of the additives. The complex containing three phenyl groups attached to the tin cation showed the most stabilizing effect on PVC. The complex containing two phenyl groups was next, with the one containing butyl substituents resulting in the least stabilization of PVC. A number of mechanisms have been proposed to explain the role of the synthesized complexes in PVC photostabilization.


Introduction
Plastics are synthetic polymers that can be molded into different shapes. Plastics are cheap, durable, lightweight, and can be used in a myriad of applications. They are effective replacements for glass, wood, and steel [1][2][3]. Plastics are mainly produced from fossil feedstocks relying on technology that was developed a long time ago. Physical properties of plastics such as strength, color, density, molecular weight, and rigidity can be manipulated during the manufacturing processes. Polyethylene, polypropylene, poly(vinyl chloride) (PVC), polyethylene terephthalate, and polystyrene are the most common synthetic plastics. In Europe, the plastics industry contributed to 1.5 million jobs and generated EUR 350 billion in 2019 [4]. China is the main producer of plastics and contributes 51% of the from Merck (Gillingham, UK). PVC with an average molecular weight of approximately 233,000 and a polymerization degree of 800 was supplied by Petkim Petrokimya (Istanbul, Turkey). The amounts of carbon, hydrogen, nitrogen, and tin within the complexes were determined using a Vario EL III elemental analyzer (Elementar Americas, Ronkonkoma, NY, USA). An FTIR 8400 Shimadzu spectrophotometer (Shimadzu, Tokyo, Japan) was used to record the FTIR spectra. A Bruker DRX-500 NMR spectrometer (Zürich, Switzerland) was used to record the 1 H and 13 C NMR at 500 and 125 MHz, respectively. The NMR spectra were recorded in deuterated dimethyl sulfoxide at 25 °C. The chemical shifts (ppm) were measured relative to tetramethylsilane as a standard. An accelerated weathermeter QUV tester (Q-Panel Company; Homestead, FL, USA) was used at 25 °C for the UV irradiation of films. The optical images of the surface of samples were captured using a Meiji Techno microscope (Tokyo, Japan). The field emission scanning electron microscopy (FESEM) images were obtained using a SIGMA 500 VP microscope (ZEISS Microscopy, Jena, Germany) in which the accelerating voltage was 15 kV at 25 °C. A Hitachi H-7100 electron microscope (London, UK) was used to record the transmission electron micrograph (TEM) images and a Veeco instrument (Plainview, NY, USA) was used to capture the atomic force microscopy (AFM) images.

Preparation of PVC Films
Complex 1, 2, or 3 (25 mg) was added to a magnetically stirred suspension of PVC (5.0 g) in tetrahydrofuran (THF; 100 mL) and the mixture was stirred for 2 h. The homogenous solution obtained was poured onto a clean and dry glass plate (4 × 4 cm 2 ) containing 15 holes with a thickness of approximately 40 µ. The solvent was removed by leaving the plate at 25 • C for 18 h. The films produced were dried in a vacuum oven (25 • C) for 18 h.

Irradiation of PVC Films
A UV light (λ max = 365 nm; light intensity = 6.2 × 10 −9 ein dm −3 s −1 ) was used for the irradiation of the PVC samples. The sample was rotated regularly to ensure that it was exposed to the light evenly from all sides. The films were irradiated for a time that ranged from 0 to 300 h. The effect the irradiation on the PVC properties was recorded every 50 h.

Measurement of PVC Functional Group Indices
PVC photodegradation leads to the formation of residues that contain C=O (carbonyl) and C=C (unsaturated carbons) groups. The growth of the intensity of the bands for both C=O and C=C groups generated by photoirradiation can be monitored using FTIR spectroscopy and compared with that for the C-H bond as a reference [40]. The indices of C=O and C=C groups (Is) were calculated using Equation (1), where A s and A r are the absorbance of the functional and reference groups, respectively.

Measurement of PVC Weight Loss
PVC photodegradation leads to weight loss due to the elimination of volatile molecules. The weight loss (%) can be calculated from the weight of the sample pre-(W pre ) and postirradiation (W post ) using Equation (2) [41].

Synthesis of Trimethoprim-tin Complexes 1-3
The reaction of trimethoprim and Ph 3 SnCl in a 1:1 molar ratio in MeOH under reflux gave complex 1 (Scheme 1) in 75% yield (Table 1). Similarly, the reaction of trimethoprim and Ph 2 SnCl 2 or n-Bu 2 SnCl 2 in a 2:1 molar ratio under similar reaction conditions gave 2 or 3 (Scheme 2) in 79 or 82% yield ( Table 1). The purity of the synthesized complexes was confirmed by elemental analyses ( Table 1).
The absorption bands due to vibration of the NH 2 , NH, and C=N groups appeared in the 3406-3408 cm −1 , 3163-3180 cm −1 , and 1680-1682 cm −1 regions, respectively (Table 2) in the FTIR spectra of 1-3 ( Figures S1-S3). In addition, the bands that appeared in the  The 1 H NMR spectra of 1-3 indicated the presence of the pyrimidine proton that appeared as a singlet at low magnetic field. The NH proton appeared as an exchangeable singlet at high magnetic field due to the shielding effect of the Sn atom. The spectra showed all other protons ( Table 3). The 13 C NMR spectra of trimethoprim-tin complexes showed the presence of carbons derived from both the trimethoprim unit and substituents (phenyl and butyl groups) attached to the Sn atom (Table 3). Representative NMR spectra are shown in Figures S4 and S5. Table 3. NMR spectral data of trimethoprim-tin complexes 1-3.

Investigation of PVC Photodegradation through FTIR Spectrophotometry
PVC photodegradation takes place mainly due to the elimination of free radicals (e.g., Cl . ) and the release of HCl. This process of dehydrochlorination is responsible for the formation of short polymeric chains containing C=O and -CH=CH-moieties [42][43][44]. As irradiation of PVC progresses, the formation of the C=O and -CH=CH-becomes noticeably significant. Therefore, FTIR spectroscopy was used to examine the growth of the absorption bands corresponding to the C=O (1722 cm −1 ) and -CH=CH-(1602 cm −1 ) groups during the irradiation process ( Figure 1). The intensities of these absorption bands increased substantially during irradiation compared with that for the C-H bonds (CH 2 ; 1328 cm −1 ) of the PVC chains. The UV light does not affect the intensity of the absorption band corresponding to the reference (C-H bonds).
Polymers 2021, 13, x FOR PEER REVIEW 6 of 16 Equation (1) has been used to calculate the indices of both C=O (IC=O) and C=C (IC=C) groups for the pure PVC film and the blends containing 1-3 after every 50 h of irradiation. The changes in IC=O and IC=C for the PVC during irradiation are shown in Figures 2 and 3, respectively. It was clear that the changes in the IC=O and IC=C were significantly higher and sharper in the case of the pure PVC film. Complexes 1-3 evidently reduced the formation of small fragments containing C=O and C=C groups and hence acted as PVC photostabilizers. For the pure PVC film, the IC=C was 0.53 after 50 h, 0.80 after 100 h, 1.01 after 200 h, and 1.51 after 300 h of irradiation. The PVC film containing 1 showed the lowest IC=O at 0.26 after 50 h, 0.40 after 100 h, 0.58 after 200 h, and 0.71 after 300 h of irradiation. The superior ability of complex 1 to act as a UV absorber is ascribed to the three phenyl group substituents. A similar pattern is observed for the changes in IC=C.

Investigation of PVC Photodegradation through Weight Loss
PVC photo-oxidation and photodecomposition lead to cross-linking and bond breaking resulting in the formation of volatiles such as HCl, and the production of small polymeric fragments. The outcome is weight loss by the PVC sample which increases with irradiation time [44]. The damage induced in the PVC chains can be assessed by measuring weight during irradiation and using Equation (2) to calculate the percentage weight loss. The percentage weight losses for the samples as a function of irradiation time (0-300 h) are shown in Figure 4. Relative to the other samples, a sharp weight loss was observed in the first 50 h for the pure PVC sample before settling to a rate fairly similar to the other samples. The weight loss (%) was 0.21% after 50 h and 0.70% after 300 h of irradiation for the pure PVC. The trimethoprim-tin complexes led to a significant drop in the weight loss of PVC. Complex 1 led to the most significant reduction followed by 2 and then by 3. The blend containing 1 showed a weight loss (%) of 0.05% after 50 h and 0.32% at the end of irradiation. These results are in agreement with those obtained from FTIR and functional group indices.

Investigation of PVC Photodegradation through Weight Loss
PVC photo-oxidation and photodecomposition lead to cross-linking and bond breaking resulting in the formation of volatiles such as HCl, and the production of small polymeric fragments. The outcome is weight loss by the PVC sample which increases with irradiation time [44]. The damage induced in the PVC chains can be assessed by measuring weight during irradiation and using Equation (2) to calculate the percentage weight loss. The percentage weight losses for the samples as a function of irradiation time (0-300 h) are shown in Figure 4. Relative to the other samples, a sharp weight loss was observed in the first 50 h for the pure PVC sample before settling to a rate fairly similar to the other samples. The weight loss (%) was 0.21% after 50 h and 0.70% after 300 h of irradiation for the pure PVC. The trimethoprim-tin complexes led to a significant drop in the weight loss of PVC. Complex 1 led to the most significant reduction followed by 2 and then by 3. The blend containing 1 showed a weight loss (%) of 0.05% after 50 h and 0.32% at the end of irradiation. These results are in agreement with those obtained from FTIR and functional group indices.

Investigation of PVC Photodegradation through Weight Loss
PVC photo-oxidation and photodecomposition lead to cross-linking and bond breaking resulting in the formation of volatiles such as HCl, and the production of small polymeric fragments. The outcome is weight loss by the PVC sample which increases with irradiation time [44]. The damage induced in the PVC chains can be assessed by measuring weight during irradiation and using Equation (2) to calculate the percentage weight loss. The percentage weight losses for the samples as a function of irradiation time (0-300 h) are shown in Figure 4. Relative to the other samples, a sharp weight loss was observed in the first 50 h for the pure PVC sample before settling to a rate fairly similar to the other samples. The weight loss (%) was 0.21% after 50 h and 0.70% after 300 h of irradiation for the pure PVC. The trimethoprim-tin complexes led to a significant drop in the weight loss of PVC. Complex 1 led to the most significant reduction followed by 2 and then by 3. The blend containing 1 showed a weight loss (%) of 0.05% after 50 h and 0.32% at the end of

Investigation of PVC Photodegradation through Surface Morphology
Changes on the surface of the PVC film can give an indication of damage during irradiation. With damage, microscope images of the surface of PVC films recorded postirradiation would show irregularities when compared with pre-irradiation images. The images for the samples are shown in Figure 5.

Investigation of PVC Photodegradation through Surface Morphology
Changes on the surface of the PVC film can give an indication of damage during irradiation. With damage, microscope images of the surface of PVC films recorded postirradiation would show irregularities when compared with pre-irradiation images. The images for the samples are shown in Figure 5.
In general, the number of cracks, dark spots, and grooves is more noticeable postirradiation. The appearance of the dark spots is mainly due to the release of HCl because of PVC decomposition [45]. The dark spots were most noticeable on the surface of the pure PVC film. The surface of the blend containing 2 was almost smooth with no groves or crakes. The surface of the blend containing 3 showed some dark spots and irregularities. The surface of the blend containing 1 showed the presence of dark areas which appeared to be rod-shaped. It is not clear why such rodlike shapes were only seen in the case of 1. However, it could have something to do with the rate of the elimination of volatiles during the photodegradation process. In an attempt to understand the changes on the surface of the PVC blend containing 1, it was decided to use an additional technique, namely FESEM.
The FESEM images gave clearer less distorted and high-resolution pictures of the PVC surface. The images provided information about the homogeneity of the surface as well as the particles size and shape for the irradiated materials [46]. Figure 6 shows that the surface of irradiated PVC films contains lumps, groves, and noticeable irregularities attributed to bond cleavage during photoirradiation and photodegradation [47]. The size of the particles varies; 42.07-132.9 nm for 1, 69.11-164.8 nm for 2, and 42.07-152.3 nm for 3. Again, the surface of the blend containing 1 shows cracks and rodlike particles, which is consistent with the microscope image ( Figure 6). It has been reported that PVC containing nickel chloride and a dithiazole Schiff base or tin-naphthalene sulfonic acid complex led to the formation of honeycomb-like structures [31,38]. In addition, PVC blended with a Schiff base containing a melamine unit showed an ice-rock structure [36]. Such observations could be attributed to the slow rate of HCl elimination from the PVC chains during photoirradiation. Figure 7 shows the FESEM images of the particles within the PVC + 1 film post-irradiation at lower magnification.  In general, the number of cracks, dark spots, and grooves is more noticeable p irradiation. The appearance of the dark spots is mainly due to the release of HCl beca of PVC decomposition [45]. The dark spots were most noticeable on the surface of the p PVC film. The surface of the blend containing 2 was almost smooth with no grove crakes. The surface of the blend containing 3 showed some dark spots and irregulari AFM has also been used to analyze the surface of the irradiated PVC films. Previous reports showed that non-irradiated polymeric films have a high level of homogeneity and smoothness [48,49]. On the other hand, the AFM images of the irradiated blends showed that the surface was badly damaged due to bond cleavage (Figure 8). The surface of the irradiated blends was rough and contained dark spots, particularly in the case of the pure PVC film. Complexes 1-3 reduced the damage caused by photoirradiation, leading to a smoother surface. The roughness factor (Rq) for the irradiated pure PVC film was 295.2, which is much higher than for the blends containing the tin complexes. The Rq for the PVC blends containing 1, 2, and 3 were 26.1, 38.6, and 51.2, respectively. Clearly, the trimethoprim-tin complexes caused a significant reduction in the Rq values. For example, complex 1 led to a reduction in the Rq by 11.3-fold, which is higher than that achieved with many other organometallic complexes ( Table 4).
The FESEM images gave clearer less distorted and high-resolution pictures of the PVC surface. The images provided information about the homogeneity of the surface as well as the particles size and shape for the irradiated materials [46]. Figure 6 shows that the surface of irradiated PVC films contains lumps, groves, and noticeable irregularities attributed to bond cleavage during photoirradiation and photodegradation [47]. The size of the particles varies; 42.07-132.9 nm for 1, 69.11-164.8 nm for 2, and 42.07-152.3 nm for 3. Again, the surface of the blend containing 1 shows cracks and rodlike particles, which is consistent with the microscope image ( Figure 6). It has been reported that PVC containing nickel chloride and a dithiazole Schiff base or tin-naphthalene sulfonic acid complex led to the formation of honeycomb-like structures [31,38]. In addition, PVC blended with a Schiff base containing a melamine unit showed an ice-rock structure [36]. Such observations could be attributed to the slow rate of HCl elimination from the PVC chains during photoirradiation. Figure 7 shows the FESEM images of the particles within the PVC + 1 film post-irradiation at lower magnification.  AFM has also been used to analyze the surface of the irradiated PVC films. Previous reports showed that non-irradiated polymeric films have a high level of homogeneity and smoothness [48,49]. On the other hand, the AFM images of the irradiated blends showed that the surface was badly damaged due to bond cleavage (Figure 8). The surface of the irradiated blends was rough and contained dark spots, particularly in the case of the pure

PVC Photostabilization Mechanisms
Trimethoprim-tin complexes 1-3 act as efficient PVC photostabilizers as they are able to absorb UV light. The complexes contain aromatic rings that stabilize the highly energetic intermediates formed on irradiation through resonance [57]. The tin complexes can also decompose hydroperoxide (Figure 9), which is produced during photo-oxidation of PVC [58]. In addition, the tin atom is capable of scavenging the HCl (Figure 9) produced during the irradiation process [52]. Both PVC and tin complexes contain polar bonds (e.g., C-Cl in PVC and C-O and C-N in tin complexes) that allow coordination between them, leading to easier energy transfer from the polymeric chains to the additives ( Figure 10). However, complications, such as steric hindrance, that may be associated with large molecules would make such a possibility less likely [53].

PVC Photostabilization Mechanisms
Trimethoprim-tin complexes 1-3 act as efficient PVC photostabilizers as they are able to absorb UV light. The complexes contain aromatic rings that stabilize the highly energetic intermediates formed on irradiation through resonance [57]. The tin complexes can also decompose hydroperoxide (Figure 9), which is produced during photo-oxidation of PVC [58]. In addition, the tin atom is capable of scavenging the HCl (Figure 9) produced during the irradiation process [52]. Both PVC and tin complexes contain polar bonds (e.g., C-Cl in PVC and C-O and C-N in tin complexes) that allow coordination between them, leading to easier energy transfer from the polymeric chains to the additives ( Figure 10). However, complications, such as steric hindrance, that may be associated with large molecules would make such a possibility less likely [53].

Conclusions
A convenient method for the production of trimethoprim-tin complexes in good yield has been established. The trimethoprim-tin complexes have been shown to act as PVC photostabilizers, leading to a significant reduction in the damage caused by photo-

PVC Photostabilization Mechanisms
Trimethoprim-tin complexes 1-3 act as efficient PVC photostabilizers as they are able to absorb UV light. The complexes contain aromatic rings that stabilize the highly energetic intermediates formed on irradiation through resonance [57]. The tin complexes can also decompose hydroperoxide (Figure 9), which is produced during photo-oxidation of PVC [58]. In addition, the tin atom is capable of scavenging the HCl (Figure 9) produced during the irradiation process [52]. Both PVC and tin complexes contain polar bonds (e.g., C-Cl in PVC and C-O and C-N in tin complexes) that allow coordination between them, leading to easier energy transfer from the polymeric chains to the additives ( Figure 10). However, complications, such as steric hindrance, that may be associated with large molecules would make such a possibility less likely [53].

Conclusions
A convenient method for the production of trimethoprim-tin complexes in good yield has been established. The trimethoprim-tin complexes have been shown to act as PVC photostabilizers, leading to a significant reduction in the damage caused by photooxidation and photodegradation of the polymeric chains. The additives contain aromatic

Conclusions
A convenient method for the production of trimethoprim-tin complexes in good yield has been established. The trimethoprim-tin complexes have been shown to act as PVC photostabilizers, leading to a significant reduction in the damage caused by photo-oxidation and photodegradation of the polymeric chains. The additives contain aromatic and heterocyclic moieties that enable them to act as absorbers for ultraviolet light, quenchers for free radicals, and decomposers for hydroperoxides. In addition, the heteroatoms (oxygen and nitrogen) within the trimethoprim-tin complexes facilitate their coordination and interaction with PVC chains. The additives led to a significant decrease in the roughness factor of the PVC surface compared with many other organotin complexes. The additives containing aromatic substituents (phenyl groups) showed better performance as PVC photostabilizers compared with the one containing aliphatic substituents (n-butyl groups).