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. 2024 Oct 7;13(11):1390–1395. doi: 10.1021/acsmacrolett.4c00550

Degradable N-Vinyl Copolymers through Radical Ring-Opening Polymerization of Cyclic Thionocarbamates

Alvaro Calderón-Díaz †, Andrew C Boggiano †, Wei Xiong †, Nadine Kaiser ‡, Will R Gutekunst †,*
PMCID: PMC11580385  PMID: 39374102

Abstract

graphic file with name mz4c00550_0007.jpg

A thiocarbonyl radical ring-opening polymerization approach was implemented with cyclic thionocarbamates to generate degradable copolymers with N-vinyl monomers. The rigid structures of cyclic N-substituted thionocarbamates have been revealed by X-ray crystallography and NMR spectroscopy. The corresponding copolymers show incorporation of the thiocarbamates within the carbon backbone of polyvinylpyrrolidone influenced by acyl substituents through radical ring-opening copolymerization. The phenyl-substituted cyclic thionocarbamate copolymerized with N-vinyl carbazole and N-vinyl caprolactam, while little to no incorporation occurred with tBu acrylate and styrene, respectively. Further, these copolymers can undergo hydrolytic degradation under mild conditions. A new family of cyclic thionocarbamates capable of radical ring-opening copolymerization with N-vinyl monomers has been established.

Cyclic thiocarbonyl compounds have recently become attractive monomers for the radical polymerization of degradable polymers.1,2 These moieties provide an advantage when compared to the analogous cyclic ketene acetals (CKAs)3 because of their stability under ambient conditions, synthetic versatility, reduced side reactions during polymerization, and the thermodynamically favored formation of the carbonyl group upon ring opening. For example, considerable attention has been placed on the copolymerization of dibenzo[c,e]oxepane-5(7H)-thione (DOT) with more activated monomers by radical ring-opening methods to install degradable thioester linkages in the backbone (Scheme 1A).1 The incorporation of this thionolactone within the C–C backbone of common polymers has also been demonstrated to produce versatile degradable architectures such as linear polymers,4−11 nanoparticles,12−15 micelles,16 bottlebrushes,17 and polymer networks.18−21 While more activated monomers have been successfully copolymerized with thionolactones, copolymerization with less activated monomers remains relatively unexplored.22,23

Scheme 1. Radical Ring-Opening Polymerization of DOT and Common Comonomers (A) and Radical Ring-Opening Polymerization of Cyclic Thionocarbamates (B).

Scheme 1

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One of the more commercially relevant less activated monomers is N-vinylpyrrolidone (NVP), and when polymerized into polyvinylpyrrolidone (PVP), this polymer possesses various industrial applications with a projected market value of $4.9 billion USD by 2030.24 A challenge to create degradable PVP copolymers results from the highly reactive NVP radical intermediate produced when compared to the stabilized radicals of more activated monomers (e.g., acrylates, acrylamide, styrene). Therefore, it is important to identify comonomer structures with more stabilized thiocarbonyl units to obtain random copolymerization. Recently, the Coughlin group was able to demonstrate that the uniform incorporation of ester fragments within a PVP backbone can be achieved through semibatch polymerization of CKAs to form degradable PVP copolymers.25 Batch copolymerization of NVP with the random incorporation and high conversion of thiocarbonyl monomers within a given system is still of interest to produce degradable PVP copolymers.

Insight into controlling the copolymerization of NVP can be obtained from reversible addition–fragmentation chain transfer (RAFT). RAFT agents of trithio- and dithioesters are not suitable for controlled polymerization of NVP because of the energetic difference between the reactive radical of NVP and the stable trithio and dithio radicals. It is, therefore, unsurprising that DOT copolymerizes poorly with NVP, such as the observed lack of control with analogous dithioester RAFT agents. Recent advances have been made through RAFT polymerization of NVP using xanthates and dithiocarbamates.26 Inspired by the versatility of RAFT agents to control the polymerization of various monomers, the design of cyclic thiocarbonyl comonomers resembling dithiocarbamates can be implemented for copolymerization with NVP using commercially available precursors for synthetic ease (Scheme 1B).

Herein, the synthesis and characterization of a new family of cyclic thionocarbamates (CTCs) MeCTC, tBuCTC, PhCTC, p-CF3–PhCTC, p-MeO–PhCTC, and tBuOCTC is reported. These monomers can undergo radical ring-opening copolymerization with NVP and less activated monomers. Moreover, incorporation of the thiocarbamate moiety within the PVP backbone induced degradation in the presence of NaOMe.

The synthesis of CTCs followed a short three-step protocol (Figure 1A). The esterification of S-(−)-4-amino-hydroxybutyric acid was carried out by the addition of acetyl chloride into a solution of MeOH to give I in an excellent yield (96%). Cyclization of I using N,N′-thiocarbonyldiimidazole in the presence of Et3N afforded cyclic thionocarbamate II (43%). Two acylation methods of II were identified to provide the corresponding target CTCs as a liquid (MeCTC) and crystalline solids (tBuCTC, PhCTC, p-CF3–PhCTC, p-MeO–PhCTC, and tBuOCTC) in yields between 47–92%.

Figure 1.

Figure 1

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Three-step synthesis of cyclic thiocarbamates (A). Crystal structures of CTCs (B).

To determine the structure and solution conformation of the CTCs, all NMR resonances were completely assigned by 1D and 2D NMR experiments in solutions of CDCl3 (Figures S11–S24 and S29–S34). The absence of NOE cross peaks between all CTC acyl and NCH2 protons suggested the amide moiety adopts a rigid conformation with the acyl substituent syn to the thiocarbonyl functionality, similar to the conformation observed in the crystal structure of a related 1,3-oxazinane-2-thione.27 Adoption of this conformation has been known to minimize the net dipole moment in N-substituted 1,3-thiazolidine-2-thiones.28 NOE cross peaks between the CH and NCH2 protons revealed that half-chair conformations in solution are favored with the ester group in the axial position for all CTC monomers prepared, which is consistent with a structurally similar tetrahydrooxazine.29

Suitable single crystals of tBuCTC, PhCTC, and tBuOCTC were grown and subjected to X-ray crystallographic structure determinations (Figure 1B). Single-crystal X-ray diffraction unambiguously highlighted the rigid isostructural configurations of the thionocarbamate monomers with ester groups in the axial positions of the half-chair conformations, consistent with 2D NMR observations. The carbonyl substituents for the PhCTC and tBuOCTC on the amide groups were exclusively anti to the ester functionality. Conformational disorder for tBuCTC showed the carbonyl substituents in the anti- and syn-configurations with respect to the ester groups in 75% and 25% occupancy, respectively.

The attempts to homopolymerize MeCTC were unsuccessful, which can be attributed to a slow ring-opening process or termination of intermediate radicals consistent with other studies.5 Further, the reversibility of radical addition may favor the chain end of the ring-opened propagating radical that resembles an acrylate group. Initial examination of CTC copolymerization with N-vinyl monomers was carried out using MeCTC and NVP at a feed ratio of 5:100 in dioxane at 60 °C with azobisisobutyronitrile (AIBN) as the radical initiator for 18 h. The copolymerization afforded a monomodal GPC trace of MeCTC-co-PVP with a Mn of 13.6 kg/mol (Figure 2A,B). The 1H NMR spectrum showed that NVP was copolymerized by the appearance of the characteristic peaks for PVP between 1.38–2.55 and 3.15–3.85 ppm (Figure S35). The disappearance of the MeCTC −CH peak at 4.95 ppm and appearance of small broad peaks between 4.82–6.96 ppm were clear indications that MeCTC was incorporated into the polymer. High conversion values were calculated by crude 1H NMR for MeCTC (≥98) and NVP (89%) (Figure 2B, entry 1). This initial data suggest that cyclic thionocarbamates with N-acyl substituents are good candidates to match the reactivity of propagating NVP radicals. Conversely, conversions determined by 1H NMR showed that no copolymerization occurred between II (0%) and NVP (99%, Mn = 38.7 kg/mol, Đ = 3.54), which can be attributed to the nitrogen lone pair conjugated strongly with the thiocarbonyl group. To investigate the degradation of the thiocarbamate groups in the PVP backbone, 5 mg of the copolymer was dissolved in CH2Cl2 (1 mL) and treated with an excess of a 25% w/v NaOMe solution in MeOH with vigorous stirring for 18 h. Subsequent neutralization using 12.1 M HCl and isolation of the copolymer showed that significant degradation had occurred with a final Mn of 1.1 kg/mol (Figure 2B). The 92% decrease in Mn is suggestive of random incorporation of the thiocarbamate fragments within the PVP backbone.

Figure 2.

Figure 2

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GPC traces of CTC-co-PVP copolymers and degradation profiles (A–E). Degradation traces are shown in red for plots B–E. Conversion of monomers over time were determined by 1H NMR (F,G).

When comparing the Mn of MeCTC-co-PVP to a PVP homopolymer (93.5 kg/mol, Figure S53), it was observed that the presence of the thiocarbamates had a decreasing effect on rate of copolymerization, which can be rationalized by the CTC behaving as a reversible deactivating group as is common in their RAFT congeners. The decreased molecular weight may also be a result of hydrolysis or chain transfer events promoted by the thiocarbamate group during polymerization when compared to the robust PVP homopolymer.

To determine the influence of the amide substituents on the copolymerization between the CTCs and NVP, the Me– group on the CTC was changed to a bulky and electron-rich tBu– group (Figure 2C, entry 2). A significant decrease in conversion of the tBuCTC (51%) and an increase in the tBuCTC-co-PVPMn (43.2 kg/mol) was observed. The lower conversion for tBuCTC when compared to MeCTC can be attributed to the sterically encumbered approach of the propagating radical toward addition to the thiocarbonyl group. This is justified by the observation of the occupancy disorder observed for tBuCTC in the solid state. Lower incorporation of the tBuCTC within the PVP backbone in turn can produce longer PVP repeat units that result in the increased Mn of tBuCTC-co-PVP. Degradation of tBuCTC-co-PVP yielded a degraded polymer with a Mn of 9.2 kg/mol, which corresponds to a 79% decrease in molecular weight.

Introducing the more electron-deficient PhCTC within the backbone of PVP while preserving the bulkiness on the amide substituent resulted in the PhCTC-co-PVP with a Mn of 11.4 kg/mol, comparable to the MeCTC-co-PVP (Figure 2D, entry 3). High conversion values were calculated for PhCTC (88%) and NVP (93%). The appearance of new thiocarbamate resonances in the 13C NMR (δ = 170 ppm) and the absence of thioketal resonances (δ = ∼113 ppm) supports a ring-opening pathway without ring-retaining side reactions (Figure S44). The degradation profile for PhCTC-co-PVP showed a degraded polymer Mn of 1.3 kg/mol similar to MeCTC-co-PVP. Because PhCTC was found to have better shelf stability than MeCTC, it was selected as the optimal monomer for further studies (vide infra). A kinetics experiment showed that the conversion over time of PhCTC and NVP were near equal, which further demonstrated the random incorporation with the thiocarbamate moieties within the PVP backbone (Figure 2F).

Surprisingly, the largest Mn was found for tBuOCTC-co-PVP at 66.1 kg/mol with high conversion values for both tBuOCTC (≥98%) and NVP (98%) (Figure 2E, entry 4). The tBuOCTC-co-PVP showed a high molecular weight shoulder similar to that of the PVP homopolymer, which suggested possible compositional drift. Moreover, the tBuOCTC-co-PVP degradation trace was bimodal with peak Mn at 26.5 and 0.7 kg/mol. Measuring conversion over time for the tBuOCTC-co-PVP revealed that 21% of the tBuOCTC was consumed within 2 h, while only 1% of the NVP was polymerized during the same period. After 4 h, the tBuOCTC and NVP conversions were 46% and 12%, respectively, which indicated that compositional drift does occur during polymerization of tBuOCTC-co-PVP (Figure 2G).

To understand the scope of copolymerization, PhCTC was subjected to the standard polymerization conditions with N-vinyl carbazole (NVC), N-vinyl caprolactam (NVCl), tBu acrylate (tBuA), and styrene (Sty) as comonomers (Figure 3A). Increasing the ring size from 5 (NVP) to 7 (NVCl) showed a decrease in the vinyl monomer conversion (73%) and an increase in the PhCTC conversion (99%) when compared to PhCTC-co-PVP (Figure 3B, entry 2). When using NVC, there was a significant decrease in vinyl monomer conversion to 49% (Figure 3B, entry 1). The thionocarbamate conversion could not be determined by 1H NMR due to a large overlap of the polymeric carbazole peaks between 4.00–6.50 ppm. As hypothesized, PhCTC had a preference to copolymerize with N-vinyl monomers when compared to the more activated tBuA (18% PhCTC conversion, Figure 3, entry 3, and Figures S43 and S56) and Sty (0% PhCTC conversion, Figure 3, entry 4). This may be due to the slow addition of the stabilized acrylate-like radical to the thionocarbamate group, which leads to acrylate homopolymers that lack degradability. Degradation profiles for PhCTC-co-PVCl and PhCTC-co-PVC showed bimodal and monomodal GPC traces, respectively.

Figure 3.

Figure 3

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Polymerization of PhCTC and vinyl monomers (A). GPC traces of PhCTC-co-PVC and PhCTC-co-PVCl (B). Degradation traces are shown in dashed lines for plot B. Table summary of PhCTC copolymerization with vinyl monomers.

To observe how the electronic modification of the nitrogen atom affects the reactivity of the cyclic thionocarbamate system, two derivatives of PhCTC were synthesized with either methoxy or trifluoromethyl substituents at the para position (Figure 4A). While modest, changes in relative rates of conversion compared to those of NVP could be readily observed. Whereas PhCTC displays near random copolymerization with NVP, p-CF3–PhCTC is fully consumed with only 81% conversion of NVP (Figure 4B, entry 2). In contrast, p-MeO–PhCTC led to slower rates of polymerization with only 79% of the cyclic thionocarbamate conversion observed upon complete consumption of NVP (Figure 4B, entry 3). This is consistent with the methoxy substituent donating electron density into the aryl amide, which leads to an increased level of resonance stabilization of the thiocarbonyl by the nitrogen atom. This effect results in slower rates of attack by the propagating radical. Conversely, the trifluoromethyl substituent destabilizes the aryl amide, which directs the nitrogen atom’s resonance contribution away from the thiocarbonyl and increases the rate of radical attack. Interestingly, increasing electron density on the para-position of the phenyl ring also increased the Mn of the copolymers from 1.9 to 21.4 kg/mol (Figure 4B, entries 1–3). This is possibly because of a correlation with the rates of chain transfer or adventitious hydrolysis during polymerization.

Figure 4.

Figure 4

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Polymerization of para-substituted CTCs (A). GPC traces of para-substituted CTC-co-PVP (B). Table summary of para-substituted CTC copolymerization with NVP.

Initial investigations using the trithiocarbonate RAFT agent DoPAT in combination with NVP and MeCTC resulted in incomplete conversions for both monomers. The controlled polymerization of the CTCs was then attempted using dithiocarbamate-based RAFT agents (Figure 5, Figures S58 and S60, and Table S14). First, universal RAFT agents R2–R3 were used because they have been proven to produce controlled PVP homopolymers.30,31 Feed ratios among the CTCs, NVP, and the RAFT agents were 5:100:1. Polymerization at 60 °C with AIBN as the radical initiator in dioxane over 16 h resulted in a copolymer Mn significantly lower than the theoretical value of 11.4 kg/mol. Optimization experiments were unable to produce an increase in the Mn. Monitoring copolymerization over time, it was seen that the copolymer reached near theoretical Mn between 2–3 h although conversion for PhCTC and NVP were <15% and <5%, respectively. It was initially believed that the pyridyl groups of R2–R3 were behaving as nucleophiles and attacking the thiocarbamate moiety; therefore, the new R4–R5 RAFT agents were synthesized to avoid the pyridyl groups but resulted in no increase in Mn. The gradual decrease in Mn observed over time and nonideal RAFT polymerization behavior was indicative of chain transfer.

Figure 5.

Figure 5

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Selected RAFT agent comparison.

In conclusion, a new family of cyclic thionocarbamate monomers has been reported for radical ring-opening copolymerization with NVP. X-ray crystallography revealed that the thionocarbamate group and acyl substituents are distorted with the introduction of bulky groups. NMR spectroscopy showed that the sterically encumbered conformation is retained in solution. The optimal monomer, PhCTC, was also successfully copolymerized with other N-vinyl monomers, while little to no copolymerization occurred with tBu acrylate and styrene, respectively. Moreover, all N-vinyl copolymers degraded at room temperature in solutions of NaOMe. This demonstrates that the incorporation of thiocarbamate moieties into the backbone of N-vinyl polymers can be degraded in the presence of chemical stimuli. Future projects will be devoted to exploring the CTC tunability to minimize undesired hydrolysis and chain transfer events.

Acknowledgments

We thank Dr. Ronald A. Smith for experimental assistance. We acknowledge the Organic Materials Characterization Laboratory at GT for use of the shared characterization facility.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmacrolett.4c00550.

  • X-ray data for tBuCTC (CIF)

  • X-ray data for PhCTC (CIF)

  • X-ray data for tBuOCTC (CIF)

  • Full experimental details, crystallographic data, 1H and 13C NMR spectra of II and CTCs, UV–vis spectra and SEC traces of CTCs (PDF)

This work was supported by funds generously provided by BASF and the Department of Education Graduate Assistance in Areas of National Need Fellowship (Award # P200A210037).

The authors declare no competing financial interest.

Supplementary Material

mz4c00550_si_001.cif (569.9KB, cif)
mz4c00550_si_002.cif (699.8KB, cif)
mz4c00550_si_003.cif (676.4KB, cif)
mz4c00550_si_004.pdf (5.7MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

mz4c00550_si_001.cif (569.9KB, cif)
mz4c00550_si_002.cif (699.8KB, cif)
mz4c00550_si_003.cif (676.4KB, cif)
mz4c00550_si_004.pdf (5.7MB, pdf)

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