Bottlebrush Polyolefin Sulfones as Triggerable and Degradable Pressure‐Sensitive Adhesives

ABSTRACT

The molecular architecture (topology) of bottlebrush polymers, featuring densely grafted side chains along a polymeric backbone, leads to unique physical properties, enabling their use as functional materials including elastomers and pressure‐sensitive adhesives (PSAs). Bottlebrush polymers can form additive‐free PSAs due to their architecture, but most are crosslinked materials that include all‐carbon backbones, have thermally unstable disulfide bonds, or do not reach the high molar masses needed for bottlebrush PSAs. Here, we applied the alternating free‐radical copolymerization of sulfur dioxide (SO₂) and norbornene‐based macromonomers in a grafting‐through approach to make bottlebrush polymers with number‐average molar masses exceeding 1100 kg/mol and backbone degrees of polymerization exceeding 900. These first examples of poly(olefin sulfone) bottlebrush polymers incorporated polyacrylate, polymethacrylate, polystyrene, and poly(lactic acid) side chains, all attached to a poly(norbornene‐alt‐SO₂) backbone. Under mild alkaline conditions, these very high molecular weight bottlebrush polymers degraded considerably within 10 min and completely within 4 h. Finally, a bottlebrush polymer synthesized using this approach behaved as a PSA with a peel strength of ∼1200 N/m, substantially higher than non‐degradable PSAs in commercial tapes. In sum, this work offers a versatile approach to synthesize triggerable and degradable bottlebrush polymer adhesives enabling end‐of‐life disposal following their intended applications.

A grafting‐through alternating copolymerization strategy yields poly(norbornene sulfone) bottlebrush polymers with high molar mass. These materials function as additive‐free pressure‐sensitive adhesives. The incorporation of cleavable sulfone units enables rapid breakdown under mild alkaline conditions, providing a pathway toward sustainable, high‐strength adhesives with improved recyclability.

1. Introduction

Bottlebrush polymers, a type of comb polymer, consist of a polymeric backbone with densely grafted side chains [1, 2, 3], and this molecular architecture (topology) imparts distinctive material properties. One particularly unusual and useful property resulting from the bottlebrush architecture is the ability to form supersoft elastomers with elastic moduli orders of magnitude lower than linear polymers [4, 5, 6], enabling their use as pressure‐sensitive adhesives (PSAs) [7, 8, 9]. Traditional PSAs derived from linear polymers are used in many consumer products and include high levels of tackifier and plasticizer additives. These additives create serious inefficiencies and problems in plastic and paper recycling streams because they break down into adhesive particles that build up on machinery or disperse into the melt phase; the particles then phase separate at lower temperatures, contaminating both the equipment and the recycling stream [10, 11]. Bottlebrush polymer PSAs can avoid these problems because they do not require additives due to their unusually low elastic moduli; however, they have not reached their full potential because they are either crosslinked materials made from all‐carbon backbone polymers or suffer from poor thermal stability or low molecular weights (MWs), limiting end‐of‐life disposal options as well as performance. A synthesis route that delivers additive‐free bottlebrush polymer PSAs with triggerable degradability, thermally stable backbones, and the high molar masses needed for effective adhesion remains a challenge.

Several strategies have been employed in efforts to make additive‐free bottlebrush polymer PSAs. For example, Sheiko and coworkers have developed the fundamental basis for controlling bottlebrush network physical properties and have recently created bottlebrush polymer networks with tunable adhesion and rheological properties (Figure 1A) [5, 9, 12]. Recognizing the lack of degradability in many of these networks, Johnson and coworkers installed degradable silyl ether units into bottlebrush polymers through a copolymerization of norbornene‐functionalized macromonomers and silyl ether cyclic olefins using ring‐opening metathesis polymerization (ROMP), although these materials were never tested as adhesives [13]. More recently, inspired by linear polymer PSAs with disulfide bonds [11, 14], separate reports led by Bates and Verduzco used α‐lipoic acid‐derived macromonomers to prepare bottlebrush polymers featuring disulfide linkages in the backbone, which enabled degradability but at the cost of relatively low MWs and with the thermal instability inherent to disulfides (Figure 1B) [15, 16]. High‐MW macromolecules are vital for bottlebrush polymer adhesives because it is the wormlike conformation and reduced entanglements that give bottlebrush polymers their unique properties [17]. Inspired by these efforts, we envisioned a new method of bottlebrush polymer synthesis using sulfur dioxide (SO₂) and norbornene‐based macromonomers to make poly(olefin sulfones) (POSs), a class of alternating copolymers well known for their depolymerizability upon exposure to alkaline conditions (Figure 1C).

FIGURE 1.

(A) Thermoplastic bottlebrush polymers were synthesized via the grafting‐from method and utilized as PSAs [18]. These PSAs were not degradable due to vinyl backbones. (B) Degradable bottlebrush polymers featuring polydisulfides backbones were synthesized via ring‐opening polymerization of lipoic acid [15, 16]. The use of these bottlebrush polymers for PSAs were not tested. (C) In this work, degradable bottlebrush polymers with high molar masses were synthesized via grafting‐through using copolymerization of norbornene‐functionalized macromonomers and SO₂.

POSs are highly alternating copolymers synthesized through free‐radical copolymerization between SO₂ and electron‐rich olefin monomers, typically 1‐alkenes or cycloalkenes [19, 20, 21, 22]. An interesting characteristic of POSs is their relatively high thermal stability under normal conditions paired with their relatively low ceiling temperatures, affording the ability to depolymerize when exposed to ionizing radiation or alkaline environments. This inherent degradability has made POSs attractive materials for applications in photoresists [23, 24, 25, 26], radiation detectors [27], and degradable polymers and composites [28, 29, 30]. POSs have also been studied as degradable adhesives, albeit thus far only as thermosetting structural adhesives [31]. Notably, free‐radical copolymerization between SO₂ and norbornene, first reported in 1959, proceeds efficiently and rapidly to high conversion when initiated by tert‐butyl hydroperoxide (tBuOOH) [32]. We recently found that poly(norbornene‐alt‐SO₂) (P(Nb‐alt‐SO₂)) formed polymers with number‐average molecular weight (M _n) values exceeding 1000 kg/mol in <10 min even at –30°C [33]. This fast propagation suggested the possibility for efficient and rapid polymerization of norbornene‐based macromonomers to make bottlebrush POSs. However, despite their potential in a variety of applications where degradable polymers are urgently needed, graft or bottlebrush polymers based on a P(Nb‐alt‐SO₂) backbone have never been reported.

We envisioned that free‐radical copolymerization between SO₂ and norbornene‐derived macromonomers could provide a straightforward and rapid grafting‐through approach to synthesize bottlebrush polymers while also enabling degradability. The grafting‐through method in bottlebrush polymer synthesis in which a macromonomer is polymerized to form the bottlebrush polymer backbone enables 100% grafting density, but grafting‐through methods that generate degradable backbones are rare. Moreover, unlike the intrinsic lability of polydisulfide backbone, the P(Nb‐alt‐SO₂) backbone remains stable above 200°C and undergoes depolymerization only upon exposure to a specific external trigger [33]. To this end, we proposed that various polymers including poly(n‐butyl acrylate) (PnBA) could be functionalized with a norbornene on one chain end and used in a subsequent copolymerization with SO₂ to produce degradable bottlebrush polymers. We further hypothesized that the poly(Nb‐alt‐SO₂) backbone would render these bottlebrush polymers degradable under alkaline conditions due to abstractable protons flanking the SO₂ group. Finally, drawing inspiration from previous reports on bottlebrush PSAs discussed above [7, 8, 9, 18], we anticipated that a bottlebrush polymer with a poly(Nb‐alt‐SO₂) backbone could function as a base‐triggered degradable PSA. Here, we set out to test these hypotheses through an approach combining synthesis, characterization, and materials testing of POS‐based bottlebrush polymers.

2. Results and Discussion

2.1. Macromonomer and Bottlebrush Polymer Synthesis and Characterization

Our synthetic approach began with the preparation of well‐defined PnBA macromonomers functionalized with a norbornene using photo‐induced atom transfer radical polymerization (PI‐ATRP) (Scheme 1A). In this ATRP mechanism, Cu(I)/L species are generated through excitation of Cu(II)/L species, followed by donation of a single electron from an amine‐based ligand [34]. We chose PI‐ATRP for this synthesis because it was simple to set up and required less copper than traditional ATRP, facilitating purification. Moreover, PI‐ATRP allows higher chain‐end functionality than traditional ATRP, ensuring minimal amounts of coupled products that could compromise the synthesis of bottlebrush polymers [35, 36, 37]. We used PI‐ATRP to synthesize a PnBA macromonomer with a target molecular weight of 1 kg/mol (N _sc value of 8). Siz‐exclusion chromatography with multi‐angle light scatterig (SEC‐MALS) analysis revealed that the macromonomer had an M _n value of 1.1 kg/mol with a narrow molar mass dispersity value (Đ) of 1.05, and end group analysis by ¹H NMR spectroscopy showed a similar molecular weight. This macromonomer is designated as MM1. Using the same method, we also synthesized another macromonomer, MM2, which was also derived from PnBA but with an M _n value of 3 kg/mol (N _sc = 23). Additionally, poly(methyl methacrylate) (PMMA) (MM3), poly(styrene) (PS) (MM4), and poly(lactic acid) (PLA) (MM5) MMs were synthesized to test the versatility of SO₂ copolymerization with diverse side‐chain chemistries. Synthesis and characterization details for all macromonomers are provided in Table 1.

SCHEME 1.

General synthesis of PnBA macromonomers using PI‐ATRP followed by synthesis of bottlebrush polymers with poly(Nb‐alt‐SO₂) backbones. Schemes showing the synthesis of macromonomers and bottlebrush polymers based on PS, PLA, and PMMA are available in the Supporting Information (Schemes S4–S6).

TABLE 1.

Synthesis and characterization of macromonomers.

Entries	Name	MM structure	Conversion (%) a	M n, NMR b (kg/mol)	M n, SEC c (kg/mol)	Đ c	N sc	T g d (°C)
1	MM1	PnBA	47	1.2	1.1	1.05	8	−43
2	MM2	PnBA	47	3.0	3.0	1.03	23	−41
3	MM3	PMMA	65	2.5	2.6	1.06	26	114
4	MM4	PS	10	3.8	3.4	1.02	33	88
5	MM5	PLA	99	1.5	1.8	1.04	25	48

^a Estimated by SEC‐MALS by dividing M _{n, SEC} values by (MW_monomer  x [monomer]/[initiator]) based on the assumption of a high level of livingness.

^b Measured using end group analysis by ¹H NMR spectroscopy (Figures S7, S8, S10, and S12).

^c Measured by SEC‐MALS in THF at 30°C using light scattering and refractive index detectors (Figures S9, S11, S13, and S15). The dn/dc values used for PnBA, PMMA, PS, PLA were 0.069, 0.084, 0.185, and 0.042 mL/g, respectively, based on literature values [38, 39].

^d Measured by DSC with 10°C/min heating and cooling ramp; data reported are from the second heat cycle (Figures S65–70).

Next, we set out to synthesize a series of bottlebrush polymers via free radical copolymerization of macromonomers with SO₂ at –20°C, which is below the −10°C boiling point of SO₂ (Scheme 1). SO₂ is an industrial byproduct, and efforts to convert SO₂ directly into useful chemicals are underway [40]. POSs are derived directly from SO₂ and typically synthesized via free radical copolymerization, where an alkene reacts with SO₂ in the presence of a radical initiator, most commonly tBuOOH due to its ability to generate a high concentration of initiating radicals even at low temperatures. In this synthesis, SO₂ served as both co‐monomer and solvent, along with a small amount of CH₂Cl₂ used to dissolve the macromonomer. Reactions were quenched after 30 min by exposing the reaction mixture to air and then pouring the contents of the Schlenk tube into methanol, which precipitated out the polymer. Bottlebrush polymers were then isolated and purified to remove small amounts of unreacted macromonomer using automated silica gel chromatography. We expected fairly broad Đ values in these free radical copolymerizations, but we envisioned that we could control molecular weight to a limited degree by changing the macromonomer to initiator molar ratio.

Using this procedure, we began by copolymerizing MM1 with SO₂ to form BB1‐A. The resulting polymer was characterized by ¹H NMR spectroscopy, SEC‐MALS, and X‐ray photoelectron spectroscopy (XPS) (Figure 2). SEC‐MALS (Figure 2A) showed a complete shift in the peak from MM1 to BB1‐A (M _n of 55 kg/mol, Đ = 1.4). Moreover, complete consumption of norbornene was evidenced by the disappearance of olefinic protons at 6.28 ppm in ¹H NMR spectroscopy after copolymerization (Figure 2B). Peaks between 2.6 and 4.0 ppm were broadened due to the presence of SO₂ in the backbone, although these signals were partially obscured by overlapping sidechain resonances. XPS was used to further confirm successful incorporation of SO₂ in the backbone. For this experiment, 10 sample locations were chosen for analysis over a 3 mm × 3 mm area. Sulfur (S2p) was measurable at all sample locations at 168 eV (Figure 2C), with mass percentage (0.4 wt%) consistent with expected values.

FIGURE 2.

(A) SEC‐MALS traces of MM1 and BB1‐A showing a complete shift from the lower molecular weight MM1 (1 kg/mol) to the higher molecular weight BB1‐A (55 kg/mol). The small peaks at ∼18.3 and ∼19.1 min are due to plasticizers leached from the syringe filter housing. (B) ¹H NMR spectra of MM1 (bottom) and BB1‐A (top) with the region near 6.5 ppm highlighted yellow showing the complete disappearance of the norbornene olefin protons and the inset region (3–4 ppm) showing new signals resulting from the POS backbone. (C) XPS spectroscopy of BB1‐A (after column purification) revealing atomic composition consistent with the expected values.

Although the ¹H NMR spectrum and the XPS analysis supported our hypothesis of a POS copolymer, the spectroscopic evidence for sulfone units in the bottlebrush polymer backbone was limited, likely due to the predominance of other functional groups (in PnBA side chains) over the C─SO₂ bonds. We therefore conducted additional control experiments to confirm the presence of the poly(Nb‐alt‐SO₂) backbone. First, we ran a polymerization under identical conditions but without SO₂ (Scheme S7). This control reaction was designed to rule out possible radical homopolymerization of the MM1 macromonomer. Unlike the previous reactions where bottlebrush polymer formed, nothing precipitated from the solution after pouring the reaction mixture into methanol. ¹H NMR spectroscopy of the reaction mixture residue showed unchanged norbornene vinyl proton integrations (Figure S26). Additionally, SEC‐MALS analysis confirmed that the macromonomer peak remained unchanged before and after the reaction (Figure S27). We also ran another control reaction, this time omitting tBuOOH to rule out potential (non‐radical) vinyl addition polymerization of the norbornene macromonomer (Scheme S8). This control reaction yielded a similar result, with both NMR and SEC confirming that no polymerization occurred (Figures S28–S30). Finally, we synthesized a model poly(norbornene sulfone) for comparison using the same reaction conditions as for the bottlebrush polymers but with norbornene in place of MM1. The ¹H NMR spectrum exhibited broadened peaks between 2.5 and 4.0 ppm (Figure S31), similar to BB1‐A, and FTIR showed peaks at ∼1300 and ∼1100 cm⁻¹, consistent with SO₂ stretches (Figure S32). Collectively, the characterization data on the bottlebrush polymers combined with the control experiments provide compelling evidence for successful bottlebrush polymer formation with P(Nb‐alt‐SO₂) backbones.

By varying feed ratios, that is, the [MM]₀:[I]₀ ratio, copolymerization between MM1 and SO₂ successfully produced four bottlebrush polymers (BB1‐A, BB1‐B, BB1‐C, and BB1‐D). Bottlebrush polymers derived from MM1 exhibited M _n values ranging from 55 to 1100 kg/mol (Table 2, Entries 1–4). SEC traces for these four bottlebrush polymers are shown in Figure 3A, which shows consistent shifts to earlier elution times with decreasing levels of tBuOOH initiator. The dispersity values remained in the range of 1.5–1.8, although the traces were not symmetrical and, in some cases, multimodal, likely due to the free radical nature of these polymerizations. We observed similarly shaped SEC traces in copolymerizations of SO₂ with small molecule cycloalkenes [33].

TABLE 2.

Synthesis and characterization of bottlebrush polymers.

Entry	Name	Structure	[MM]0: [I]0	M n, SEC a (kg/mol)	Đ a	N sc	N bb b	T g c (°C)
1	BB1‐A	P(Nb‐alt‐SO2)‐g‐MM1	1:1	55	1.4	8	55	0
2	BB1‐B	P(Nb‐alt‐SO2)‐g‐MM1	5:1	145	1.8	8	145	0
3	BB1‐C	P(Nb‐alt‐SO2)‐g‐MM1	50:1	338	1.6	8	338	−1
4	BB1‐D	P(Nb‐alt‐SO2)‐g‐MM1	200:1	1100	1.5	8	959	−1
5	BB2‐A	P(Nb‐alt‐SO2)‐g‐MM2	50:1	103	1.1	23	35	−1
6	BB2‐B	P(Nb‐alt‐SO2)‐g‐MM2	100:1	329	1.5	23	109	0
7	BB2‐C	P(Nb‐alt‐SO2)‐g‐MM2	200:1	617	1.5	23	218	0
8	BB3	P(Nb‐alt‐SO2)‐g‐MM3	100:1	149	1.2	26	57	144
9	BB4	P(Nb‐alt‐SO2)‐g‐MM4	100:1	140	1.2	33	40	104
10	BB5	P(Nb‐alt‐SO2)‐g‐MM5	100:1	763	1.2	25	423	70

^a Measured by SEC‐MALS. dn/dc values of 0.036, 0.024, 0.047, and 0.030 mL/g were obtained by assuming 100% mass elution for bottlebrush polymers with PnBA, PMMA, PS, and PLA side chains, respectively.

^b Measured by dividing M _{n, SEC} of the bottlebrush polymers by the M _{n, SEC} of the corresponding macromonomer.

^c Measured by DSC with 10°C/min heating and cooling ramp; data reported are from the second heat cycle (Figures S66–S70).

FIGURE 3.

Stacked SEC traces with normalized traces showing light scattering (red dashed trace) and differential refractive index (blue solid line) signals of (A) bottlebrush polymers derived from MM1 with PnBA side chains of 1 kg/mol, (B) bottlebrush polymers derived from MM2 with PnBA side chains of 3 kg/mol, and (C) bottlebrush polymers derived from MMs 3–5 with PMMA (top), PS (middle), PLA (bottom) side chains.

We then moved on to MM2, another PnBA MM with a molecular weight of 3 kg/mol. Bottlebrush polymers BB2‐A, BB2‐B, and BB2‐C (Table 2, Entries 5–7) were successfully synthesized with M _n values ranging from 103–617 kg/mol. SEC traces of BB2‐A, BB2‐B, and BB2‐C (Figure 3B) had similar shapes to those for the BB1 series. We also attempted to synthesize bottlebrush polymers with PnBA side chains of 5 kg/mol (Figure S33), but this macromonomer yielded only bottlebrush polymers with short backbones, which is likely due to the larger macromonomer size creating steric hindrance. This topological effect has been previously reported in both ROMP and ATRP systems [41, 42].

Finally, we successfully prepared three bottlebrush polymers, denoted BB3, BB4, and BB5, from MM3 (PMMA), MM4 (PS), and MM5 (PLA), respectively. These polymerizations also yielded bottlebrush polymers, with M _n values ranging from 140–763 kg/mol (Figure 3C). Collectively, these results demonstrate the versatility of this approach to bottlebrush polymer synthesis.

2.2. Thermal Properties

Differential scanning calorimetry (DSC) analysis demonstrated markedly different T _g values for the macromonomers and bottlebrush polymers (Tables 1 and 2). MM1 exhibited a T _g of –43°C, which is close to the reported literature value for PnBA [7]. All four bottlebrush polymers derived from MM1 displayed T _g values around 0°C (Figure S66). Bottlebrush polymers derived from MM2 also had similar T _g values (Figure S67). These results indicate that the POS backbone, despite being only a small percentage of the bottlebrush polymer by mass, greatly influenced the T _g. As a comparison, the T _g value for poly(norbornene sulfone) (without PnBA side chains) was 176°C (Figure S64) [33]. We attribute the increase in T _g upon converting MM1 into the BB1 series to the effect of restricting one end of each PnBA unit on the POS backbone, yielding a T _g value that is intermediate between the low‐T _g side chains and the high‐T _g backbone. Bottlebrush polymer molecular weight had a negligible effect on T _g. Our DSC analysis is also consistent with reports by Kennemur and coworkers for bottlebrush polymers consisting of a polynorbornene backbone by ROMP, in which the bottlebrush polymer T _g was largely determined by the side chain chemistry [43]. Similar results were observed for bottlebrush polymers with PS and PMMA side chains (BB3 and BB4, respectively, Figures S68,S69).

2.3. Degradation of Poly(Nb‐alt‐sulfone)‐g‐PnBA Bottlebrush Polymers

Next, we investigated the degradation of these POS‐based bottlebrush polymers. Previous studies have shown that POSs undergo depolymerization through multiple stimuli including thermal treatment, irradiation, and exposure to basic conditions [25, 31]. Both elevated temperature and irradiation degrade POSs by cleaving the relatively weak C─S bonds within the polymer backbone. Additionally, bases can abstract acidic protons on carbons adjacent to the electron‐withdrawing sulfonyl groups, leading to subsequent chain scission and complete depolymerization with concomitant regeneration of the original monomer and SO₂. In fact, various bases such as piperidine, pyridine, triethylamine, and NaOH can depolymerize linear POSs within minutes [28, 30, 44].

To test the degradability of poly(Nb‐alt‐SO₂)‐g‐PnBA bottlebrush polymers, a model bottlebrush polymer BB2‐C (Table 2, Entry 7) was chosen because its high M _n value of 654 kg/mol allowed us to assess its degradability by SEC‐MALS. Based on initial kinetic studies, we conducted all degradation tests for 16 h, and reaction crudes were analyzed by SEC‐MALS. Results for degradation of BB2‐C under various conditions are summarized in Table S1. Contrary to previous results for linear POSs, a strong organic base like DBU demonstrated limited effectiveness for BB2‐C, with 5 equiv (with respect to SO₂ units) achieving only 35% degradation of the bottlebrush polymer (Table S1, Entry 6, Figure S43). DIPEA was even less effective, failing to induce depolymerization as evidenced by minimal shifts in the SEC‐MALS traces (Table S1, Entries 7 & 8, Figure S44). This reduced sensitivity of the POS bottlebrush polymer to organic bases likely stems from restricted accessibility of the bases to protons on the backbone due to the densely grafted side chains. Degradation of BB2‐C using a small inorganic base, NaOH, dramatically enhanced degradation efficiency with conversion of BB2‐C into 99% macromonomer fragments (Table S1, Entries 9 & 10, Figure S45). Monitoring the degradation kinetics by SEC‐MALS revealed that BB2‐C degraded into oligomers in just 10 min and fully into macromonomer fragments within 4 h (Figure 4).

FIGURE 4.

(A) Schematic illustration of degradation of poly(Nb‐alt‐SO₂)‐g‐PnBA; Yellow circles: SO₂ unit; Blue squares: norbornene unit; Black line: MM. (B) SEC‐MALS traces (dRI signal only) of the degradation study of BB2‐C using 5 equiv NaOH in THF at room temperature over 4 h. Dark blue solid lines represent the reaction crude at various time points, with BB2‐C eluting at 13.5 min; light blue lines represent MM2 with a peak near 17 min.

We also evaluated degradation using heat and UV irradiation as the triggers. Thermal treatment of BB2‐C in DMF without base yielded moderate degradation, with macromonomer peak integration increasing from 10% at 75°C to 31% at 90°C (Table S1, Entries 11–12, Figure S46). Irradiation using UV light at 280 nm for 4 h resulted in degradation and in situ formation of crosslinked products (Table S1, Entry 13, Figure S47). However, only minimal depolymerization (8% and 5%, respectively) was achieved under irradiation at 365 and 395 nm wavelengths (Table S1, Entries 14–15, Figure S47). These results confirm that base‐induced degradation, particularly with inorganic bases such as NaOH, represents the most effective pathway for complete degradation and recovery of the side‐chain polymer.

To probe the degradation mechanism of BB2‐C, we employed a diethylaminocoumarin–hemicyanine probe, synthesized following the methodology reported by Guo and co‐workers [45]. This probe absorbs strongly at 545 nm, which shifts to 410 nm upon nucleophilic addition of SO₃ ²⁻, the product of the reaction of SO₂ with H₂O (Scheme 2). We conducted a series of UV–vis spectroscopy experiments to examine the proposed degradation mechanism (Figure 5A). The probe alone (Figure 5A, black curve) and the probe in the presence of NaOH (Figure 5A, dark orange curve) both displayed the expected absorbance at 545 nm, indicating that it did not degrade under basic conditions. Addition of NaHSO₃ produced the characteristic peak shift to 410 nm, consistent with the report by Guo and co‐workers (Figure 5A, cyan curve) [45]. When BB2‐C was exposed to NaOH in the presence of the probe, we observed the emergence of absorbance at 410 nm (Figure 5A, solid green curve), indicating in situ SO₂ release. Importantly, a control experiment using a structurally analogous bottlebrush polymer synthesized by ROMP (PNb‐g‐PnBA) (termed here as Ctrl‐Poly), (Scheme S13 and Figure S54) showed no absorbance at 410 nm under identical conditions.

SCHEME 2.

Reaction of ratiometric SO₂‐responsive probe. ^a The reaction is shown with SO₃ ^2–, the product of the reaction of SO₂ with water, which changes the absorption peak from 545–410 nm.

FIGURE 5.

(A) UV–vis absorbance for SO₂ detection. (B) Bar graph showing I₄₁₀/I₅₄₅ ratios for the five samples in Panel A. Ctrl‐Poly in the graph is a PNb‐g‐PnBA bottlebrush polymer synthesized by ROMP. In these experiments, BB2‐C or Ctrl‐Poly (final repeating unit conc. 50 µM) and the probe (final conc. 10 µM) were dissolved in CH₃CN. Then, NaOH (final conc. 50 µM in H₂O) was added, and the reactions were allowed to stir for 20 min before performing absorption measurements.

Figure 5B shows a bar graph depicting the ratio of the absorption at 410–545 nm (I₄₁₀/I₅₄₅) for each condition or analyte. A near‐zero I₄₁₀/I₅₄₅ ratio indicates no reaction between SO₃ ²⁻ and the probe. The probe alone and a mixture of probe and the control in NaOH exhibited I₄₁₀/I₅₄₅ values near 0, indicating no SO₃ ²⁻ in the solution. However, BB2‐C in NaOH degraded and released SO₂ in situ, causing a shift from 545–410 nm. As a result, the I₄₁₀/I₅₄₅ value increased to 30, which was not quite as high as probe + NaHSO₃ but still provided clear evidence of SO₂ release. These UV–vis results support the presence of SO₂ in the backbone and demonstrate SO₂ release during the degradation of the poly(norbornene sulfone) bottlebrush polymer in NaOH.

The degradation studies confirmed depolymerization to release SO₂ and PnBA oligomer, but the fate of the imide unit linking the PnBA side chains to the norbornene was unclear. We therefore investigated whether the imide anchor groups were susceptible to base‐catalyzed hydrolysis under our degradation conditions. The control bottlebrush polymer (Ctrl‐Poly), prepared by ROMP (M _n = 515 kg/mol, Đ = 1.06), was subjected to 5 equiv NaOH, identical to the conditions in Entry 10. SEC‐MALS analysis revealed a very different degradation profile, including progressive disappearance of the high molecular weight bottlebrush polymer peak over 4 h, accompanied by the emergence of multiple lower molecular weight species (Figure S55). We also subjected macromonomer MM2 to the same conditions, and the SEC‐MALS traces remained unchanged throughout the study (Figure S53). Based on these results, we conclude that the observed molecular weight reduction in Ctrl‐Poly stems from hydrolysis of the imide linkage connecting the side chains to the backbone, rather than backbone degradation. This imide hydrolysis reaction likely also occurred in the NaOH‐mediated degradation of POS bottlebrush polymer BB2‐C. ¹H NMR spectroscopy of the BB2‐C decomposition products showed peaks corresponding to the PnBA MM. However, our attempts to identify the amine were unsuccessful, presumably because its signal overlapped with those of the macromonomer backbone (Figures S56–57). Therefore, elucidating the complex degradation mechanism of these norbornene macromonomer–based POSs will require a more detailed study. Based on the available SEC and ¹H NMR spectroscopy data, we surmise that degradation of BB2‐C occurs through a synergistic mechanism: Hydrolysis of the imide anchor groups releases the PnBA side chains, thereby increasing backbone accessibility to hydroxide ions. Excess NaOH then promotes rapid degradation of the poly(Nb‐alt‐SO₂) backbone through the known degradation mechanism.

2.4. Adhesion Properties of a Bottlebrush POS

To evaluate a potential application of BB‐POSs as degradable pressure‐sensitive adhesives, we synthesized a bottlebrush polymer featuring P(nBA_0.9‐stat‐AA_0.1) side chains with an average N _sc value of 12. The macromonomer contained 10 mol% poly(acrylic acid) (PAA) to promote adhesion and cohesive strength. The resulting bottlebrush POS (termed here as BB‐PSA) exhibited a molecular weight of 574 kg/mol corresponding to a backbone degree of polymerization of 359 (Figures 6B, S63). DSC revealed glass transition temperatures of −45°C and −36°C for the macromonomer and BB‐PSA, respectively (Figure S71). We envisioned that the low T _g of the BB‐PSA would facilitate easy application and wetting on substrate surfaces.

FIGURE 6.

(A) Chemical structure of a degradable bottlebrush PSA (BB‐PSA) containing a poly(Nb‐alt‐SO₂) backbone. (B) SEC‐MALS chromatogram of BB‐PSA with M _n = 574 kg/mol and Đ = 2.1 (dashed red: LS signal; blue: dRI signal). (C) Rheological characterization of BB‐PSA (filled squares: storage modulus; empty squares: loss modulus). (D) 90° peel test results for BB‐PSA (blue trace), commercial duct tape (black trace), and masking tape (gray trace).

In addition to the glass transition behavior, the thermal stability of the BB‐PSA material was also evaluated. Thermogravimetric analysis (TGA) was performed on representative materials, including BB2‐B, BB3, BB4, BB5, and BB‐PSA (Figures S76–S78). The TGA trace for BB‐PSA in particular showed minimal mass loss below 200 °C and a 5% mass loss temperature (T _d,5%) of 235 °C, indicating that the material is thermally stable under conditions relevant to typical processing and application. These data are consistent with the known thermal robustness of POS materials studied earlier in our group [33]. We attribute the high thermal stability of bottlebrush POSs to the inherent thermal stability of the sulfone unit, which makes them comparable to thermally stable (T _c ∼180°C) but reconfigurable dynamic covalent bottlebrush networks [46]. In addition, compared with dynamic covalent bottlebrush networks and bottlebrush polymers with disulfide backbones [15, 47], the BB‐PSA could achieve reusability exhibited by dynamic bottlebrush networks while avoiding the thermally labile and oxidation‐prone disulfide bonds, either through direct recovery of the polymers via thermal or solvent treatments or via depolymerization. However, unlike these directly repolymerizable systems with disulfide backbones, regeneration of the macromonomer after bottlebrush polymer depolymerization/degradation may require a second step to reinstall the norbornene unit.

Rheological characterization of the BB‐PSA was performed by measuring the storage (G′) and loss (G″) moduli as a function of frequency at room temperature (Figure 6C). At low frequencies (< 0.2 Hz), the BB‐PSA behaved as a viscous liquid with G″ > G′, consistent with the requirements for effective bonding to the substrate, which typically occurs at low frequencies (∼0.01 Hz). Furthermore, both G′ and G″ remained below 3 × 10⁵ Pa at ∼1 Hz, satisfying the Dahlquist criterion for PSAs [48]. We observed a crossover point at 0.2 Hz where the storage modulus became larger than the loss modulus, which we anticipated would improve cohesion and provide resistance to substrate debonding. Temperature sweep experiments revealed that the storage modulus decreased with increasing temperatures (Figure S74). These results suggest that the material undergoes a pronounced shear‐stiffening effect, presumably due to some degree of chain entanglement from the relatively high N _bb/N _sc ratio, a phenomenon recently discussed by Kennemur, which is consistent with the suitable cohesive strength that we observed in our additive‐free PSA formulation [43, 49].

Next, we evaluated the adhesive properties of the BB‐PSA using 90° peel tests. A film of the BB‐PSA sample was laminated between a PET sheet and a PTFE sheet, preheated at 30°C for 4 min without pressure, and then pressed. Figure 6D shows adhesion strength as a function of peeling distance. The BB‐PSA, with no added tackifiers, demonstrated high and stable peel strength as seen from a steady plateau force of approximately 1200 N/m. The steady plateau indicates consistent viscoelastic energy dissipation throughout the peeling process. The adhesion strength of the BB‐PSA is threefold greater than that of commercial duct tape and sixfold higher than that of commercial masking tape. These results demonstrate that the unique combination of viscoelastic properties and the bottlebrush architecture in the BB‐PSA achieves an optimal balance between surface wetting, interfacial adhesion, and cohesive strength, resulting in superior peel resistance. Notably, the BB‐PSA exhibits excellent cohesive strength without the need for chemical crosslinking. Collectively, the BB‐PSA possesses adhesion strength comparable to crosslinked, non‐degradable bottlebrush polymer PSAs reported recently by Sheiko and coworkers [8]. To further test its adhesion, the load‐bearing capability of a sample of the BB‐PSA was evaluated using a glass–glass bonding test. A 50 mg sample of the adhesive was deposited between two glass substrates and pressed manually to ensure contact and then secured with binder clips to maintain uniform adhesion, generating a 25 cm² bonded area. After equilibration at room temperature, the assembly was fitted with mechanical attachments and subjected to a 15 lb (6.8 kg) static load. The adhesive exhibited sufficient shear strength to sustain the applied load without failure, demonstrating its effectiveness as a general adhesive (Figure S75 and Movie S1). Additional tests with a 20 lb load led to failure of the apparatus but not debonding of the adhesive.

3. Conclusions

This work establishes a versatile and efficient methodology for synthesizing degradable bottlebrush polymers with P(Nb‐alt‐SO₂) backbones. Our approach successfully accommodated norbornene‐functionalized macromonomers with diverse chemical compositions and degrees of polymerization while maintaining control over molecular weight through adjustment of macromonomer‐to‐initiator ratios. The degradability of these high‐molecular‐weight bottlebrush POSs was demonstrated through base treatment, where excess NaOH yielded oligomers within 10 min and achieved nearly complete degradation within 4 h via synergistic backbone cleavage and imide hydrolysis. Furthermore, we demonstrated that a bottlebrush polymer synthesized using this methodology behaved as a PSA with viscoelastic properties suitable for surface wetting and interfacial adhesion without the need for tackifier or plasticizer additives. Peel tests characterized a peel strength of approximately 1200 N/m, which is three‐fold higher than non‐degradable commercial tapes and comparable to recently reported crosslinked, non‐degradable bottlebrush PSAs. Collectively, this approach enables the preparation of degradable bottlebrush polymers with high molar masses and fully degradable backbones. Future comparative studies on the viscoelastic, mechanical, and thermal properties of bottlebrush polymers with P(Nb‐alt‐SO₂) backbones will provide fundamental insights and further applications for these new degradable bottlebrush polymers.

Author Contributions

Clark Vu: conceptualization, investigation, writing – original draft, formal analysis. Isaac D. Addo: conceptualization, investigation, writing – original draft, formal analysis. Woojung Ham: investigation, writing – original draft, formal analysis. Udita Choudhury: investigation, writing – original draft, formal analysis. Joshua C. Worch: writing – review and editing, project administration, supervision. John B. Matson: writing – review and editing, project administration, supervision, funding acquisition, conceptualization.

Conflicts of Interest

The authors have filed a provisional patent on this work.

Supporting information

The authors have cited additional references within the Supporting Information [50].Supporting File 1: anie73729‐sup‐0001‐SuppMat.docx.

Supporting File 2: anie73729‐sup‐0002‐MovieS1.mp4.

Data Availability Statement

The data that support the findings of this study are openly available in Figshare at https://figshare.com/s/72451935960f3f382fa0.