## 2 Methods
### Synthesis  
#### 2.2.1 Synthesis of Macromonomer Norbornenyl-Terminated Polystyrene (NB-Pst) [42] (Figure S1a)  
_NB-OH_: To a 500 mL flask, cis-5-norbornene-exo-2,3-dicarboxylic anhydride (16.41 g, 164.15 g/mol, 0.1 mol), ethanolamine (6.01 mL, 61.08 g/mol, 0.1 mol), DMAP (12.21 g, 122.17 g/mol, 0.1 mol), and 100 mL toluene were added. A Dean-Stark trap was attached to the flask, and the reaction mixture was heated at reflux (150 \({}^{\circ}\)C) for 10 h. Then the toluene was concentrated and removed to yield a faint yellow solid. The crude product was recrystallized three times in an appropriate amount of toluene at 135 \({}^{\circ}\)C. Finally, it was dried in a 40 \({}^{\circ}\)C vacuum oven to yield 19.68 g of white solid (yield: 95%). The product was used for the following step without further purification. The chemical compositions of NB-OH were confirmed by \({}^{1}\)H NMR (CDCl\({}_{3}\)) (Figure S2a).  
_NB-Br_: The protocol for the synthesis of NB-Br is similar to that reported in another study with some modifications [43]. NB-OH (19.68 g, 207.23 g/mol, 0.095 mol) was added to a 200 mL Schlenk flask and protected by N\({}_{2}\). Then, 150 mL of anhydrous N-methyl-2-pyrrolidone (NMP) was added to the above-mentioned Schlenk flask via a syringe to completely dissolve it, and it was cooled to 0 \({}^{\circ}\)C. \(\alpha\)-bromoisobutyryl bromide (14.83 mL, 229.91 g/mol, 0.12 mol, 1.2-fold compared to the hydroxyl groups) was then added dropwise to the mixture solution under stirring and protection by N\({}_{2}\). The reaction temperature was maintained at 0 \({}^{\circ}\)C for 2 h and then slowly increased to ambient temperature, after which the reaction was allowed to continue for 24 h. The mixture was then incubated in an oil bath at 40 \({}^{\circ}\)C for 12 h. The brown solution was poured into saturated NaHCO\({}_{3}\) aqueous solution under stirring. The separated oil mixture at the bottom of the beaker was collected and dissolved in 200 mL of DCM. The organic yellow solution obtained was washed three times with 0.1 N HCl aqueous solution (200 mL), 0.1 M Na\({}_{2}\)CO\({}_{3}\) aqueous solution (200 mL), and ultrapure water (200 mL) and dried over MgSO\({}_{4}\). The solvent was evaporated andrecrystallized three times in EtOH to yield 30.68 g of white solid (yield: 86%). The chemical compositions of NB-Br were confirmed by \({}^{1}\)H NMR (CDCl\({}_{3}\)) (Figure S2b).  
_NB-PSt_: St (34.8 mL, 104.15 g/mol, 0.3 mol), CuBr (42.0 mg, 143.45 g/mol, 0.30 mmol), and NB-Br (0.34 g, 1.0 mmol) were added to a 100 mL Schlenk flask. Then, the mixture was purged with N\({}_{2}\) for 30 min under stirring, followed by the addition of PMDETA (63.4 \(\mu\)L, 173.3 g/mol, 0.30 mmol) via a syringe for another 15 min purging of N\({}_{2}\). The mixture was stirred at 65 \({}^{\circ}\)C for the desired time, and then the reaction was terminated by cooling it quickly down to room temperature using liquid nitrogen. The product was passed through a short silica column to remove the catalyst and precipitated into MeOH three times. The obtained white powder weighed around 2.38 g. The chemical compositions of NB-PSt were confirmed by \({}^{1}\)H NMR (CDCl\({}_{3}\)), as shown in Figure S2c. The conversion determined by \({}^{1}\)H NMR was ca. 8.3%. GPC (DMF): \(M_{\rm n}=2.4\) Kg/mol and \(B=1.21\).  
Other NB-PSt\({}_{\rm n}\) polymers with varying lengths were synthesized by increasing the ATRP time, with the structural parameters listed in Table S1.