*2.2. Methods*

#### 2.2.1. Emulsifier-Free Copolymerization of Acrylate Monomers

In a typical example illustrated in Figure 1A, BA (1.027 g, 8.01 mmol), BMA (6.162 g, 40.33 mmol), MMA (0.822 g, 8.21 mmol), St (1.233 g, 11.83 mmol), HEMA (2.618 g, 20.12 mmol), 9.138 g of deionized water, and 273 mg (1.074 mmol) of I2 were added to a flat-bottom flask (100 mL), and the mixture was stirred by a magnetic stirrer for 15 min. Thereafter, a neutralized MAA solution comprising MAA (1.664 g, 19.33 mmol), deionized water (4.141 g), and ammonia solution (1.49 g, 21.87–24.50 mmol NH3) was added to the flask and stirred for 15 min. Next, 3.461 g (1.235 mmol) of ACPA 10 wt.% solution (2.25 g ACPA, 642 mg NaOH, and 19.608 g H2O) was added. The reaction system was deoxygenated by bubbling with high-purity nitrogen for 25 min at room temperature while stirring. Afterward, the ingredients were placed under the atmosphere of high-purity nitrogen provided by a balloon, and the mixture was heated to 80 ◦C while stirring; the total heating time was 210 min. The reaction was ceased by exposure to air. The total theoretical monomer mass content by weight was 40 wt.%. The monomer conversion and the solid content were determined by gravimetric analysis.

When the mass ratio of the monomer or the mass of the I2 was changed, the procedures were as described above.

#### 2.2.2. Chain Extension Reaction with BA and St

Firstly, BMA (6.162 g, 40.33 mmol), HEMA (2.618 g, 20.12 mmol), deionized water (9.892 g), and I2 (164 mg, 0.644 mmol) were added to a flat-bottom flask (100 mL), and the mixture was stirred by a magnetic stirrer for 15 min. Thereafter, a PEGMA solution comprising PEGMA (0.832 g, 1.75 mmol) and H2O (2.27 g) was added into the flask and stirred for 15 min; then, a neutralized MAA solution comprising MAA (0.832 g, 9.66 mmol), deionized water (3.80 g), and ammonia solution (0.708 g, 10.39–11.64 mmol NH3) was added to the flask and stirred for 10 min. Afterward, 3.461 g (1.235 mmol) of ACPA 10 wt.% solution was added. The reaction system was deoxygenated by bubbling with high-purity nitrogen for 25 min at room temperature while stirring. Next, the ingredients were placed under the atmosphere of high-purity nitrogen provided by a balloon, and the mixture was heated to 80 ◦C while stirring. The sample was removed by a syringe under a nitrogen atmosphere after heating for 130 min (the first stage). Thereafter, the nitrogen gas-saturated mixture comprising BA (1.027 g, 8.01 mmol) and St (2.054 g, 19.72 mmol) was immediately added to the flask, and polymerization continued for 90 min (the second stage).

When the mixture comprising BA and HEMA was added at the second stage, the procedures were as described above. The reaction time of the first stage was 168 min, and that of the second stage was 33 min.

#### 2.2.3. Modification of the Emulsion Polymer

Firstly, the emulsion (2.5 g) and deionized water (2.08 g) were added to a round flask (25 mL) at room temperature, and then stirred for 5 min. Afterward, HMMM (0.64 g) was added while stirring for 10 min. TsOH solution (85 μL, 10 wt.%) was added dropwise to the mixture, and the mixture was stirred for 1 h.

To measure the hardness, adhesive properties, and water resistance, the ingredients of the modified film were coated on a clean tinplate and coverslip, and these samples were placed in room temperature for 2 h to evaporate water naturally. Thereafter, the tinplate and coverslip were heated at 80 ◦C for 120 min and then 150 ◦C for 40 min. After heating, the tinplate and coverslip were cooled to room temperature via natural cooling.

To measure tensile strength, the ingredients of the modified film were coated on a horizontal tetrafluoroethylene plate, and the samples were placed in room temperature for 2 h to evaporate water naturally. The plate was heated at 80 ◦C for 120 min and then 150 ◦C for 40 min. Afterward, the modified film could be torn off the plate.

#### 2.2.4. Preparation of Dried Emulsion Films

To measure tensile strength, the emulsion mixture was poured onto a horizontal tetrafluoroethylene plate, and the emulsion was dried under the radiation of an infrared lamp for at least 24 h; then, the film could be torn off the plate. At least three samples of the layer could be obtained per piece of emulsion film.

### *2.3. Characterizations*

#### 2.3.1. Viscosity

The emulsion viscosity was measured using a DV-79 digital viscometer (Shanghai Ni Run Intelligent Technology Co., Ltd., Shanghai, China) with a rotor (E-type rotor, F-type rotor, or G-type rotor) at 25 ◦C. The rotational rate of the rotor was 75 or 750 rpm when the viscosity was in the corresponding measurement range of the rotor type and rotational rate.

#### 2.3.2. Monomer Conversion

The determination of monomer conversion was as follows:

```
Conversion% = m (Dried emulsion)/(m (Emulsion solution) × w (Total monomer)) (1)
```
where w (Total monomer) is the total mass content of the monomer in weight.

#### 2.3.3. Particle Diameter

After adding deionized water to the emulsion according to the volume, the emulsion sample was diluted 1000-fold. Then, the particle size diameter and the particle size distribution of the diluted emulsion sample were measured using a Zetasizer Nano ZS laser particle sizer, Malvern Instruments Ltd. (Shanghai, China) at 25 ◦C; the Malvern Zetasizer ZS device that we used worked at 173◦ in backscattering mode.

#### 2.3.4. Molecular Weight

The emulsion sample was further neutralized using DMEA, and the excess DMEA could be removed via rotary evaporation under reduced pressure. Afterward, the sample was dispersed in THF (20 mg/mL), and the mixture was filtered using an organic phase filter head. Then, 50 μL of the filtered sample was injected into the device. The number average molecular weight (*M*n), the weight average molecular weight (*M*w), and the index of the molecular weight distribution (*Ð*) of the polymer were measured using a gel permeation chromatograph (GPC) from US WATERS Corporation (Milford, MA, USA). The GPC device was equipped with a Waters-2414 Refractive Index Detector, and the eluent THF flowed at a rate of 1.0 mL/·min at 25 ◦C. The material of the GPC columns was styrene–divinylbenzene copolymer. The GPC columns used were WATERS Styragel

HR1 + Styragel HR3 + Styragel HR4. The calibrated material in the chromatographic column was polymethyl methacrylate.

#### 2.3.5. Glass Transition Temperature

*T*<sup>g</sup> was tested using a TA Q20 DSC Instrument (TA Instruments, New Castle, DE, USA), and the measurement was conducted under nitrogen atmosphere. The measuring temperature range was −15 ◦C to 120 ◦C, and the heating/cooling rate was 10 ◦C/min.

#### 2.3.6. Fourier-Transform Infrared Spectroscopy

The sample was prepared by drying the emulsion under the radiation of an infrared lamp. The infrared spectrum of the sample was tested using a Thermo Nicolet Fouriertransform infrared spectroscope (FTIR) (Thermo Fisher, Waltham, MA, USA), and the device was equipped with a diamond from Smiths Detection for attenuated total reflectance (ATR). The scanning range was 4000–500 cm<sup>−</sup>1.

#### 2.3.7. Transmission Electron Microscopy

The 1000-fold diluted sample was added to the copper mesh, and then stained with 3% phosphotungstic acid (PTA) solution. After evaporation of water in the mesh, the mesh was observed using a JEM-2100 transmission electron microscope from JEOL Corporation (Tokyo, Japan), with an accelerating voltage of 220 kV. The sample morphology could be observed after vacuuming.

#### 2.3.8. Hardness Rank of the Modified Film

The hardness of the modified film was tested by sliding pencil lead on the film surface. The procedures and the hardness rank were conducted according to China National Standard GB/T 6739-2006/ISO 15184:1998.

First, the pencil was pressed down on the film surface at an angle of 45◦ by hand. The pencil was shifted at a rate of 0.5–1 mm/s and for at least 7 mm. If the length of the marking was longer than 3 mm, a pencil with lower hardness was used until no marking longer than 3 mm existed. When a marking shorter than 3 mm existed, the pencil hardness rank was regarded as the hardness rank of the film. This measurement was conducted at a temperature of 23 ± 2 ◦C. The measurement was conducted at least three times.

#### 2.3.9. Adhesive Property Rank of the Modified Film

The adhesive property was evaluated by the affected area of cuts on the surface of the modified film or dried emulsion film adhered to the tinplate piece. The procedures and the evaluation standard were conducted according to China National Standard GB/T 9286-1998 equivalent ISO 2409:1992.

Six cuts were made in each direction of the lattice pattern, and the lattice was made by scratching the knife under the surface of the film with the use of ruler. The spacing of the cuts in each direction were equal to 1 mm (0–60 μm thick film) or 2 mm (60–120 μm thick film). The tape was removed on the surface of the cuts by grasping the free end and pulling it off in 0.5–1.0 s at about 60◦. When none of the lattice squares were detached, classification was regarded as rank 0. When 0–5%, 5–15%, 15–35%, and 35–65% of the area was affected, classification was regarded as 1, 2, 3, and 4, respectively. Any degree of flaking that could not be classified as 4 was classified as 5. This measurement was conducted at a temperature of 23 ± 2 ◦C. The measurement was conducted at least three times.

#### 2.3.10. Water Resistance of the Modified Film

The water resistance of the modified film was measured by the change in the film immersed in boiling water. The test was based on a comparison with a modified film not immersed in boiling water. The procedures and the evaluation standard were conducted according to China National Standard GB/T 1733-1993.

Water resistance of the films was determined by immersing three sample pieces into boiling water for 2 h, and then the immersed part was dried by filter paper. Next, the appearance of these three sample pieces in terms of gloss, whitening, wrinkling, etc. was compared with those not immersed in boiling water.

#### 2.3.11. Water Absorption of the Modified Film

The modified film was weighed. Then, the same piece of modified film sample was immersed in deionized water for 24 h. After 24 h, the sample immersed in water was weighed. The determination of water absorption was as follows:

Water absorption% = 100% × m (Dried film)/(m (Film immersed in water)). (2)

#### 2.3.12. Toluene Absorption of the Modified Film

The modified film was immersed in toluene for 24 h. The determination of toluene absorption was as follows:

Toluene absorption% = 100% × m (Dried film)/(m (Film immersed in Toluene)) (3)

#### 2.3.13. Tensile Strength

Tensile strength of the dried emulsion film or modified film was measured using an Electronic Universal Testing Machine from MTS SYSTEMS (China) Co., Ltd. (Shanghai, China) with SANS-Power Test software, and the measurement was conducted at 10 mm·min−<sup>1</sup> at 20 ± <sup>2</sup> ◦C. The shape of the film was rectangular in general. The length of the film was longer than 12 mm, and the width ranged from 5–12 mm.
