*3.5. Chain Extension Reaction with St and BA in Emulsion Polymerization*

A random copolymer was prepared, and monomer conversion was over 99.5%, as shown in Table 12. The emulsion of the random copolymer was stable and milky white with a weak blue color. After the first polymerization stage, the monomer mixture comprising water-insoluble monomer BA and St was added at the second polymerization stage. The emulsion at the end of the second polymerization stage was stable and milky white with a weak blue color, indicating that the random copolymer poly (PEGMA-*co*-MAAa-*co*-HEMA-*co*-BMA) can be used as a macro-emulsifier in the chain extension reaction. The measured *M*<sup>n</sup> of the copolymer at the second stage was higher than that of the random copolymer at the first polymerization stage, indicating the living polymerization of the seed polymer chain. The molecular weight distribution index (*Ð*) at the second stage was lower than that at the first stage. In Tonnar's work [38,60], *Ð* was decreased after the chain extension reaction in emulsion polymerization, and these results further indicate living RITP. Therefore, the molecular weight distribution decreased after the chain extension reaction in this paper, proving living polymerization. The diameter of the seed polymer was lower than that of the block copolymer, due to the growth of the seed polymer chain in the chain extension reaction. The solid content of the copolymer at the second stage was higher than that of the random copolymer at the first polymerization stage due to the addition of monomer mixtures at the second stage. These results indicate that the random copolymer poly (PEGMA-*co*-MAAa-*co*-HEMA-*co*-BMA) can be used as macro-chain transfer agent to

control the chain extension reaction containing water-insoluble monomer BA and St in this polymerization system, and the seed random copolymer exhibited living polymerization.



MAA<sup>a</sup> was neutralized by ammonia solution. Conditions: *m*(PEGMA)/*m*(MAA) = 1/1 and *m*(MAA) + *m*(PEGMA) = 1.664 g; no MMA in the polymerization system; ammonia solution (0.70 g); *n*(PEGMA)/*n*(MAA)/*n*(St)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/ *n*(I2) = 2.72/15/18.38/30.62/31.22/67.28/12.44/1.92/1; *m*(BA)/*m*(St) = 1/2; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

> As shown in Figure 7, the core–shell microstructure of the random copolymer was obvious. The core microstructure of the block copolymer was vague, and the shell layer was relatively thinner than that of the random copolymer. The shape of both random copolymer and block copolymer micelles was regular, and the core layer was completely covered by the shell layer. The diameter of the block copolymer in Figure 7b was generally larger than that of the random copolymer in Figure 7a. These results indicate that the random copolymer can be used as a seed polymer to stabilize the polymerization and as a nanoreactor for emulsion polymerization. However, the diameter according to the TEM micrograph ranged from 100 to 190 nm, smaller than that measured by dynamic light scattering (DLS), as shown in Table 12. The diameter measurement by TEM was conducted in an environment without liquid water and represents the true radius of the particles [61]. On the other hand, the diameter measurement by DLS is conducted in an environment with liquid water and represents the hydrodynamic size of the particles [62,63]. The aggregation state of the particles affects the measured results; thus, the hydrodynamic size of particles or the size of agglomerated particles measured by DLS is often larger than the true radius of particles measured by TEM [63]. Agglomerated particles can be seen in the TEM micrograph (Figure 7); hence, the diameter measured by DLS was larger than that measured by TEM.

**Figure 7.** *Cont*.

**Figure 7.** TEM micrograph of (**a**) the seed polymer and (**b**) the block copolymer described in Table 12.

We investigated whether the random copolymer can be used as macro-chain transfer agent or macro-emulsifier in the chain extension reaction when BA and HEMA were added in at second polymerization stage. As shown in Table 13, the emulsion of the random copolymer poly (PEGMA-*co*-MAAa-*co*-BMA-*co*-MMA-St) was stable and milky white with a weak blue color. When the monomer mixture comprising BA and HEMA was added at the second polymerization stage, the emulsion was not stable, and a lot of coagulation existed after 33 min of reaction time. The pH value of the withdrawn sample ranged between 6.5 and 7, and there were no nonionic MAA monomers that participated in random copolymerization with HEMA, thereby restraining the formation of an interpolymer complex. Crosslinking caused by transesterification among the pendent hydroxyl groups can lead to crosslinked polymers [23,24], and these crosslinked polymers may lead to instability of the emulsion. Thus, the emulsion was not stable, and a large amount of gel existed after 33 min. The instability of the emulsion in the chain extension reaction period indicated that HEMA could not be copolymerized fluently in the chain extension reaction of this emulsion polymerization system.



MAA<sup>a</sup> was neutralized by ammonia solution. Conditions: *m*(PEGMA)/*m*(MAA) = 12/7 and *m*(MAA) + *m*(PEGMA) = 3.1611 g; *m*(BA)/ [*m*(St) + *m*(MMA)] = 1/2; ammonia solution (1.10 g); *n*(PEGMA)/*n*(MAA)/*n*(St)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(MMA)/*n*(ACPA)/ *n*(I2) = 6.52/21/18.38/31.22/67.28/12.44/12.74/1.92/1; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

> In conclusion, the living polymerization of the random copolymer chain was proven by the chain extension reaction containing BA and St in the emulsion. Moreover, the copolymer prepared via the chain extension reaction exhibited a higher measured *M*n, lower *Ð*, larger particle size, and higher solid content than the random copolymer at the first polymerization stage. The TEM results indicate the increase in particle size and the regular shape of the micelle. However, the chain extension reaction with BA and HEMA

was not successful; thus, there exists some limitation to the chain extension reaction of the seed random copolymer with other kinds of monomer.

### *3.6. Infrared Spectra of Polymer*

The FTIR spectra of the copolymer are illustrated in Figure 8. The wide absorption peak at 3430 cm−<sup>1</sup> was caused by O–H stretching vibration. The wide absorption peak at 3224 cm−<sup>1</sup> was derived from N–H stretching vibration of NH4 <sup>+</sup> located in the ammonium salt of MAA units. The two peaks at 2955 and 2868 cm−<sup>1</sup> were ascribed to C–H stretching vibrations of –CH3 and –CH2 groups, respectively. The two peaks at 1451 and 1383 cm−<sup>1</sup> were caused by C–H bending vibrations of –CH3 and –CH2 groups, respectively. The strong absorption peak at 1723 cm−<sup>1</sup> was derived from the stretching vibration of the carbonyl ester C=O. The peak at 1544 cm−<sup>1</sup> was caused by the asymmetric stretching vibration of carbonyl anion COO−. The bands of 1240 and 1068 cm−<sup>1</sup> were derived from asymmetric stretching vibration and symmetric stretching vibration of the ester group C–O–C, respectively. The strong absorption peak at 1150 cm−<sup>1</sup> was ascribed to the bending vibration of the ether group C–O–C from PEGMA units and that of C–O–H from HEMA units. With the increase in mass ratio of PEGMA/MAA, the peak intensity at 1544 cm−<sup>1</sup> was deceased, indicating a decrease in MAA content in the polymer chain. The FTIR spectra indicate the influence of the mass ratio of PEGMA/MAA on the signal changes representing polymeric groups.

**Figure 8.** FTIR spectra of copolymer with different mass ratios of PEGMA/MAA. Polymerization conditions: *m*(MAA) + *m*(PEGMA) = 1.664 g; no MMA in the polymerization system; the MAA solution was neutralized by ammonia solution; *m*(BA)/*m*(St) = 1/2; *n*(St)/*n*(HEMA)/*n*(BMA)/ *n*(BA)/*n*(ACPA)/*n*(I2) = 18.36/18.74/40.36/7.46/1.15/1; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

#### **4. Conclusions**

Emulsifier-free styrene–acrylic emulsions prepared via RITP were studied in this paper. When the ammonium salt of MAA was used as the only kind of polymeric surfactant and St was added into the polymerization system containing HEMA monomer

component, the emulsion did not flow fluently and was sticky. Thus, it was necessary to explore a method to prepare stable styrene–acrylic emulsions with a high content of HEMA monomer component. In this paper, a stable styrene–acrylic emulsion with low viscosity and an analogous *<sup>M</sup>*<sup>n</sup> range to previous work (from 23,000 to 30,000 g·mol−1) could be prepared when PEGMA was combined with the ammonium salt of MAA. The acrylatebased polymer emulsion with HEMA units in the polymer chain was not stable when only PEGMA was used as a polymeric emulsifier. St is more hydrophobic than MMA; hence, the polymerization containing St monomer units was conducted in combination with the ammonium salt of MAA and PEGMA. The influence of PEGMA and I2 on the emulsion was studied. With the increase in PEGMA amount, the *T*<sup>g</sup> of the polymer decreased; the largest maximum tensile strength (5.39 MPa) in the polymer with *T*g = 29.9 ◦C was larger than that of the polyacrylate polymer with *T*g = 43.9 ◦C (2.98 MPa) in our previous paper. When polymerization was conducted in 1.4 times the reference amount of initiator, *M*<sup>n</sup> was obviously increased with the decrease in I2, and the highest *M*<sup>n</sup> of the polymer with HEMA units (40,700 g·mol<sup>−</sup>1) was larger than that in our previous paper (32,700 g·mol<sup>−</sup>1) when the mass ratio of BA/BMA was 1/6, while the largest maximum tensile strength of the dried styrene–acrylic emulsion polymer film with the highest *M*<sup>n</sup> was more than 5.5 MPa. The living polymerization of the random copolymer chain was proven by a kinetics experiment and chain extension reaction; the uniformly shaped particles and the increase in particle diameter according to TEM indicated that the random copolymer can serve as a seed emulsifier in the polymerization. The novelty lies in that the polymerization was conducted with a high solid content (ranging from 40 wt.% to 46 wt.%) and a high content of hydroxyl monomer component, while St took part in the polymerization; a moderate *<sup>M</sup>*<sup>n</sup> range (20,000–41,000 g·mol<sup>−</sup>1) was achieved in a short period of time (<4 h), and a stable emulsion with moderate viscosity (ranging from 100 mPa·s to 700 mPa·s) was successfully prepared. The polymerization was conducted in water, and no costly nonpolymeric emulsifier or organic solvent was used. Furthermore, the amount of MAA could be reduced when MAA was combined with PEGMA, and this strengthened the stability of the emulsion when ammonium hydroxide used for neutralizing MAA was volatilized quickly in hot weather. This protocol represents an environmentally friendly system tailored for the direct preparation of an emulsion used for maintenance coating. In summary, the work in this article provides a synthetic method for preparing high-solidcontent styrene–acrylic emulsions with HEMA units in the polymer chain, and the synthetic method may have application in preparing coatings or polymer materials with excellent mechanical properties.

**Author Contributions:** Conceptualization, T.H. and S.G.; methodology, T.H. and S.G.; validation, T.H.; formal analysis, T.H.; investigation, T.H.; writing—original draft preparation, T.H.; writing review and editing, T.H.; visualization, T.H.; supervision, S.G.; funding acquisition, S.G. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

