**3. Results and Discussion**

#### *3.1. Participation of Styrene in Emulsifier-Free Polymerization*

As shown in Table 1, when all hard monomer MMA was replaced by St, the emulsion was not stable throughout the reaction when only the ammonium salt of MAA was used as a polymeric emulsifier in the emulsion system. In Tables 1 and 2, the total mass of MMA and St was constant, while the mass ratio of MMA/St was changed; the mass of other ingredients was constant. When hard monomer MMA participated in the polymerization, the emulsion was stable, the monomer conversion was higher than 95%, and the solid content was over 40 wt.%. The monomer conversion was not changed obviously for the four mass ratios. The emulsion was stored and remained stable for 6 months. The emulsion polymer had a measured *<sup>M</sup>*<sup>n</sup> range from 23,000 to 30,000 g·mol−1, which is analogous to the range in our previous paper. The measured *M*<sup>n</sup> was higher than the theoretical *M*n. Iodine is hydrolyzed in water [41], and this hydrolysis decreases the amount of iodine that is involved in the synthesis of the chain transfer agent in situ [42–44]. Furthermore, ammonia solution facilitates hydrolysis. Thus, the deviation of measured *M*<sup>n</sup> from the theoretical *M*<sup>n</sup> was caused by the hydrolysis of I2 in water. The measured *M*<sup>n</sup> increased with the increase in MMA. The index of the molecular weight distribution decreased overall with the increase in MMA. According to the work of Tonnar et al. [38,45], the index of the molecular weight distribution with iodine in the polymerization was smaller than that without iodine, and the index with iodine in the polymerization ranged from 1.40 to 2.20 in most cases. In comparison with typical controlled radical emulsion polymerization such as RAFT polymerization [8,36] or atom transfer radical polymerization (ATRP) [46], the index of the molecular weight distribution in RITP was relatively large. However, the diameter of the particle (*d*p) was not increased with the increase in MMA, and the *d*<sup>p</sup> ranged from 350–430 nm. When the mass ratio of MMA to St was 4:6, the viscosity value of emulsion was 2350 mPa·s at 750 rpm. This viscosity was larger than that (525 mPa·s at 750 rpm) in our previous paper with the mass ratio of BA to MMA kept at 1:2 [28]. More MMA led to an overall decrease in the emulsion viscosity. Side products are unavoidable because chain transfer occurs due to collision between two active centers attached to polymer chains in polymerization [47] or emulsion polymerization [48–50]. Experimentally, an emulsion with

lower viscosity can improve the diffusion of the added hydrophobic monomer or monomer mixture, and this may ensure that random copolymerization or block copolymerization with St monomer units is conducted more fluently. The results in Table 1 show that, when all MMA was replaced by St, the emulsion was not conducted fluently with only the ammonium salt of MAA used as a polymeric emulsifier, whereas when both St and MMA were present, the emulsion was stable and of low viscosity.


**Table 1.** Results of emulsion with St units in polymer chain.

<sup>a</sup> *<sup>M</sup>*n,th = (mass of monomer) × (monomer conversion)/(2 × *<sup>n</sup>*I2,initial) + *<sup>M</sup>*AI, in which *<sup>M</sup>*AI = 275.02 g·mol<sup>−</sup>1. <sup>b</sup> *<sup>d</sup>*p: particle size diameter; PDI: polydispersity index of particle size diameter. Conditions: MAA solution was neutralized by 1.39 g of ammonia solution; *m*(BA)/[*m*(MMA) + m(St)] = 1/2; *m*(MMA) + *m*(St) = 2.054 g; *n*(MAA)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2) = 18/18.74/40.36/7.46/1.15/1; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

**Table 2.** Properties of the cured film synthesized by emulsion with St units in polymer chain.


Conditions: MAA solution was neutralized by ammonia solution; *n*(MAA) /*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2) = 18/18.74/ 40.36/7.46/1.15/1; *m*(BA)/[*m*(MMA) + *m*(St)] = 1/2; *m*(MMA) + *m*(St) = 2.054 g; ammonia solution (1.39 g); the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

> Before modification by MF resin, the pencil hardness rank of the three kinds of dried emulsion film was 1 H in room temperature, and their adhesive property rank was 1. Furthermore, the dried film was translucent, and it could be dispersed in water when the mixture was stirred. The cured film surface became hard with a pencil hardness rank of 2 after the modification in Table 2. The reason is that a crosslinked structure polymer was prepared via the reaction of hydroxy functional acrylics with MF resin [22], and this crosslinked structure limited the movement of the segment [18], thus leading to the hardness of the polymer film being higher than that of the dried emulsion film. Furthermore, the adhesiveness rank of the cured film was 0, suggesting that the adhesive property of the modified film was better than that of the dried emulsion film. Toluene absorption at the mass ratio of 10/0 was higher than that for the other three mass ratios; hence, the toluene resistance of the cured film was increased when St was added. The cured film was not dissolved in toluene, as polymers with a crosslinked structure only swell in some kinds of organic solvent [51]. Water absorption decreased when the mass ratio of MMA/St increased. The cured film was translucent after immersing in boiled water when the monomer mass ratio was 6/4 or 10/0, indicating water resistance. The best results in terms of good pencil hardness, good adhesive property, low toluene absorption, low water absorption, and good water resistance of the modified film were obtained when the mass ratio of MMA/St was 6/4.

Styrene is a hydrophobic monomer, and the phenyl structure of St units in the polymer promotes chain rigidity. Poly (ethylene oxide) (PEO) in the pendant group of a polymeric emulsifier can form a dense protective layer around the surface of the latex particles [29,52] prepared via conventional radical emulsion polymerization. Some reports have proposed that hydrophilic polymeric emulsifiers such as poly (ethylene glycol) ethyl ether methacrylate (PEG-EEA) can be copolymerized with St to prepare a stable emulsion [29,30,53] via conventional radical polymerization. Furthermore, MAA can cooperate with PEGMA to stabilize an emulsion with St units [40] via conventional radical polymerization. Considering the above properties of polymers containing St monomer units, a high-solid-content styrene–acrylic emulsion may be prepared by adding PEGMA to an RITP system. As shown in Table 3, when there was no PEGMA in the polymerization system, the emulsion was of high viscosity and poor fluidity. In Tables 3 and 4, the total mass of PEGMA and MAA was constant, while the mass ratio of PEGMA/MAA was changed; no MMA was added, and St was added; the mass of other ingredients was constant. Increasing the mass content of PEGMA resulted in lower monomer conversion, lower molecular weight, higher index of molecular weight distribution (*Ð*), lower diameter, lower viscosity, and good fluidity. The dense protective layer derived from PEGMA on the latex particles may hinder the access of hydrophobic monomers such as St, BA, and BMA, which may not be beneficial for the increase in particle size. The steric effects depend upon the size of the substituents [54], whereby larger substituents in the PEGMA may lead to a larger steric effect; thus, propagating chain radicals with PEGMA leads to a larger steric effect than that with ammonium salt of MAA, resulting in lower reactivity, a lower propagating reaction rate, and lower monomer consumption. Therefore, an increase in PEGMA may also lead to a lower molecular weight and lower monomer conversion. Furthermore, the dense protective layer may decrease the frictional effect between particles, which may decrease the viscosity of the emulsion. The emulsion was stable when the mass ratio of PEGMA/MAA was 5/5, indicating that half the referenced mass amount of MAA could be used to prepare a stable emulsion with analogous *M*<sup>n</sup> range and low viscosity. However, when the mass ratio of PEGMA/MAA was 7/3, the stability of the emulsion was not good, and a little white precipitate existed in the emulsion. This phenomenon indicated that enough MAA is needed to guarantee higher monomer conversion and maintain the stability of the emulsion with St units in the polymer chain. Furthermore, a high mass content of PEGMA may not guarantee high monomer conversion, high molecular weight, and low polydispersity index in this polymerization system. The above results show that the cooperation of PEGMA and MAA facilitated stabilization of the styrene–acrylic emulsion, and all MMA monomers could be replaced by St.

**Table 3.** Results of emulsion with PEGMA units in polymer chain.


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


**Table 4.** Properties of the cured film synthesized by emulsion with PEGMA units in polymer chain.

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

From the perspective of the chemical structure of PEGMA, no groups in PEGMA monomer units exist that can react with MF resin. This property is different from that of MAA units or HEMA units. Thus, it is necessary to research the influence of the mass ratio of MAA/PEGMA on the properties of the cured film. The pencil hardness rank of the cured film was not changed by the mass ratio of MAA/PEGMA, as shown in Table 4. The adhesive property worsened when the mass ratio of PEGMA/MAA was 7/3. The water absorption decreased with the increase in mass ratio, which may be because less MAA decreased the hydrophilicity of the chain segment in the crosslinked film when no MMA participated in polymerization. The best results in terms of good pencil hardness, good adhesive property, and low water absorption were obtained when the mass amount of PEGMA was equal to that of MAA.

From the perspective of the chemical structure of PEGMA, it can be used as a polymeric emulsifier. However, the emulsification ability of PEGMA in this emulsion polymerization was unknown. As shown in Table 5, the emulsion was stable when the polymeric emulsifier consisted of neutralized MAA, and all St monomer units were replaced by MMA. However, the emulsion was not stable when all MAA was replaced by PEGMA, and there existed flocculation that could not be dispersed in water or THF. These phenomena suggest that the emulsification ability of neutralized MAA units in the polymer chain was stronger than that of PEGMA, and the emulsion was not stable when all neutralized MAA was replaced by PEGMA. The hydrophilic/lipophilic balance (HLB) value of the ammonium salt of MAA is 21.25, while that of PEGMA is 9.68. Thus, the lipophilic property of PEGMA is stronger than that of neutralized MAA. However, the steric hindrance due to the side group of PEGMA is larger than that of neutralized MAA, which may result in a lower reactivity of propagating radical with PEGMA in the chain than that of the neutralized MAA. In summary, the emulsion ability of neutralized MAA throughout the polymerization period was stronger than that of PEGMA. St is more hydrophobic than MMA; hence, the emulsion polymerization with St units must be conducted with the addition of neutralized MAA.

**Table 5.** Results of emulsion with MMA units in polymer chain.


Conditions: MAA solution was neutralized by ammonia solution; *m*(MAA) + *m*(PEGMA) = 1.664 g; *n*(HEMA)*/n*(BMA)*/n*(BA)*/n*(ACPA) */n*(I2) = 18.74/40.36/7.46/1.15/1; *m*(BA)*/m*(MMA) *=* 1/2; no St in the emulsion; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

> As shown in Table 3, when the mass ratio of PEGMA/MAA was 3/7, the viscosity was much higher than that of the others. As mentioned in Table 1, changing the mass ratio of MMA/St could tune the viscosity when no PEGMA units existed in the polymer chain. Similarly, changing the mass ratio of MMA/St could tune the viscosity when MAA and PEGMA were used as polymeric emulsifiers. This experiment was done, and the results are shown in Table 6. In Tables 6 and 7, the total mass of MMA and St was constant, while

the mass ratio of MMA/St was changed; the mass of other ingredients was constant, and the monomer conversion was higher than 95%. The monomer conversion decreased when the mass ratio was increased from 0/10 to 8/2. The measured molecular weight decreased when the mass ratio of MMA/St was increased from 0/10 to 4/6, while the measured molecular weight was reduced when the mass ratio of MMA/St was increased from 4/6 to 8/2. The measured molecular weight ranged between 21,000 and 28,000 g·mol<sup>−</sup>1, and the index of molecular weight distribution ranged between 1.55 and 1.87. The largest *d*p existed when the mass ratio of MMA/St was 8/2, and *d*p at the six mass ratios ranged from 330 to 430 nm. When no MMA or no St took part in the polymerization, the viscosity was higher than 1100 mPa·s. The smallest viscosity existed when the mass ratio of MMA/St was 4/6. The viscosity of the emulsion with no St units in the polymer chain was larger than that with both St and MMA units in the polymer chain. This indicates that a styrene–acrylic polymer with analogous molecular weight and lower viscosity could be prepared when MAA was combined with PEGMA, and the emulsion with some MMA replaced by St was stable when PEGMA was added to the system.

**Table 6.** Results of emulsion with MMA and St units in polymer chain.


Conditions: *m*(MMA) + *m*(St) = 2.054 g, *m*(MAA) + *m*(PEGMA) = 1.664 g, and *m*(PEGMA)/*m*(MAA) = 3/7; *n*(MAA)/*n*(PEGMA)/*n*(HEMA)/ *n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2) = 12.60/0.98/18.74/40.36/7.46/1.15/1; ammonia solution (1.10 g); *m*(BA)/[*m*(St) + *m*(MMA)] = 1/2; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.



Conditions: *m*(MMA) + *m*(St)= 2.054 g, *m*(MAA) + *m*(PEGMA) = 1.664 g, and *m*(PEGMA)/*m*(MAA) = 3/7; *n*(MAA)/*n*(PEGMA)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2) = 12.60/0.98/18.74/40.36/7.46/1.15/1; *m*(BA)/ [*m*(St) + *m*(MMA)] = 1/2; ammonia solution (1.10 g); the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

In Table 7, the properties of the cured film synthesized by emulsion with MMA units and St units in the polymer chain are shown. Here, the pencil hardness of the cured films was at rank 2 in most cases, and the adhesive property was good for all six mass ratios of MMA/St. The toluene absorption of the cured film ranged between 12.3 wt.% and 16.1 wt.%, and the difference was overall minimal. The water absorption of the cured film ranged between 2.20 wt.% and 5.75 wt.%. The best results in terms of good pencil hardness, good adhesive property, low toluene absorption, low water absorption, and good water resistance of the modified film were obtained when the mass ratio of MMA/St was 10/0.

In summary, PEGMA could be combined with MAA in the reaction system to stabilize a styrene–acrylic emulsion with HEMA units in the polymer chain, and an emulsion polymer with analogous molecular weight range and relative low viscosity was synthesized. Moreover, the cured film exhibited a good hardness property, good adhesive property, low toluene absorption, low water absorption, and good water resistance.

#### *3.2. Influence of the Amount of PEGMA on Emulsifier-Free Polymerization*

In our previous paper, MAA accounting for at least 8.6 wt.% of the total monomer mass was added to stabilize the emulsion; however, the viscosity of the polymerization upon changing the amount of MAA was higher than 1200 mPa·s for most of the experiment [28]. As mentioned before, the viscosity of the emulsion without MMA units in the polymer chain could be decreased when the proportion of PEGMA was increased. Furthermore, the emulsion with St units in the polymer chain was stable and of relatively low viscosity for the combination of MAA and PEGMA. Therefore, it is necessary to study the influence of the amount of PEGMA on the emulsion when both MMA and St are involved in the polymerization.

In Tables 8 and 9, the mass of PEGDMA was changed, and the mass ratio of MMA/St was constant; the mass of other ingredients was constant, and the total mass content was constant. Monomer conversion was over 98%, and the solid content of the emulsion could reach 45 wt.%, as shown in Table 8. The monomer conversion did not change obviously with the change in mass ratio. The measured molecular weight decreased when the mass ratio PEGMA/MAA was increased from 1/7 to 5/7. The reason may be that PEGMA could be used as an emulsifier and monomer, and the polymeric emulsifier favored the generation of oligomers, whereas the generated oligomers increased the index of molecular weight distribution (*Ð*) and the PDI; *Ð* or PDI was increased at this mass ratio. The molecular weight was increased when the mass ratio was increased from 5/7 to 12/7. The reason may be that PEGMA was used as a monomer, and the concentration of monomers increased with the increase in PEGMA amount, while the increase in monomer concentration could increase the average degree of emulsion polymerization [55]. The measured molecular weight ranged between 18,000 and 22,000 g·mol−1, and the index of molecular weight distribution ranged between 1.70 and 1.91. Larger-diameter particles existed when the mass ratio of PEGMA/MAA was 5/7 or 7/7. The viscosity ranged from 520 mPa·s to 770 mPa·s in most cases. When the mass ratio of PEGMA/MAA was between 5/7 and 12/7, the viscosity was increased with the increase in PEGMA, but the tendency was opposite for the molecular weight distribution index and the diameter. The emulsion was stable throughout the reaction and remained stable for 6 months.

The *T*<sup>g</sup> of the emulsion polymer decreased with the increase in PEGMA, as shown in Figure 2. According to the Fox equation of *T*<sup>g</sup> [14], *T*<sup>g</sup> of a random copolymer can be changed as a function of the weight fractions of the monomer unit and *T*g of the component monomer, whereby more of the soft monomer unit leads to a decrease in the polymer *<sup>T</sup>*g. The *<sup>T</sup>*<sup>g</sup> of the PEGMA homopolymer (*M*<sup>n</sup> of PEGMA monomer is 475 g·mol−1) is −62.8 ◦C [56]; hence, PEGMA is a soft monomer. Thus, a greater amount of PEGMA would lead to a decrease in the *T*<sup>g</sup> of a random polymer. Moreover, the *Tg* at all six mass ratios was in the range of room temperature applied for acrylic resin, indicating that the emulsion has a potential application in coating.


**Table 8.** Influence PEGMA on emulsion with St units and MMA units in polymer chain.

Conditions: *m*(MMA) + *m*(St) = 2.054 g and *m*(MMA)/*m*(St) = 4/6; *m*(MAA) = 1.165 g; *n*(MAA)/*n*(HEMA)/*n*(MMA)/*n*(St)/*n*(BMA)/ *n*(BA)/*n*(ACPA)/*n*(I2) = 12.60/18.74/7.64/11.02/40.36/7.46/1.15/1; ammonia solution (1.10 g); *m*(BA)/[*m*(St) + *m*(MMA)] = 1/2; the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.


**Table 9.** Properties of the cured film with different amounts of PEGMA.

Conditions: *m*(MMA) + *m*(St) = 2.054 g and *m*(MMA)/*m*(St) = 4/6; *m*(MAA) = 1.165 g; *n*(MAA)/*n*(HEMA)/ *n*(MMA)/*n*(St)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2) = 12.60/18.74/7.64/11.02/40.36/7.46/1.15/1; *m*(BA)/[*m*(St) + *m*(MMA)] = 1/2; ammonia solution (1.10 g); the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

As illustrated in Table 9, the pencil hardness rank of the cured film was at 2 H in most cases, and the adhesive property was good for all six mass ratios of PEGMA/MAA. The hardness and the adhesive property were not influenced by the mass ratio of PEGMA/MAA. Water absorption ranged between 2.80 wt.% and 7.90 wt.%. Water absorption was increased between the mass ratios of 1/7 and 7/7. The reason may be that, when the amount of MAA was constant and the amount of PEGMA was less than MAA, more PEGMA was located on the surface of the film, as PEGMA units are hydrophilic. PEGMA is a soft monomer, and PEGMA units favor the movement of chain segments in the crosslinking reaction, thereby allowing more polymer to react with amino resin. This may have enhanced the crosslinking reaction and decreased the hydrophilicity of the modified film. Thus, water absorption was decreased between the mass ratios of 7/7 and 12/7.

**Figure 2.** *T*g of the emulsion polymer with different mass ratios of PEGMA/MAA.

The tensile strength and the elongation of dried emulsion polymer films are shown in Figure 3. In Figure 3A, the dried emulsion polymer film with *T*<sup>g</sup> = 29.9 ◦C exhibited the largest maximum tensile strength (over 5.0 MPa). The maximum tensile strength of the dried emulsion polymer film was increased when the *T*<sup>g</sup> of the emulsion polymer was increased, but the trend was opposite for the elongation at break. Below *T*g, there is insufficient energy for whole segments of the polymer chains to move; hence, the polymer

film is stiff and deformation is resisted [57]. When the film is elongated at a temperature lower than *T*g, a higher *T*<sup>g</sup> of the tested polymer leads to higher energy, enabling whole segments to move and more outside force to elongate the polymer film. Thus, the maximum tensile strength of the dried emulsion film increased with the increase in *T*<sup>g</sup> when the tested temperature was below *T*g. The largest maximum tensile strength (5.39 MPa) in the polymer with *T*<sup>g</sup> = 29.9 ◦C was higher than that of the polyacrylate polymer with *T*g = 43.9 ◦C (2.98 MPa) in our previous paper, indicating that the styrene–acrylic emulsion has potential application in preparing materials with mechanical properties.

**Figure 3.** Stress–strain curves of (**A**) dried emulsion polymer film, and (**B**) the film modified via reaction of emulsion polymer with MF resin.

The maximum tensile strength of the modified film was higher than that of the film unmodified, as shown in Figure 3B. When half the mass of the reference MF resin was used, the maximum tensile strength of the modified film was more than 5.5 MPa. These results indicate that the tensile strength property of the emulsion film could be improved via modification of the polymer with HEMA units. Hydroxyl groups from HEMA units in the polymer chain react with MF resins to form a crosslinking structure [22], which restricts the motion of the polymer chains [18]; therefore, the strength of the modified film was higher than that of the emulsion film without modification.

In conclusion, the best results in terms of measured molecular weight, index of molecular weight distribution, particle size, viscosity, solid content, adhesive property rank, pencil hardness rank, and maximum tensile strength were obtained when the mass ratio of PEGMA/MAA was 7/7.

#### *3.3. Influence of Iodine on Copolymerization*

In Table 10, no MMA was added, hard monomer St was added, and the mass ratio of PEGMA/MAA was 1/1; the mass of iodine was changed. As shown for runs 1a to 5a in Table 10, the molar ratio of ACPA/I2 was 1.15 when 1.0 times the mass amount of ACPA was added, and the measured *M*<sup>n</sup> increased overall with the decrease in iodine, while the highest measured *<sup>M</sup>*<sup>n</sup> was no more than 30,000 g·mol−1. However, the increment of *M*<sup>n</sup> was not significant. The monomer conversion was no more than 97%, and the solid content was no more than 41 wt.%. In Tonnar's work [38], the molar ratio of ACPA/I2 was 1.6, and the measured *M*<sup>n</sup> increased obviously with the decrease in I2 in the presence of ACPA, while the highest measured *<sup>M</sup>*<sup>n</sup> was 47,000 g·mol−1. This molar ratio of ACPA/I2 could be used to prepare a polymer with a measured *<sup>M</sup>*<sup>n</sup> of more than 30,000 g·mol−1. When 1.4 times the mass amount of ACPA was added, the molar ratio of ACPA/I2 was 1.61, the measured *<sup>M</sup>*<sup>n</sup> was increased from 19,400 g·mol−<sup>1</sup> to 32,900 g·mol−<sup>1</sup> (as shown for runs 1b to 5b in Table 10), and the largest measured *<sup>M</sup>*<sup>n</sup> was more than 30,000 g·mol<sup>−</sup>1. As shown for runs 3a to 3b or runs 5a to 5b, the measured *M*<sup>n</sup> for 1.4 times the reference mass amount of ACPA was higher than that for 1.0 times the reference mass amount of ACPA; the reason for this phenomenon is unknown. The monomer conversion was more than 98%, and the solid content was over 42 wt.%. The monomer conversion in Table 10 did not change obviously overall when the iodine amount was increased. The monomer conversion, diameter, and solid content for 1.4 times the reference mass amount of ACPA were higher than those for 1.0 times the reference mass amount of ACPA.

**Table 10.** Influence of iodine on emulsion with St units in polymer chain.


<sup>α</sup> *m*(I2)0 = 0.273 g; *m*(ACPA) = *m*(ACPA)0 = 0.346 g. Conditions: *m*(PEGMA)/*m*(MAA) = 1/1 and *m*(MAA) + *m*(PEGMA) = 1.664 g; no MMA in the polymerization system; *n*(St)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2)0 = 18.36/18.74/40.36/7.46/1.15/1; *m*(BA)/*m*(St) = 1/2; ammonia solution (0.70 g); the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory. <sup>β</sup> *m*(I2)0 = 0.273 g; *m*(ACPA) = 1.4*m*(ACPA)0 = 0.485 g. Conditions: *m*(PEGMA)/*m*(MAA) = 1/1 and *m*(MAA) + *m*(PEGMA) = 1.664 g; no MMA in the polymerization system; *n*(St)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2)0 = 18.36/18.74/40.36/7.46/1.61/1; *m*(BA)/*m*(St) = 1/2; ammonia solution (0.70 g); the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory.

The hard monomer St is usually combined with MMA in radical polymerization for determination of the copolymerization parameters based on kinetic data and quantumchemical considerations [58]. Thus, we investigated whether the above molecular weight tendency in Table 10 existed in the styrene–methyl methacrylate-based styrene–acrylic emulsion polymer. In Table 11, the mass ratio of MMA/St was 4/6, the mass ratio of PEGMA/MAA was 3/7, and the mass of iodine was changed. As shown for runs 1b to 3b in Table 11, the monomer conversion was not changed obviously with the increase in the iodine amount, but the measured *M*<sup>n</sup> for 1.4 times the mass amount of ACPA increased obviously with the decrease in iodine, and the highest measured *M*<sup>n</sup> could reach 40,000 g·mol−1. This molecular weight tendency could have led to some changes in the mechanical property of the polymer film. Thus, it was necessary to measure the tensile strength of the dried emulsion film with different measured *M*n. The maximum tensile strength was increased with the increase in polymer *M*n, and the largest maximum tensile strength was more than 5.5 MPa, as shown in Figure 4. Polymer chains with a high molecular weight become large and are, hence, entangled [18]; thus, a higher molecular weight promotes entanglements, which can act as junction points and govern the material's mechanical response [59]. Thus, a polymer with a high molecular weight exhibits high strength, including tensile strength. The elongation at break of the dried emulsion film with *<sup>M</sup>*<sup>n</sup> 40,700 g·mol−<sup>1</sup> was more than 100%, indicating flexibility of the polymer film. Therefore, the polymer with the highest *<sup>M</sup>*<sup>n</sup> over 40,000 g·mol−<sup>1</sup> has some significance, and this polymerization methodology may provide potential application for preparing styrene–acrylic emulsions used in materials with excellent mechanical properties.

**Table 11.** Influence of iodine on emulsion with MMA and St units in polymer chain.


<sup>α</sup> *m*(I2)0 = 0.273 g; *m*(ACPA) = *m*(ACPA)0 = 0.346 g. Conditions: *m*(MMA)/*m*(St) = 4/6 and *m*(MMA) + *m*(St) = 2.054 g; *m*(BA)/ [*m*(MMA) + *m*(St)] = 1/2; *m*(PEGMA)/*m*(MAA) = 3/7; *m*(MAA) + *m*(PEGMA) = 1.664 g; *n*(MAA)/*n*(PEGMA)/*n*(HEMA)/*n*(BMA)/ *n*(BA)/*n*(ACPA)/*n*(I2)0 = 12.60/0.98/18.74/40.36/7.46/1.15/1; ammonia solution (1.10 g); the total mass of ingredients without ACPA solution and I2 was maintained at 30.08 g in theory. <sup>β</sup> *m*(I2)0 = 0.273 g; *m*(ACPA) = 1.4*m*(ACPA)0 = 0.485 g. Conditions: *m*(MMA)/*m*(St) = 4/6 and *m*(MMA) + *m*(St) = 2.054 g; *m*(BA)/[*m*(MMA) + *m*(St)] = 1/2; *m*(PEGMA)/*m*(MAA) = 3/7; *m*(MAA) + *m*(PEGMA) = 1.664 g; *n*(MAA)/*n*(PEGMA)/*n*(HEMA)/*n*(BMA)/*n*(BA)/*n*(ACPA)/*n*(I2)0 = 12.60/0.98/18.74/40.36/7.46/1.15/1; ammonia solution (1.10 g); the total mass of ingredients without ACPA solution and I2 was kept at 30.08 g in theory.

> The above result indicates that a styrene–acrylic emulsion polymer with a relatively higher molecular weight could be prepared by reducing the I2 amount when 1.4 times the mass amount of initiator was used, and the dried emulsion film could exhibit a larger maximum tensile strength at higher molecular weight. Therefore, the polymer film has potential application in materials with excellent mechanical properties.
