## 3 Results and Discussion  
### Solution Polymerization  
The polymer solutions resulted after removing the low molecular mass compounds had final concentrations around 5 wt.% and pH about 2.5-3. The number of carboxyl groups for **PAA** and its copolymers with HEMA was determined by pH and conductometric titration with NaOH solution (\(4.76\times 10^{-2}\) M). Typical titration curves for weak acids were obtained, as can be seen in Figure 1. For a weak acid such as **PAA** or its copolymers, the conductometric titration was more suitable for identifying the equivalence point (Figure 1b), while the pH titration curves were used to find out the apparent acidity exponent of polymer pKa,app (Figure 1a). Literature data indicated a pKa of 4.25 for AA, while values starting from 4.5 to 6.8 were reported for PAA [33; 34; 35; 36; 37]. Nevertheless, covalently binding of the AA residues within the polymer chain negatively affected the degree of dissociation due to the repulsive interactions between the neighboring carboxylate groups. Indeed, a pKa,app of 6.36 was obtained for **PAA**. For **PAA** the number of moles of carboxyl groups determined from the volume of NaOH solution used to attain the equivalence point (\(1.33\times 10^{-2}\) mole/g of polymer) was very close to that theoretically expected (\(1.39\times 10^{-2}\) mole/g of polymer). For copolymers, the differences between the two quantities (experimentally and theoretically obtained) could offer some insights about the ratio between AA and HEMA units in copolymer backbone. It can be easily observed in Figure 1a,b that the volume of NaOH solution used to neutralize all carboxyl groups was much lower for copolymers compared with **PAA** in spite of the fact that samples having the same concentration of polymer were used. Thus, the number of carboxyl groups determined from titration curves represented only 60% from the theoretical number calculated for **PAH9** and 45% for **PAH18**. Even though the method was not very accurate, it could be concluded that the molar ratio between AA and HEMA units in copolymer was smaller than in the initial mixture of monomers. Thus, although the ratio between AA and HEMA in the initial monomer mixture was very high for both copolymers (18 and 9, respectively), due to the very uneven reactivity ratios of these monomers, the copolymer composition was different, with much smaller ratios between the same monomer units. The values reported for reactivity ratios being 4.382 for HEMA and 0.131 for AA [38], it was expected that the copolymer would be enriched in HEMA units when compared with the substrate. This was proved also by the fact that for a molar ratio of AA/HEMA equal to 3/1, the resulting copolymer was insoluble in water at low pH. Such behavior was due to a larger content of HEMA monomer units than AA ones in **PAH3** which were responsible for reducing the water solubility of this copolymer. On the other hand, based on the product value of the two reactivity ratios (0.574), the copolymerization of the discussed comonomer mixtures might be considered as nonideal, being placed almost symmetrically between the alternating and ideal type of radical copolymerization. This conclusion is in good agreement with that arising from the Q-e scheme. Even though the reactivity ratios calculated according to this empirical model (1.460 for HEMA and 0.355 for AA) [39] were significantly different from the same quantities experimentally determined, the product of them (0.518) practically indicated a quite similar nonideal kind of copolymerization. Copolymers **PAH18** and **PAH9** are stronger acids than **PAA**, as can be seen in Figure 1. As the number of HEMA monomer units increased in the copolymer, the distance between carboxyl groups increased, favoring their dissociation. As a consequence, pK\({}_{\text{a,app}}\) was expected to be lower for copolymer **PAH9**. Indeed, these values experimentally obtained were 6.18 for **PAH18** and 5.83 for **PAH9**.