A novel initiating system for wool grafting

2.1 Materials

Scoured and bleached Marino 590 wool fabric (100%) (323 g/m², 2/1 twill) was supplied by Misr Spinning and Weaving Co., El-Mahalla El-Kobra, Egypt. The plain weave fabric [22 picks (weft) and 25 ends (warp)] was used without further purification and cut into samples of known weight (≈1 g).

2.2 Chemicals

2.3 Procedures

3.1 Tentative mechanisms of grafting reaction

The most probable mechanisms of initiation, propagation and termination reactions that fit the results may be written as shown under.

3.1.1

3.1.1 Initiation

First of all, SPB should be represented by the formula NaB0₂·H₂0₂·3H₂0, since its properties appear to be those corresponding to an addition product of hydrogen peroxide rather than to a salt of some hypothetical peroxyboric acid.

SPB is produced by the reaction between sodium metaborate (SMB) and hydrogen peroxide in aqueous solution (Eq. (1)):

(1)

$$\underset{\left(\text{SMB}\right)}{{\text{NaBO}}_2}+{\text{H}}_2{\text{O}}_2+3{\text{H}}_2\text{O}\to \underset{\left(\text{SPB}\right)}{{\text{NaBO}}_3.4{\text{H}}_2\text{O}}$$ When SPB is dissolved in water, it is then broken down (hydrolyzed) into the starting materials, viz., SMB and H₂O₂, from which SPB is formed. SBP, upon hydrolysis, exhibits an alkaline reaction, because one of the resultant components, SMB, is converted into metaboric acid (MBA, weak acid) and sodium hydroxide (strong base) (Eq. (2)). The equilibrium of this hydrolysis shifts, according to Le Chatelier’s principle, to the direction of the base formation. This explains the high alkalinity of the initial aqueous solution of SPB (pH ≈ 10.5).

Strictly speaking, when SPB was coupled with Fe²⁺- treated wool fabric in the presence of MAA (or either of the acrylic monomers in this work) in aqueous acidic medium, it releases H₂O₂ (Eq. (3)). Once the latter is generated, it subtends Fe²⁺ ions deposited all-over the wool fabric surface. Hence, a very efficient Fe²⁺-H₂O₂ redox initiation system is well established (Eq. (4)). This redox pair is well-known as Fenton’s reagent. The ferrous ion initiates and catalyzes the decomposition of H₂O₂ liberated from SPB, resulting in the generation of highly reactive hydroxyl radicals (HO^•) (Sawyer, 1991; Wörner and Braun, 1998; Yoon et al., 2001). The mechanism involves a one-electron transfer from the ferrous ion to the peroxide with the dissociation of the oxygen–oxygen bond and the generation of one hydroxyl radical and one hydroxyl ion:

(2)

$$\underset{\left(\text{SMB}\right)}{{\text{NaBO}}_2}+{\text{H}}_2\text{O}\rightleftharpoons \underset{\left(\text{MBA}\right)}{{\text{HBO}}_2}+\text{NaOH}$$

(3)

$$\underset{\left(\text{SPB}\right)}{{\text{NaBO}}_3.4{\text{H}}_2\text{O}}\to \underset{\left(\text{SMB}\right)}{{\text{NaBO}}_2}+{\text{H}}_2{\text{O}}_2+3{\text{H}}_2\text{O}$$

(4)

$${\text{Fe}}^{2+}+{\text{H}}_2{\text{O}}_2\to {\text{Fe}}^{3+}+{\text{HO}}^{-}+{\text{HO}}^{\text{•}}(\text{Chain initiation})$$

(5)

$${\text{Fe}}^{2+}+{\text{HO}}^{\text{•}}\to {\text{Fe}}^{3+}+{\text{HO}}^{-}(\text{Chain termination})$$ Moreover, the newly formed ferric ions may catalyze hydrogen peroxide, causing it to be decomposed into perhydroxyl radical ( ${\text{HO}}_2^{\text{•}}$ ), peroxide radical anion ${\text{O}}_2^{-\text{•}}$ , water and oxygen (this reaction is referred to as a Fenton–like reaction). Ferrous ions and radicals are also formed in the reactions. The generation of perhydroxyl radical involves a complex reaction sequence in an aqueous solution (Eq. (6)) (Neyens and Baeyens, 2003). The reactions are as shown in Eqs. (6)–(12):

(6)

$${\text{Fe}}^{3+}+{\text{H}}_2{\text{O}}_2\leftrightarrow \text{Fe}-{\text{OOH}}^{2+}+{\text{H}}^{+}$$

(7)

$$\text{Fe}-{\text{OOH}}^{2+}\to {\text{Fe}}^{2+}+{\text{HO}}_2^{\text{•}}$$

(8)

$${\text{Fe}}^{2+}+{\text{HO}}_2^{\text{•}}\to {\text{Fe}}^{3+}+{\text{HO}}_2^{-}$$

(9)

$${\text{Fe}}^{3+}+{\text{HO}}_2^{\text{•}}\to {\text{Fe}}^{2+}+{\text{O}}_2+{\text{H}}^{+}$$

(10)

$${\text{Fe}}^{3+}+{\text{HO}}_2^{-}\to {\text{Fe}}^{2+}+{\text{O}}_2^{\overline{\text{•}}}+{\text{H}}^{+}$$

(11)

$${\text{HO}}^{\text{•}}+{\text{H}}_2{\text{O}}_2\to {\text{H}}_2\text{O}+{\text{HO}}_2^{\text{•}}$$

(12)

$${\text{HO}}_2^{\text{•}}+{\text{HO}}^{-}\to {\text{O}}_2^{\overline{\text{•}}}+{\text{H}}_2\text{O}$$ As seen in Eq. (11), H₂O₂ can act as an HO^• scavenger as well as an initiator (Eq. (4)). The oxidation rate of ferrous ion by hydroxyl radical (Eq. (8)) is about 1000 times faster than the reduction rate of ferric ion (Eq. (9)) (Walling et al., 1974; Walling and El-Taliawi, 1973). The continued catalytic decomposition of hydrogen peroxide is usually limited by the regeneration of ferrous ion. In most reaction conditions, it is correlated with the solubility of ferric salts, since ferric ions usually rapidly precipitate and therefore become unavailable.

H₂O₂ extricated from SPB is also catalyzed by the other reducible transition metal ions in the current work. A generalized mechanism is propounded to describe the initiation role of H₂O₂ catalyzed by these metal ions (Mⁿ⁺):

(13)

$${\text{M}}^{\mathbf{n}+}+\text{HOOH}\to {\text{M}}^{(\mathbf{n}+\mathbf{1})+}+{\text{HO}}^{-}+{\text{HO}}^{\text{•}}$$

(14)

$${\text{M}}^{(\mathbf{n}+\mathbf{1})+}+\text{HOOH}\to {\text{M}}^{\mathbf{n}+}+{\text{H}}^{+}+{\text{HO}}_2^{\text{•}}$$ It is worth mentioning, that in addition to the reactions given in Eqs. (13) and (14), it is necessary to take into consideration metal ion (Mⁿ⁺) interactions with monomer with the formation of radicals capable of initiating acrylic monomer homopolymerization (Eq. (17)). Interaction of (Mⁿ⁺) with water in the grafting medium to form hydroxyl radicals (Eq. (18)) cannot be ruled out.

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Of the free radical species (R^•⚌HO^•, ${\text{HO}}_2^{\text{•}}$ , ${\text{O}}_2^{-\text{•}}$ ) formed, HO^• radicals are extremely reactive intermediates that are capable of several reactions: hydrogen radical abstraction, electrophilic addition, radical coupling, and electron transfer reactions (Legrini et al., 1993).

Generation of these active species is greatly accelerated in strongly acidic medium. These radicals have no ability to abstract H-atoms from the wool peptide chain, disinclination of the wool itself to be grafted with MAA, or any of the acrylic monomers in the current work, using SPB alone proves this. Nevertheless, the active radical species (R^•) are capable of abstracting the more mobile H-atoms from the Fe²⁺-wool peptide chain to form primary wool macroradicals (Eq. (19)) (Wool^•). Studies on native and modified (reduced, oxidized, alkylated, etc.) wool fibers permitted to ascertain that the thiol cysteine groups are the preferred grafting sites (Nayak and Lenka, 1990; Arai et al., 1968).

(17)

$$\text{Wool-H}+{\text{R}}^{\text{•}}\to {\text{Wool}}^{\text{•}}+\text{RH}$$ (R^• = HO^• , ${\text{HO}}_2^{\text{•}}$ , ${\text{O}}_2^{-\text{•}}$ )

Since the grafting reactions are carried out in the aqueous medium, it is likely that the competitive reaction for grafting, viz., the homopolymerization reaction may also be initiated by the active species like H^• and/or OH^• which may be suggested by Eqs. 20, 29, 30:

(18)

$$\text{Wool-H}+{\text{R}}^{\text{•}}\to {\text{Wool}}^{\text{•}}+\text{RH}$$

(19)

$${\text{Wool}}^{\text{•}}+\text{HOH}\to \text{Wool-H}+{\text{HO}}^{\text{•}}$$

(20)

$${\text{Wool}}^{\text{•}}+\text{HOH}\to \text{Wool-OH}+{\text{H}}^{\text{•}}$$

3.1.2

3.1.2 Propagation

The free radical sites formed on the peptide chain backbone may readily interact with the acrylic monomer in the immediate vicinity to initiate chain propagation (Eq. (31)):

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where X = –H or –CH₃, Y = COOH , CONH₂, COOCH₃, or COOC₂H₅.

Hence, conversion of monomer to grafted polymer will rely upon the aptitude of the wool macroradicals to capture monomer molecules. As against this is the ability of the active species (R^•) to homopolymerize the monomer molecules. Homopolymerization may also take place through a chain transfer process involving wool macroradicals. At any event, however, current data suggest that the grafting reaction prevails considerably over the homopolymerization reaction, an advantage which categorizes the Fe²⁺-wool/SPB redox system among the most efficient initiation systems for grafting of MAA and other acrylic monomers in this work onto wool fabric.

3.1.3

3.1.3 Termination

On the line of our previous studies (Zahran, 1996), the redox initiation system containing transition metals, whether reductants or oxidants, are characterized by several termination possibilities. Termination mechanism possibilities are strongly related to the complexity of redox processes involving an organic substrate, an inorganic salt activator, an initiator as well as the experimental conditions of the graft copolymerization reaction.

The most probable termination mechanisms for the propagating grafted chain of wool involve one or more of the following routes (Eqs. 32–(28):

(i) primary radical termination.

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(ii) metal oxidative termination.

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where A⁻ represents the acidic radical of the transition metal salt used, viz., the sulfate anion and/or the anions produced from decomposition of H₂O₂ liberated from SPB (i.e. HO¯ and ${\text{HO}}_2^{-}$ )

(ii) metal reductive termination.

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(iii) bimolecular or coupling termination.

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(iv) disproportionation termination.

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With the above in mind and aiming to optimize the yield of wool graft copolymers initiated by the Fe²⁺-wool/SPB redox system, experiments were designed to study major factors affecting the graft polymerization of MAA. Factors studied include SPB concentration, ferrous ion concentration, pH of polymerization medium, grafting temperature, grafting time, MAA concentration, nature of the reductant, influence of the inhibitor as well as type of the acrylic monomer. The optimum grafting conditions were obtained by applying the grafting reaction scheme as one condition varied for a set of reactions whilst keeping the other reaction parameters constant.

3.2 SPB concentration

Graft copolymerization of MAA onto wool fabric was carried out at 70 °C for 120 min using the Fe²⁺-wool/SPB redox system for initiation. The concentration of the ferrous ammonium sulfate activator in the polymerization system was set at 0.5 mmol/L, whereas the SPB concentration varied from 5 to 150 mmol/L. Fig. 1 shows variation of the polymer yield including GY, GE, TC and HP with concentration of SPB. As is manifested in this figure, these polymer criteria, except %HP, increase with the increase of SPB concentration up to 110 mmol/L and decrease with further increase of SPB concentration. In other words, a SPB concentration of 110 mmol/L constitutes the optimal concentration of grafting under the conditions studied. The significant heightening of graft yield, grafting efficiency and MAA total conversion by enlarging SPB concentration up to 110 mmol/L could be interpreted in terms of the redox process under question. Logically, this process is greatly accelerated as SPB increases gradually until the said optimum concentration with successive production of free radical active species (Eqs. 4, 7, 10, 11, and 12) capable of initiating and propagating the grafting reaction (Eqs. (20) and (31)).

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Figure 1

Graft yield, grafting efficiency, total conversion and homopolymer as a function of SPB concentration: [FAS], 0.5 mmol/L, [MAA], 4%, grafting temperature 70 °C, grafting time, 120 min, pH, 3, M/L, ratio, 1:50.

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On the other hand , deportment of the GY, GE and TC with SPB concentration higher than 110 mmol/L is expected and due to repletion of the grafting medium by the active species R^• (R^•⚌HO^•, ${\text{HO}}_2^{\text{•}}$ , ${\text{O}}_2^{-\text{•}}$ ) whereupon they tend to participate in the termination reaction of the growing grafted wool chain (Eq. (32)), beside to their tendency to homo-combine with each other or with other reactive intermediates to form inactive species (Sawyer, 1991; Bossmann et al., 1999) (Eqs. 5,8,9, and 29–34) :

3.3 Ferrous ion concentration

Samples of wool fabric were independently treated with a solution containing a different FAS concentration (0.01–2 mmol/L) at 30 °C for 30 min using the M/L ratio of 1:30. The samples were then subjected to graft polymerization with MAA in the presence of 110 mmol/L SPB (initiator). The polymerization reaction was fixed at 70 °C for 120 min using the material-to-liquor ration of 1:50. The percentage polymer yield (%GY, %GE, %TC and %HP) was determined. The results obtained are shown in Fig. 2 . Evidently, the %GY greatly enlarges by increasing the concentration of FAS up to 1 mmol/L. Thereafter, grafting diminishes. The same holds true for %GE and %TC. On the other hand, the magnitude of homopolymerization is very low irrespective of the FAS (activator) concentration used, a point which signifies the idealistic of the Fe²⁺-Wool/SPB redox system for initiating grafting of acrylic monomers in general onto wool fabric. That the MAA graft yield and grafting efficiency attain an average values of ca. 114% and 94%, respectively, speak of this.

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Figure 2

FAS concentration versus percentage of polymer yield: [SPB], 110 ml mole/L, MAA, 4%, grafting temperature 70 °C, grafting time, 120 min, pH 3, M/L ratio, 1:50.

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These findings could be explained in terms of the roles played by Fe²⁺ ions, deposited all over the wool fabric surface, during the whole course of the polymerization reactions. It is recognized that the roles of Fe²⁺ ion during polymerization embrace different chemical features, i.e., it acts as an activator for the free radical formation, a Fe³⁺ ion-generator, and a propagating chain-terminator. The pre-eminence of one feature, or more, over another would explain the difference in the behavior of the polymer yield with increasing FAS concentration.

On the other hand, the free radical formation-activator feature of Fe²⁺ ion (Eq. (4)) seems to prevail, within the FAS concentration ranging from 0.01 mmol/L up to 1 mmol/L, thereby becoming responsible for the increase of polymer yield. The latter, encompassing GY, GE and TC, reaches maximum with the FAS concentration of 1 mmol/L. Beyond this concentration, these polymer criteria decrease, due to prevalance the other two features of Fe²⁺ ion. That is with the highest FAS concentration, the Fe²⁺ ions may participate in termination of the growing grafted wool chain (Eq. (36)) and may bring forth more and more Fe³⁺ ions. The latter may also react with the growing grafted chain bringing in termination (Eq. ()()()(32)–(34)) and convert the active initiating species to the inactive ones (Eq. (9)).

Besides, with the highest FAS concentration (2 mmol/L), the Fe²⁺ ions may attack MAA molecules to enhance homopolymerization (Eq. (17)). This explicates for the trivial increment in the homopolymer magnitude.

3.4 pH of the polymerization system

To investigate the effect of pH, graft polymerization of MAA onto wool fabric was carried out at various pH’s ranging from 1 to 5. Sulfuric acid and ammonium hydroxide were used for pH adjusting. Concentrations of SPB (initiator), FAS (activator), MAA were set at 110 mmol/L , 1 mmol/L and 4%, respectively. Polymerization was conducted at 70 °C for 120 min using a M/L ratio of 1:50. Results of this investigation are shown in Fig. 3. A perusal of the results indicates that the percentages of grafting and grafting efficiency increase markedly at lower pH’s and reaches a maximum at pH3. The main implication of this deportment is based on favoring the Fe²⁺-wool/SPB redox reaction at lower pH due to: (a) the rapid decomposition of SPB to its constituents , H₂O₂ and sodium metaborate (SMB) (Eq. (3)). The latter, in acidic medium, forms metaboric acid (MBA) with shifting the reaction 2 to right (Le Chatelier’s principle(. Accumulation of MBA assists in decomposition of SBP itself to generate more H₂O₂, the latent initiator, (b) the enhancement of the redox reaction established between Fe²⁺ deposited along with the wool chains and H₂O₂ released from SPB, (c) production of excessive amounts of ferric iron which, in strongly acidic media, in particular at pH3, participate correctly in creation of active initiating species (Eqs. 7, 10, (11), and (12)), and (d) generation of successive reactive species (R^•) capable of attacking the wool chain to form the wool macroradical (Eq. (4), (7), (10)–(12), (and) (20)).

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Figure 3

Effect of pH on polymer yield : SPB 110 mmol/L, [FAS] 1 mmol/L, [MAA], 4%, temperature 70 °C, time, 120 min, M/L ratio, 1:50.

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The lower GY and GE at pH higher than 3 are most probably due to the conflicting effects of (a–c) in the grafting medium. Moreover, at higher pH, the congregated ferric ions, produced from the primary Fe²⁺-H₂O₂ redox reaction, begin to precipitate as ferric hydroxide (Eq. (35)):

The effect of pH on the homopolymerization and total conversion is depicted in Fig. 3. Conversely to grafting and grafting efficiency, both HP and TC decrease as the pH value increases within the studied range. This substantiates that the created active species are predominantly directed to generate the wool macroradicals which, in turn initiate grafting, rather than to initiate homopolymerization. Since the total conversion is the sum of grafting as well as homopolymer, this finding implies that, lowering the latter may affect adversely on the percentage of total conversion. Besides, unfavorable influence of the polymerization medium heterogeneity on both homopolymerization and total conversion at higher pH’s should be taken into consideration.

3.5 Polymerization temperature

Figs. 4a–c show the effect of temperature on the progress of the polymerization (GY, GE, TC and HP). The graft copolymerization was carried out at three different temperatures ranging from 60 °C to 80 °C at different lengths of time (15–180 min), keeping the other variables constant. A perusal of the results indicates that the percentages of GY (Fig. 4a), GE (Fig. 4b) and TC (Fig. 4d) enlarge by raising temperature within the studied range following the order 80 °C > 70 °C > 60 °C. The opposite holds true for the demeanor of HP (Fig. 4c).

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Figure 4

(a–c) Dependence of graft yield, grafting efficiency, total conversion and homopolymer on the polymerization temperature: [SPB] 110 mmol/L, [MAA], 4%, [FAS], 1 mmol/L, pH, 3, M/L ratio, 1:50.

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The acceleration of the rates of polymerization, expressed as GY, GE and TC, upon raising the temperature from 60 °C to 80 °C could be associated with (Hebeish et al., 1996) (i) the higher rate of dissociation of SPB and thereof H₂O₂ (Eq. (3)), (ii) promotion of all redox reactions and thence the rate of the active species production increases (Eqs. (4),(6),(7), (10), (11), and (12)) which increase the number of grafting sites at a higher rate (Eqs. (20) and (31)), (iii) acceleration of the kinetic energy of the MAA molecules which resulted in the faster diffusion of greater number of monomer molecules and monomer radicals onto the wool fabric backbone and (iv) heightening the swellability of wool fabric and solubility of monomer.

The behavior of the HP as the polymerization temperature elevates from 60 °C to 80 °C (Fig. 4c) is to be anticipated since it will be in competition with grafting and is due the adverse effects of the reasons (ii)–(iv).

3.6 Duration of polymerization

Figs. 4a–d show the percentage polymer yield {expressed as graft yield (%GY), grafting efficiency (GE%), homopolymer (%HP) and total conversion (%TC)} for the graft polymerization of MAA as a function of reaction time using the Fe²⁺-wool/SPB redox system at 60 °C, 70 °C and 80 °C. It is seen (Fig. 4a–b and Fig. 4d) that for a given temperature, the %GY, %GE and %TC enlarge by lengthening the reaction time within the studied range. Furthermore, these polymer criteria are characterized, in particular at higher temperatures, by an initial fast rate followed by a slower rate.

The slower rate of polymerization observed during the later stages of the polymerization reactions could be associated with dwindling in SPB and MAA concentrations, as well as reduction in available sites for grafting on the wool backbone, as the reaction continues.

Fig. 4c shows that the %HP diminishes successively by prolonging the polymerization time from 15 up to 180 min., irrespective of the temperature applied. As mentioned earlier, the behavior of HP is foretold as it is the converse of GE, i.e. the homopolymerization decreases as the grafting efficiency increases.

3.7 Monomer concentration

The effect of MAA concentration on the percentages of GY, GE, TC, and HP was investigated by varying the monomer concentration from 1% to 6%, keeping the concentrations of other reagents constant. The results are depicted in Fig. 5. As it is obvious from this figure that, the aforesaid polymer criteria play different situations by increasing the MAA concentration. That is as the latter enlarges, the %GY is greatly ameliorated and reaches maximum value (i.e. GY ≈ 163%) with the highest MAA concentration (i.e. 6%). The %TC behaves similarly as %GY, but to a limited extent. On the other hand, the %GE, contrary to HP, increases by increasing MAA concentration and attains maximum with the concentration of 4%, then falls.

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Figure 5

Influence of methacrylic acid concentration on the polymer criteria: [SPB], 110 mmol/L, [FAS] 1 mmol/L, grafting temperature 70 °C, grafting time, 120 min, pH, 3, M/L ratio, 1:50.

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Since the copolymerization reaction was carried out at the same pH, time and temperature and at fixed concentrations of Fe²⁺ and SPB, it is possible to assume that the concentration, nature and efficiency of the free radical, and other active species generated during the polymerization reaction would the same. Hence, increasing the MAA concentration within the studied range could be attributed to the greater heaping up of MAA molecules in the close proximity of wool backbone. The MAA molecules in the immediate vicinity of reaction sites become acceptors for the wool macroradicals, resulting in chain initiation, and thereafter become free radical donors to the neighboring MAA molecules (Eq. (31)). This also explains the increments in GE percentage by increasing the MAA concentration up to 4%. However, beyond this concentration, the MAA homopolymerization starts to increase in a trivial amounts due to enhancing combination and disproportionation of poly(MAA) macroradicals causing the GE percentage to marginally diminishes.

3.8 Nature of the reductant

The influence of reducing agents (variable valency metal salts) on the polymer yield has been investigated by using the transition metal sulfates, namely, ferrous ammonium sulfate [(NH₄)₂Fe(SO₄)₂·6H₂O], manganous sulfate (MnSO₄·H₂O) and cobaltous sulfate (CoSO₄·7H₂O). These reducers were individually used to treat the wool fabric just prior to the grafting process, as described under the metathesis procedure. To disclose the activator-roles of these metal salts, temperate polymerization conditions including [SPB], 110 mmol/L, [MAA], 4%, pH 3, grafting temperature 70 °C and M/L ratio, 1:50, were applied. The reductant concentration of 1 mmol/L was used for different lengths of polymerization time. The results are manifested in Figs. 6a–d. Clearly, all the reductants examined considerably ameliorate both MAA grafting (Fig. 6a) and total conversion (Fig. 6d) percentages, following the order: Fe²⁺ ≫ Co²⁺ > Mn²⁺. However these metal ions accelerate the percentage of grafting efficiency (Fig. 6b) according to the order: Fe²⁺ > Mn²⁺ > Co²⁺.

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Figure 6

(a–d) Effect of the activator nature on the rates of graft yield, total conversion, grafting efficiency and homopolymer: [reductant], 1 mmol/L [SPB], 110 mmol/L, [MAA], 4%, pH, 3, grafting temperature 70 °C, M/L ratio, 1:50.

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Fig. 6c shows the effect of Fe²⁺, Co²⁺, Mn²⁺ salts on the MAA homopolymer formation. Obviously, the homopolymer percentage is sequentially decreases by lengthening polymerization time in presence of Fe²⁺ salt. The opposite holds true for Mn²⁺ and Co²⁺ reductant salts. The influence of Co²⁺ ion for bringing about MAA homopolymer is the highest as compared to Mn²⁺, however, the maximum value of %HP does not exceed 21% at the end of polymerization reaction (i.e. at 180 min.).

The aforementioned results may be explained in terms of the efficiency of each metal-ion reductant:

1. To enhance decomposition of SPB and thence H₂O₂ to generate the reactive species R^• (Eqs. 3,4,7),(10)–(12).

2. To autocatalyze the initiating system (Eqs. (13) and (14)).

3. To interchange into the corresponding metal oxidant (Eqs. 4,5,8 and 13).

4. To homoplymerize MAA by the metal ion – oxidant formed.

5. To reductive and/or oxidative terminate the grafting reaction (Eqs. (31)–(36)).

The first two factors are responsible for enhancement in the graft percentage. In addition, the creation of free-radical species under the influences of these reductant metal ions would be in the proximity of wool, thus assisting the formation of wool macroradicals which, in turn, initiate MAA grafting. This positively reflects on the percentage of MAA total conversion.

The last three factors restrict the growing grafted wool chain and this adversely affects the rate of grafting and enhances homopolymerization. Apparently, the factors 3–5 do not operate with Fe²⁺ ion which presents itself as the most efficient reductant transition metal salt for activation wool/SPB initiation system.

3.9 Influence of Inhibitor

Fig. 7 shows the effect of hydroquinone (HQ) concentration on the polymerization reaction of MAA using the Fe²⁺-wool/SPB redox system. In this respect, the polymerization reaction was monitored via quantitative determination of graft yield (%GY), grafting efficiency (%GE), homopolymer (HP) and total conversion ((%TC). As the results, inclusion of HQ in the polymerization medium enormously affects these polymer criteria and this proves the free radical nature of the Fe²⁺-wool/SPB redox system. Unlike HP and TC, both GY and GE successively decline by increasing the HQ concentration within the studied range (1–10 mmol/L). The suppressive effect of HQ on both GY and GE is ascribed to its strong efficacy to kill the wool macroradicals (Eq. 36) and the growing MAA grafted wool chain radicals (Eq. 37). HQ reacts with these fabric active centers via hydride radical abstraction with the formation of the highly delocalization of electron charge density throughout the hydroquinone aromatic ring. The resulting semiquinone radical cannot initiate further polymerization (grafting and homopolymerization).

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Figure 7

Influence of hydroquinone concentration on the polymer criteria: [SPB], 110 mmol/L, [FAS], 1 mmol/L, [MAA], 4%, grafting temperature, 70 °C, grafting time, 60 min, pH, 3, M/L ratio, 1:50.

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In Fig. 7 it can also be seen that that both MAA homopolymerization and total conversion unexpectedly deport in presence than in absence of HQ in the polymerization medium. That is, these two polymer criteria increase as the HQ concentration increases within the studied range. This surprising behavior for both HP and TC is possibly due to the decomposition of the latent H₂O₂ just it liberates from SPB by HQ to form HO^• (very reactive) and SQ^• (very unreactive) (Eq. 40):

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3.10 Nature of the monomer

The ability of the Fe²⁺-wool/SPB redox initiating system for individually inducing grafting AA, Aam, MA, MMA and EMA may be realized from Fig. 8. Results of the graft yields obtained with MAA are also given in the same figure for comparison. The grafting reaction of these monomers was performed under the optimum conditions arrived at with MAA and including [FAS], 1 mmol/L, [SPB], 110 mmol/L grafting time, 150 min, grafting temperature, 80 °C and M/L ratio, 1:50.

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Figure 8

Grafting of wool fabric using different acrylic monomers: [SPB], 110 mmol/L, [FAS], 1 mmol/L, pH, 3, grafting temperature 80 °C, grafting time, 150 min, M/L ratio, 1:50.

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The data of Fig. 8 reveal clearly that (1) nature and concentration of the acrylic monomer brought about differences in the magnitude of grafting onto the wool fabric , (2) the graft yield increases sequentially as the acrylic monomer concentration heightens from 1% up to 4% , irrespective of the monomer used, (3) water-soluble and water-miscible (hydrophilic) acrylic monomers, viz. Aam, AA and MAA, yield higher grafting as compared to that obtained with very sparingly water-miscible (hydrophobic) acrylic monomers, viz. MA, MMA and EMA, (4) Aam has the highest grafting value over all the acrylic monomers used, whether hydrophilic or hydrophobic, whereas MA grafting is superior to that of other hydrophobic monomers, (5) presence of the methyl group (–CH₃) attached to the unsaturated C-atom and/or the carboxylate group of the acrylic esters monomers curtails the grafting efficiency of these monomers, (6) increasing the length of the alkyl carboxylate group (by replacing the –CH₃ by –CH₂CH₃), in the molecular monomer structure suppresses its efficiency toward grafting onto the wool backbone chain, and (7) the graft yield onto the wool fabric decreases by increasing the molecular weight water-solubility of the monomer used, and follows the order :

The above findings may simply be attributed to the following:

1. As the molecular weight, and thence the molecular size of the monomer increases, its mobility within the aqueous phase and its diffusibility from this phase to the wool fabric surface is reduced. Furthermore, penetrability of monomer from the fabric surface within the fiber pores and crevices is also restricted by enlarging the monomer molecular size.

2. Introducing –CH₃ group in the molecular structure of monomer enlarges its molecular size, in one side, and establishes the steric hindrance effect in the other one. This effect is greatly accelerated by massiveness of the alkyl group, viz. by replacing –CH₃ with -CH₂CH₃.

3. Homopolymerizability of the monomer molecules which is strongly related to the structural features of the monomer molecule. That is, the monomer of strongest steric effect and weakest water-solubility, has the highest capability for homopolymerization, as EMA behaves. Homopolymerization is also hastened as the monomer concentration increases.

4. Polarizability and frangibility of the acrylic monomer double bond play a further key role on its grafting efficiency. Frangibility of the acrylic double bond homolytically is significantly influenced by the inductive effects of the substituents attached to the unsaturated C-atom.

5. Degree of water-solubility/miscibility markedly affects the monomer grafting efficiency. As the monomer solubility escalates, the ambit of the polymerization medium homogeneity broadens. This facilitates the collision between the reacting active species, monomer molecules and wool macromolecules. In addition, as monomer water-solubility/miscibility increases, the swellability of wool fabric enhances.

Indeed, reasons (i)–(v) explain the differences in the magnitude of grafting onto the wool fabric brought about by nature and concentration of acrylic monomers using the Fe²⁺-wool/SPB redox initiation system.