Polymerization of acrylamide in styrene containing inverse microemulsions: polymerization kinetics and polymer product composition studies

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1 Polymer International Polym Int 49:1483±1491 (2000) Polymerization of acrylamide in styrene containing inverse microemulsions: polymerization kinetics and polymer product composition studies Jaroslav Barton * and Viera Juranic ová Department of Polymerization Reactions, Polymer Institute, Slovak Academy of Sciences, Bratislava, Slovak Repulic Astract: The kinetics of free-radical homopolymerization and copolymerization of acrylamide (AAm) and styrene (S) initiated y water-solule ammonium peroxodisulfate, (APS) or y toluenesolule dienzoyl peroxide (DBP) in inverse microemulsion toluene/s/aot (sodium is(2-ethylhexyl)sulfosuccinate)/water/aam characterized y a low volume fraction of the aqueous phase (F aw 0.08) as a function of the concentration of S in the oil phase of the inverse microemulsion system have een studied. S strongly decreases the rate of AAm/S (co)polymerization. This is valid for oth APS and DBP initiators. Kinetic measurements indicate the important role for cross-initiation of water solule AAm growing chains and of oil solule S analogues activated y the primary free-radicals generated from APS (or from DBP) in the dispersed water droplets (or in the continuous oil-phase) of the inverse microemulsion, respectively. With inverse microemulsions containing toluene (70.73%)/S (2.44%)/AOT(17.56%)/water (7.32%)/AAm (1.95%), after polymerization (initiator APS, mol dm 3 of water) and separation of the polymeric components, the following yields were otained: AAm/S (co)polymer (89.20 mass%; ie 62.24mass% of AAm structural units and 26.96mass% of S structural units), polyacrylamide (9.4 mass%) and polystyrene (1.4 mass%). # 2000 Society of Chemical Industry Keywords: acrylamide; styrene; inverse microemulsion; free-radical (co)polymerization; kinetics; polymer product analysis INTRODUCTION The simultaneous polymerization and copolymerization in inverse microemulsion of monomers differing reasonaly in their water soluilities was studied recently. 1±9 Acrylamide (AAm), styrene (S) and methyl methacrylate (MMA) were selected for these studies. Conversion curves of copolymerization of acrylamide with S and/or MMA in inverse microemulsion show the typical course of a `dead end' polymerization. 3 It was speculated that the initial ascending part of the conversion re ects polymerization at two different loci of initiation, homopolymerization of AAm within the micelles and copolymerization of AAm and MMA (or S) at the water±toluene interface. 1,3,4 It was shown that the soluility of the monomer 10,11 in the toluene oil phase of the inverse microemulsion, even for such a highly water solule monomer as AAm, was suf cient to consider the existence of an oil-phase formation of low-molecular mass polyacrylamide chains, especially if an oil-solule initiator such as dienzoyl peroxide (DBP), was used. For this reason, an alternative mechanism for polymer particle formation and growth was proposed. 8,12 Kinetic studies on the effect of the oil-solule monomer S located in the continuous oil phase on AAm polymerization in an inverse microemulsion, 7 showed that competition took place etween the monomers in the oil phase (slow homopolymerization of S and/or (co)polymerization of S with AAm dissolved in the oil phase) and the monomers in the water pools of inverse micelles towards reaction with the initiator primary radicals or with the co-oligomer growing radicals. S also decreases the rate of terpolymerization in the termonomer system AAm/ N,N'-methyleneisacrylamide (MBAAm)/S, initiated y water-solule APS, or y toluene-solule DBP in the inverse microemulsion toluene/s/aot//water/aam/mbaam. 9 Our aim was to study the effect of an oil-solule monomer, such as S in inverse microemulsion on the kinetics of AAm/S (co)polymerization initiated y water-solule APS and/or oil-solule DBP free-radical initiators. The inverse microemulsion was characterized y the low volume fraction of the aqueous phase (aout 0.08) and also y the relatively low [water]/ [AOT] molar ratio of The effect of the selected * Correspondence to: Jaroslav Barton, Department of Polymerization Reactions, Polymer Institute, Slovak Academy of Sciences, Bratislava, Slovak Repulic Contract/grant sponsor: VEGA; contract/grant numer: 2/5006/98 (Received 10 Decemer 1999; revised version received 24 April 2000; accepted 15 May 2000) # 2000 Society of Chemical Industry. Polym Int 0959±8103/2000/$

2 J BartonÆ, V JuranicÆova S/AAm (co)monomer pair and of its concentration in inverse microemulsion on the composition of the resulting polymer product was also studied y selective toluene and/or water extraction and y nitrogen analysis of the products thus otained. EXPERIMENTAL Chemicals Acrylamide (AAm) (puriss, recrystallized; from Serva, Feiniochimica GmH, Heidelerg, Germany), sodium is(2-ethylhexyl) sulfosuccinate (AOT) (purum, from Fluka Chemie AG, Buchs, Switzerland), toluene (pa, from Lachema, Prague, Czech Repulic) and ammonium peroxodisulfate (APS) (purum, from Lachema, Prague, Czech Repulic) were used without further puri cation. Dienzoyl peroxide (DBP) (purum, from Lachema, Prague, Czech Repulic) was recrystallized from ethanol. Styrene (S) (99%, from Aldrich-Chemie, Steinham, Germany) was rendered free of inhiitor y distillation under reduced pressure of argon. Distilled water used for preparation of inverse microemulsions was deprived of oxygen y heating to oiling point and cooling under a stream of argon. Inverse microemulsions were prepared y mixing toluene, S and/or toluene/s solutions containing AOT with an aqueous solution of AAm at room temperature. In the preparation of inverse microemulsions for polymerization experiments, the water-solule initiator APS was added to the aqueous solution of acrylamide and the oil-solule initiator DBP was added to toluene, S and/or toluene/s solutions of AOT. Procedures The dilatometric technique has een descried elsewhere. 13 By derivation of the conversion curves expressing the volume contraction DV corresponding to the initial volume of AAm and S in the inverse microemulsion system V M (V M =V AAm V S ) at a given polymerization time t, polymerization rates R p =~V/ (V AAm V S )1/Dt (t is in minutes) were otained. For simpli cation they are given as R p /au (aritrary units). The percentage conversion of monomers was otained from the caliration curve expressing the variation of DV/V M with the gravimetrically determined conversion of the monomers. The polymer product otained through inverse microemulsion (for gravimetric and compositional studies) was precipitated in an excess of ethanol. The precipitated polymer was thoroughly washed with ethanol to remove the AOT surfactant. After drying in an oven at 40 C, the puri ed polymer was weighed (for gravimetric determination of the conversion of monomers) and extracted y water and/or toluene at room temperature for 240 h. The concentration of AAm structural units in the puri ed polymer product after polymerization and also after extractions was determined y elementary analysis (determination of nitrogen content). RESULTS AND DISCUSSION To evaluate the polymerization and (co)polymerization kinetics, it is necessary to consider the nature of the monomers involved with respect to their polymerization rate constants 14±16 in addition to the partitioning of the monomers and initiators etween the oil and water phases of the inverse microemulsion. Thus, for example, if initiator radicals cannot escape from the water pools of the inverse micelles and enter the oil phase, their role must take over monomer, solvent or emulsi er radicals which can e formed y addition or transfer reactions of monomer, solvent or emulsi er with radicals generated from APS. The extent of the aove-mentioned initiation mechanism depends on the concentration of the monomer present in the water pools or entering the water pools of the inverse micelles (closely related to the soluility of S in the water pools), the rate of monomer initiation in the water pools and the exit rate of the monomer radicals formed. It follows from the values of the ratios of propagation and termination rate constants k p /k t 0.5 for AAm 14 (4.72mol 0.5 dm 1.5 s 0.5 at 25 C) and S 14,15 ( mol 0.5 dm 1.5 s 0.5 at 50 C and mol 0.5 dm 1.5 s 0.5 at 60 C) that AAm will polymerize at a sustantially higher rate than S. Reactivity ratios for copolymerization of AAm (monomer 1) and S (monomer 2) are 16 r 1 =k p11 /k p12 =0.33 and r 2 =k p22 /k p21 =1.49. This means that oth monomers readily copolymerize. In other words, S, even in low quantities, owing to the two order difference etween the k p /k t 0.5 ratios for AAm and S, should effectively decrease the overall copolymerization rate of the AAm/S comonomer pair compared to the rate of AAm homopolymerization (ie in the asence of S). Tale 1 gives the asic information on the composition of individual toluene- and/or S-ased inverse microemulsions and some of their characteristics, together with the maximum homo- and (co)polymerization rates otained. Evidently, the presence of S in inverse microemulsions consideraly decreases the (co)polymerization rates. This conclusion is valid for oth water-solule APS and oil-solule DBP initiators. The results listed in Tale 1 also show the very low rates of the homopolymerization of S initiated y DBP and/or APS. Low rates of AAm/S (co)polymerization were also found in the case of APS initiator for purely S-ased inverse microemulsions. Polymerized inverse microemulsions (Tale 1, runs 1 and 1') are Winsor IV type dispersion systems, with water swollen polyacrylamide particles as dispersed aqueous phase in the continuous toluene oil phase. The initially single-phase Winsor IV inverse microemulsion structure is retained after polymerization of the S monomer present in relatively low concentrations in the toluene-rich oil phase (Tale 1, runs 2 and 3); the polymerized system is a single-phase Winsor IV 1484 Polym Int 49:1483±1491 (2000)

3 Acrylamide polymerization in styrene containing inverse microemulsions Tale 1. Composition of toluene (T, runs 1 and 1'), styrene (S, runs 4, 4', 5 and 5') and toluene/styrene (runs 2 and 3)-ased single phase Winsor IV inverse microemulsions a and the oserved maximum rate of polymerization of acrylamide and styrene at a polymerization temperature of 60 C Run T S AAm n S Φ aw c R p d (au) R p d (au) ' ' ' a All components in grams. For preparation of inverse microemulsions in all runs esides the components indicated in the tale, water (1.5g) and AOT (3.6g) were also used. APS initiator (0.0104g) for runs 1, 2, 4 and 5 was used. DBP initiator (0.050g) was used for runs 1', 3, 4' and 5'). Molar fraction of styrene with respect to the amount of acrylamide in inverse microemulsion. c Volume fraction of the aqueous phase (sum of the volumes of water and acrylamide, ie (V water V AAm ) for the preparation of inverse microemulsion efore polymerization. The oil phase is given y the sum of the volumes of toluene, styrene and/or toluene styrene, and AOT, ie (V T, or V S, or V T V S and V AOT ). d Maximum attainale polymerization rate (see Experimental). Initiator APS (runs 1, 2, 4 and 5); initiator DBP (runs 1', 3, 4' and 5'). Arithmetic average of at least two measurements. The maximum polymerization rates were otained at conversions of around 30% (homopolymerization of AAm, runs 1 and 1'); around 5% (copolymerization of AAm/S, runs 2 and 3), and elow 2% (copolymerization of AAm/S, runs 4 and 4'; homopolymerization of S, runs 5 and 5'). inverse microemulsion with water swollen AAm/S copolymer in polymer particles that are dispersed in a continuous toluene oil phase. The Winsor IV inverse microemulsion with S monomer as the main component of the oil phase (no toluene present in the system) and AAm in dispersed water droplets, upon polymerization up to low conversions of S (5%) yields a Winsor IV inverse microemulsion (Tale 1, runs 4 and 4'). At higher conversions of S, this Winsor IV inverse system changes into a viscous or even solid system containing a water swollen AAm/S copolymer mixed with S homopolymer. In the asence of AAm and toluene in the Winsor IV inverse microemulsion (Tale 1, runs 5 and 5', no toluene and AAm present in the system) the polymerization yields only S homopolymer, which is, according to the conversion of S, either dissolved in S or swollen y S. At higher conversions of S (>90%) practically ulk polystyrene is formed. Surfactant AOT micelles lled with water form an admixture in the polymer reaction product. The nature of the initiator affects the polymerization and/or (co)polymerization rates in inverse microemulsion. This is clearly seen from data of Tale 1. In the case of AAm homopolymerization (Tale 1, runs 1 and 1') and under conditions of approximately the same rate of initiation for oth APS and DBP initiators, the oserved rates of polymerization are nearly the same (within experimental error; see also ref 12. Different (co)polymerization rates for DBP and APS initiators were found for AAm/S in inverse microemulsion (Tale 1, runs 2 and 4 versus runs 3 and 4'). Here, the rates are clearly higher for DBP initiator. In the asence of AAm, ie for homopolymerization of S in inverse microemulsion, a rate value ve times as high was oserved for DBP. These results indicate transfer of the oil-solule monomer radicals of AAm and/or of S from the oil phase into inverse micelles containing the major part of AAm monomer. For homopolymerization of S initiated y APS initiator residing in inverse micelles (Tale 1, run 5), a possile explanation includes the formation of S monomer radicals via addition reaction of sulfate anion radicals onto S molecules dissolved in inverse micelles. Styrene monomer radicals exit from the inverse micelles and enter the continuous oil phase, which contains the major part of S monomer, and initiate the polymerization of S. The low soluility of S in water results in a relatively low yield of S monomer radical concentration in the inverse microemulsion, and thus in the low polymerization rate oserved. On the asis of the experimental results presented, the formation of toluene or AOT emulsi er radicals and their participation in the polymerization process, seems improale. In spite of the fact that the concentration of AOT emulsi er in inverse microemulsion is sustantially higher than the overall concentration of monomer(s), no reasonale effect of AOT concentration on the polymerization and/or (co)polymerization rates was oserved. Figure 1 shows the conversion curves of polymerizing inverse microemulsions versus polymerization time. The very high rate of AAm polymerization in the asence of S (an AAm conversion of >99% was otained in less than 30 min) strongly contrasts with Figure 1. Dependence of the conversion C, for AAm/S copolymerization in inverse microemulsion toluene/s/aot/water/aam on polymerization time for runs 1, 2, 4 and 5 in Tale 1; temperature 60 C: *, run 1; ~, run 2; &, run 4; *, run 5. Polym Int 49:1483±1491 (2000) 1485

4 J BartonÆ, V JuranicÆova the rates otained in the presence of S. The shape of the conversion curve for n s =0.460 (Tale 1, run 2) points to the period of rapid AAm/S (co)polymerization and the susequent relatively slow period of S homopolymerization. The latter is also well documented y the curves descriing the (co)polymerization of AAm/S in inverse microemulsion with a relatively very high S content n s =0.962 (Tale 1, run 4), or the homopolymerization of S for n s =1.00 (Tale 1, run 5). Figure 2 shows the (co)polymerization rates as a function of the conversion of inverse microemulsion systems with (n s =0.460) and without S (n s =0.000). It is interesting that the AAm/S maximum (co)polymerization rate in inverse microemulsion is shifted to low conversion (aout 5%) while homopolymerization of AAm reaches its maximum value at conversions of around 30% (see also Tale 1, footnote d). The value of around 5% for the overall conversion shows that only a minor part of AAm and S was polymerized at this moment. For oth homo- and (co)polymerization inverse microemulsion systems, the rates of AAm homopolymerization and of AAm/S (co)polymerization increase from the eginning of polymerization as a result of the increasing numer of polymer particles formed (a constant numer of polymer particles in any interval of conversions, connected with the constant rate of polymerization, typical for emulsion polymerization, is missing here). The effect of molar fraction of S (n s ) on the maximum (co)polymerization rates of AAm/S in inverse microemulsion is shown in Fig 3. The (co)polymerization rate decreases exponentially and its value eyond n s 0.60 corresponds practically to the rate for S homopolymerization in toluene solution. Styrene as a monomer component of the oil phase can fully sustitute toluene (T) in the oil phase (Tale Figure 3. Dependence of the maximum rate (R p /au) of AAm/S copolymerization in toluene/s/aot/water/aam inverse microemulsion on the molar fraction of n St (n St =[S]/[S] [AAm]). Molar ratio [toluene]/ [AOT]=20.1; mass ratios AAm/water=0.267; APS/AAm=0.026; volume fraction of aqueous phase, F aw =0.09; temperature, 60 C. 2; see also Tale 1). In the region of high mass ratios of S/T, the maximum value of F aw2 necessary for the formation of a two-phase dispersion system was reached at S/T=13.5/1.5, and in the low S/T region F aw2 reached its highest value at S/T=1.5/13.5. This means that the addition of a relatively small amount of T to the S- rich oil phase and/or S to the T-rich oil phase signi cantly improves the amount of compatile water in the inverse microemulsion. The data in Tales 3 and 4 illustrate the results ased on the experimentally found and calculated (with the help of selective extraction and the results of elementary analysis of the polymer product) overall conversion of oth (co)monomers. Some of these data were used for material alance calculations (see Tale 5 and the Appendix). The results con rmed the hypothesis 3 of practically 100% conversion of AAm in inverse microemulsion system. It was proved independently y elemental analysis that the watersolule part of the (co)polymer product was AAm homopolymer (AAm > 99%). Similarly, the toluenesolule part of the (co)polymer product contained only S homopolymer (nitrogen content elow 0.1%). Tale 2. Effect of partial sustitution of toluene (T) y styrene (S) in the oil phase of the inverse microemulsion toluene/styrene/aot/water/aam a on the volume fraction of aqueous phase F aw2 at which the two-phase system was formed (temperature 20 C) Mass composition S/T Φ aw2 Mass composition S/T Φ aw2 15.0/ / / / / / / / / / / Figure 2. Dependence of the rate (R p /au) of AAm/S copolymerization in toluene/s/aot/water/aam inverse microemulsion on conversion for runs 1 and 2 (see Tale 1); temperature 60 C: *, run 1; ~, run 2. a T/S/3.6/1.5/0.4 (T S=15g, system without initiator). Volume fraction of aqueous phase of inverse microemulsion was changed y step-wise addition of the water solution of acrylamide (mass ratio AAm/ water=0.267) Polym Int 49:1483±1491 (2000)

5 Acrylamide polymerization in styrene containing inverse microemulsions S feed a AAm feed a C TG C TN C SG C SN AAm prod c S prod c a Mass% of styrene (S) and of acrylamide (AAm) of the monomer mixture (AAm S) in the feed. Overall conversion of monomers C TG determined gravimetrically. Overall conversion of monomers C TN determined from the nitrogen content in the polymer product (72.2 mass% of AAm structural units), was calculated from eqn (1): C TN ˆ 44: :2 55:55=100 ˆ 59:9% 1 Tale 3. Overall conversion of monomers and concentration of acrylamide and styrene structural units in polymer product (for recipe of inverse microemulsion and further details see run 2 of Tale 1) Conversion of styrene C SG using overall conversion C TG detrmined gravimetrically, was calculated from eqn (2), and conversion of styrene C SN using overall conversion C TN otained from nitrogen analysis, was calculated from eqn (3): C SG ˆ 100 C TG AAm feed =S feed ˆ :2 44:45 =55:55 ˆ 39:2% 2 C SN ˆ 100 C TN AAm feed =S feed ˆ :9 44:45 =55:55 ˆ 27:8% 3 It was assumed that the total amout of acrylamide was already converted to polymer (ie conversion of AAm=100%) at the time when polymerization was stopped. 3 c Mass% of styrene structural moieties in polymer product, calculated from eqn (4): S prod ˆ 100 AAm prod ˆ 27:8% W ins. a AAm W,ins S W,sol c T ins. a AAm T,ins AAm T,sol c a Mass fractions of polymer product: water insolule, W ins. ; toluene insolule, T ins.. Mass% of AAm structural units in water-insolule (AAm w,ins ) or toluene insolule (AAm T,ins ) mass Tale 4. Results of water and/or toluene extraction of the polymer product otained y inverse microemulsion (for recipe of inverse microemulsion and further details see run 2 of Tale 1) fractions of polymer product, determined y nitrogen analysis. c Mass% of S structural units in water solule (S W,sol ) and mass% of AAm structural units in toluene solule (AAm T,sol ) fractions of polymer product. Water solule fraction is practically AAm homopolymer. Toluene solule fraction is practically S homopolymer. The comparison of the overall monomer (AAm S) conversion C TN calculated from the acrylamide content in the (co)polymer product with the value of C TG determined gravimetrically (see Tale 3) shows that the latter seems to e overestimated, proaly ecause of the presence of traces of water and/or unseparated AOT emulsi er in the hygroscopic AAm/S (co)polymer. The comparison of the values of AAm structural units found y elemental analysis for the polymer product AAm prod (72.2mass%) and of the AAm structural units for the toluene-insolule part of the polymer products AAm T,ins (72.0mass%) shows a very good agreement etween the data otained y comination of the selective extractions and elemental analysis. For these reasons, the value of the overall monomer conversion calculated from the AAm content in the polymer product was considered to e more reliale and thus was used for further calculations. The ratio etween the polymerized AAm structural units of the AAm/S copolymer and the polymerized AAm structural units of the AAm homopolymer is 6.62 (0.8756:0.1322). Analogously, for the polymerized S structural units, this ratio is (0.2586:0.0134). These values show that a relatively small part of the polymerized AAm structural units (13.1 mol%) was used for the formation of AAm homopolymer. For S homopolymer, the respective value of S structural units forming S homopolymer was only 4.9 mol%. Extraction in Copolymer (C) Homopolymer (H) Total (C H) W ins T ins AAm S W sol AAm T sol S AAm S Water 90.6 ± ± Toluene ± ± Tale 5. Results of the material alance calculations for water and toluene extractions of the polymer product (for recipe of inverse microemulsion polymerization refer to run 2 of Tale 1; Values refer to 100g of polymer product a ) a For further details and explanation of symols see Tale 4. Acrylamide and styrene homopolymers form a mixture with AAm/S copolymer in the polymer product efore extraction. After water extraction, styrene homopolymer remains in the mixture with AAm/S copolymer in extracted polymer product, while AAm homopolymer was extracted y water. Toluene extraction of polymer product removes S homopolymer, while the toluene insolule mixture of AAm/S copolymer with AAm homopolymer remains in polymer product. Thus the polymer product contains 89.20mass% of AAm/S copolymer (composed from structural units of AAm (62.24mass%) and S (26.96mass%), 9.4mass% of polyacrylamide and 1.4mass% of polystyrene. Polym Int 49:1483±1491 (2000) 1487

6 J BartonÆ, V JuranicÆova This indicates that most AAm and S monomers were used for (co)polymerization reaction, leading to the formation of AAm/S copolymer. The kinetics and polymer product composition studies in this work, corroorated y our results pulished earlier, 7,12 have enaled us to draw conclusions regarding the mechanism of initiation and propagation of radical (co)polymerization in inverse microemulsions containing polymerizale waterand/or oil-solule monomers. These conclusions are summarized in the following paragraphs. Homopolymerization of water-solule monomers in inverse microemulsion (eg acrylamide) In the case of AAm monomer and APS initiator, oth initiation and propagation loci are in the dispersed water phase (inverse micelles). Two mechanisms for the formation of polymer particles can e proposed. The rst mechanism supposes the formation of initiating monomer radicals in inverse micelles and propagation reactions with monomer in the same micelles. Thermal decomposition of APS initiator in water pools of inverse micelles yields a pair of sulfate anion radicals which, esides cage recomination (regenerating the parent APS molecule), escape from the cage and undergo addition reactions with AAm monomer. AAm monomer (oligomer) radicals then have three possiilities to react: mutual termination which leads to the loss of radical centres and to the formation of low molecular weight products; propagation reaction with AAm monomer (formation of AAm oligomers and high molecular weight AAm polymer); or they may exit from the water pools of inverse micelles through the phase oundary into the oil phase and initiate radical reactions there. It is important to recognize that the formation of growing polyacrylamide chains of high molecular weight is possile only if the termination of monomer and oligomer radicals is suppressed and propagation of the said radicals is favoured. Simple tools for favouring the propagation reactions of monomer radicals with monomer at the expense of mutual termination reactions of monomer radicals in the water pools of inverse micelles are either decreasing of the rate of initiation (y decreasing the concentration of initiator or reaction temperature) and/or increasing the concentration of the reactive monomer in the water pools of inverse micelles. 17 From the point of view of elementary reactions of the polymerization process in any heterogeneous reaction system, the dimensions of the reaction locus are also very important. Thus, for example, the diameter of aout 5 nm of water pools of inverse micelles is already tenfold greater than the known limiting value of the maximum separation distance for ef cient cage termination of small radicals in noninteracting solvent (ie in the asence of competition etween termination and propagation reactions of small radicals). The dimensions of the water pools of inverse micelles can e reasonaly enlarged in percolating inverse microemulsion. 18 The formation of dynamic percolation clusters upon collisions of micelles is connected with the creation of water channels among water pools of individual micelles of the cluster. The volume of the water pools of inverse micelles connected y water channels in the cluster of micelles is, of course, sustantially greater than the volume of the water pool of an individual micelle. AAm monomer radicals can thus e separated y longer distances and the instantaneous mutual termination of AAm monomer radicals may not e applicale. Next, the necessary amount of water solule monomer for propagating reactions and formation of polymer of high molecular weight is also availale. The growing polymer chain is fed with a suf cient amount of monomer to reach its nal molecular weight. The expansion (efore formation of water channels among micelles of the cluster) of monomer radical from the water volume elonging to micelle 1 of the cluster into water volume elonging to micelle 2 of the cluster is analogous (with the exception that the diffusing monomer radical through the water channel is not crossing the phase oundary) to entering of monomer and/or oligomer radicals from the oil phase into water pools of dormant inverse micelles not containing monomer or oligomer radicals (see elow). The alternative mechanism for initiation and propagation reactions in inverse micelles supposes rst the exit of AAm monomer radicals from the water pools of inverse micelles into the oil phase. (Direct exit of sulfate anion radicals from the water phase into the oil phase is not realistic, as shown y Lamla et al; 19 for further support of this reasoning see elow). Next, the entry of an AAm monomer radical from the oil phase into the water pool of another (non-polymerizing, dormant) inverse micelle initiates AAm polymerization in an inverse micelle. Such a mechanism needs the transportation of monomer radicals through the two phase oundaries (water/oil and oil/water). This mechanism of initiation of AAm polymerization in water pools of inverse micelles is thus analogous to the mechanism proposed for DBP initiated AAm polymerization in water pools of inverse micelles y AAm monomer radicals generated in the oil phase (see elow). Both mechanisms for polymer particle formation in inverse microemulsions include the transportation of water pool components (eg monomer and/or AAm monomer radicals) among individual micelles also during inelastic collisions of micelles. 20 In the case of an oil-solule initiator (eg DBP), the small part of AAm dissolved in the oil phase enales the formation of AAm monomer (oligomer) radicals in the oil phase itself. Some of these AAm monomer radicals enter the dispersed water droplets, ie inverse micelles containing water and the major part of monomer AAm, and start and/or terminate the polymerization of AAm in the growing and/or dormant micelles (ie micelles containing or not containing growing polymer chain) or later in the formed polymer particles. The main propagation locus is the polymer particle, analogously to the case of APS initiator. The 1488 Polym Int 49:1483±1491 (2000)

7 Acrylamide polymerization in styrene containing inverse microemulsions direct entry of enzoyloxy radicals formed in the oil phase into the water pools of inverse micelles is improale. 21 The rates of the addition reaction of most common vinyl monomers with enzoyloxy radical 22 are more than 10 7 mol 1 dm 3 s 1, and thus enzoyloxy radicals are effectively consumed in reactions with AAm monomer and transformed into AAm monomer radicals. AAm monomer radicals enter the micelles and/or propagate with AAm monomer in the oil phase. The increased hydrophilicity of AAm oligomer radicals helps their capture y inverse micelles. Captured AAm monomer and oligomer radicals then start the polymerization of AAm in inverse micelles. Irrespective of the nature of the initiator and the possile reaction mechanism of polymer particle formation, after polymerization the inverse microemulsion contains water swollen polyacrylamide particles as the dispersed phase. 12,23 Homopolymerization of oil-solule monomers in inverse microemulsion (eg styrene) For S homopolymerization initiated y DBP, oth initiation and propagation loci are con ned to the continuous oil phase. The situation is similar to the solution polymerization of S in the presence of AOT water swollen inverse micelles. Polystyrene remains dissolved in the continuous toluene phase. For homopolymerization of oil-solule monomers, eg MMA, 1 S 7 or N,N-dimethylacrylamide, 8 initiated y APS, the part of the oil-solule monomer dissolved in dispersed water droplets transfers the radical activity from the aqueous phase into the oil phase via monomer radicals, exiting the inverse micelles and entering the oil phase. These radicals then initiate the polymerization of the oil-solule monomer and homopolymer (eg polystyrene dissolved in the continuous toluene oil phase) is formed. In neither case (APS or DBP initiators) are polymer particles formed. Copolymerization of water- and oil-solule monomers in inverse microemulsion (eg acrylamide and styrene) In the case of APS initiator, primary radicals are formed y thermal decomposition of APS in dispersed water droplets, ie in micelles containing water and dissolved monomer(s) (mainly AAm and to some extent S). Some of the formed AAm monomer radicals exit from the water pools of inverse micelles, enter the oil phase (rememer that the soluility of AAm in toluene oil phase supports this process) and start the addition reactions with S and with AAm dissolved in toluene. The formed hydrophilic AAm/S (co)oligomer and also AAm oligomer radicals are then captured y micelles and later y the polymer particles, and initiate or terminate the radical reactions in dormant or growing micelles and/or polymer particles. In this case also, the radical activity comes from the dispersed water droplets (inverse micelles). The main locus of polymerization is inside the polymer particles and/or on the polymer particle surface. The proportion of propagating reactions in these two loci depends on the degree of hydrophilicity/hydrophoicity of the polymeric chains eing formed (formation of partly hydrophoicized polyacrylamide particles 7,9 ). In the case of DBP initiator, AAm monomer (oligomer) and (co)oligomer radicals of AAm and S generated in the oil phase, enter the dispersed water droplets, ie micelles containing water and dissolved monomer(s) and later the dormant and/or growing micelles or polymer particles. Also here the main propagation locus is the polymer particle as indicated aove; see also BartonÆ. 6 As a result of polymerization and copolymerization processes initiated either y APS or DBP, and depending on the initial composition of the inverse microemulsion, the polymerized reaction system presents either dispersed water swollen polymer particles composed of a mixture of AAm/S copolymer and AAm homopolymer in continuous oil phase or viscous and/ or solid mixture, or water swelled AAm/S copolymer and AAm homopolymer in ulk polystyrene. A graphical visualization of the mechanism of initiation and/or growth reactions of homo- and (co)polymerization of monomers in the oil and dispersed water phases of inverse microemulsion, initiated y APS and/or DBP initiator, is given in Scheme 1. Horizontal arrows symolize the transfer of monomer and/or initiator together with that of their radicals from dispersed water-phase into continuous oil-phase, and vice versa. Monomer and monomer Scheme 1. Mechanism of monomer-radical formation and monomerradical transfer from water-phase to oil-phase and vice versa for homo- and (co)polymerization of water-solule acrylamide (AAm) and oil-solule styrene (S) initiated y water solule (ammonium peroxodisulfate, APS) and/or oil solule (dienzoylperoxide, DBP) initiators in toluene/sodium is(2-ethyl hexyl)sulfosuccinate (AOT)-ased single-phase inverse (w/o) microemulsions. For explanation see text. Polym Int 49:1483±1491 (2000) 1489

8 J BartonÆ, V JuranicÆova radicals shown in round rackets indicate that these species are in a minority in a given phase with respect to those of the other monomer from the (co)monomer pair of the dispersion system. Thus, for example, the mechanism of S homopolymerization in the oil, continuous phase of inverse microemulsion initiated y the water solule initiator APS includes the formation of S monomer (oligomer) anion radicals RÐS. via addition reaction of sulfate anion radical R. arising from thermal decomposition of APS with S monomer dissolved in the water phase. S monomer radicals exit the water phase and initiate the homopolymerization of S in the oil phase of inverse microemulsion. All the other steps of free-radical polymerization (propagation, termination, etc) are then realized in the oil phase of the inverse microemulsion. ACKNOWLEDGEMENTS Financial support from VEGA, the Grant Agency of the Slovak Academy of Sciences (Project No 2/5006/ 98), is gratefully acknowledged. APPENDIX: Results of compositional analysis of the polymer product otained from inverse microemulsion; see Tale 1, run 2, initiator APS and Tale A1. Monomer feed Tale A1. Monomer feed composition Monomer Quantity g mol mol% AAm S Total Polymer product Contains 72.2% of AAm structural units (elementary analysis) and thus =27.8% of S structural units. Water extraction of polymer product (related to 100g of polymer product) W ins. =90.6%; AAm=68.7% (elementary analysis) W sol. =100 W ins. =9.4%. W sol. =100% AAm homopolymer AAm: W ins. W sol =90.6g g1.00= = g of AAm structural units in copolymer and homopolymer, ie = 0.56 g (difference etween AAm structural units determined y elementary analysis of polymer product efore water extraction and of the sum of the recovered AAm structural units in water-insolule and water-solule parts after water extraction of polymer product). Similarly for S: 90.6 ( )=28.36g of S structural units in water insolule part of polymer product (as a part of copolymer and S homopolymer), ie =27.8g PS; = 0.56g./ Thus: =0. Toluene extraction of polymer product (related to 100g of polymer product) T ins. =98.6%; AAm=72.0% (elementary analysis) T sol. =100 T ins. =1.4; T sol. =100% S homopolymer (AAm=0.0%) AAm: =70.99g of AAm structural units in copolymer and AAm homopolymer forming T ins (no homopolymer of AAm was separated). Difference of AAm structural units is =1.01 S: 98.6 (1 0.72) = = g of S structural units in (co)polymer and homopolymer. Difference of S structural units is = Thus: =0. Overall composition of polymer product Tale A2. Composition of polymer product Product Unit Quantity mass% mol mol% AAm/S copolymer AAm S Total AAm homopolymer AAm S homopolymer S Total REFERENCES 1 VasÆkova V, JuranicÆova V and BartonÆ J, Makromol Chem 191:717 (1990). 2 Candau F, Polymerization in inverse emulsions and microemulsions, in Scienti c Methods for the Study of Polymer Colloids and their Applications, Ed y Candau F and Ottewill RH, NATO ASI Series, Kluwer Academic, Dordrecht. p 73 (1990). 3 VasÆkova V, JuranicÆova V and BartonÆ J, Makromol Chem 192:989 (1991). 4 BartonÆ J, Makromol Chem Macromol Symp 53:289 (1992). 5 Candau F, Polymerization in microemulsions, In Polymerization in Organized Media, Ed y Paleos CM, Gordon & Breach, Philadelphia. p 215 (1992). 6 BartonÆ J, Prog Polym Sci 21:399 (1996). 7 BartonÆ J and JuranicÆova V, Macromol Chem Phys 197:3177 (1996). 8 JuranicÆova V, Kawamoto S, Fujimoto K, Kawaguchi H and BartonÆ J, Angew Makromol Chem 258:27 (1998). 9 BartonÆ J, Kawamoto S, Fujimoto K, Kawaguchi H and Capek I, Polym Int 49:358 (2000). 10 Barton J and Capek I, Radical Polymerization in Disperse Systems, Ellis Horwood, London. p 140 (1994). 11 Graillat C, Pichot C, Guyot A and ElAasser MS, J Polym Sci Part A Polym Chem 24:427 (1986). 12 BartonÆ J, Polym Int 30:151 (1993). 13 BartonÆ J, Stillhammerova M and LezÆovicÆ M, Angew Makromol Chem 237:99 (1996). 14 Kamachi M and Yamada B, Propagation and termination constants in free-radical polymerization, In Polymer Handook, 4th edn, Ed y Brandrup J, Immergut EH and Grulke EA, John Wiley, New York. p II/77 (1999) Polym Int 49:1483±1491 (2000)

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