Dispersion polymerization of styrene and methyl methacrylate initiated by poly(oxyethy1ene) macromonomeric azoinitiators

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1 Die Angewandte Makromolekulare Chemie 231 (1995) (NK 4054) Karadeniz Technical University, Department of Chemistry, llabzon 61080, lbrkey Polymer Institute, Slovak Academy of Sciences, DubravskA cesta, Bratislava, Slovakia Dispersion polymerization of styrene and methyl methacrylate initiated by poly(oxyethy1ene) macromonomeric azoinitiators Ufuk Yildiz', Baki Hazer', IgnAc Capek2* (Received 9 January 1995) SUMMARY Macromonomeric poly(oxyethy1ene) azoinitiators (macroinimers) MIM-400 and MIM-1500 were synthesized and characterized by IR and NMR spectroscopy and DSC techniques. The dispersion polymerizations of styrene and methyl methacrylate (MMA) initiated by poly(oxyethy1ene) macroinimers (MIM-400 and MIM-1500) in watedethanol were investigated at 60 C. The rate of polymerization was found to increase with increasing concentration of MIM and the increase was more pronounced in the styrene system. In the range of medium conversions the rate of polymerization was found to be proportional to the 1.7th and 1.6th power of [MIM-400] and [MIM-1500] for MMA and to the 25th power of [MIM-400] for styrene, respectively. ZUSAMMENFASSUNG Makromonomere Polyethylenoxid-Azoinitiatoren (Makroinimere) MIM-400 und MIM-1500 wurden synthetisiert und IR- und NMR-spektroskopisch sowie mit Hilfe der DSC-Analyse charakterisiert. Die mit diesen Makroinimeren initiierte Dispersionspolymerisation von Styrol bzw. Methylmethacrylat (MMA) in EthanoVWasser bei 60 "C wurde untersucht. Dabei wurde gefunden, da8 die Polymerisationsgeschwindigkeit mit zunehmender MIM-Konzentration ansteigt, wobei der Anstieg im Styrolsystem ausgepragter war. Im Bereich von mittleren Umsatzen konnte gezeigt werden, da8 die Polymerisationsgeschwindigkeit von MMA proportional der Potenz 1,7 bzw. 1,6 von [MIM-400] bzw. [MIM-1500] ist, wahrend fur Styrol eine Potenz von 2,5 bezuglich [MIM-400] gefunden wurde. * Correspondence author Hiithig & Wepf Verlag, Zug CCC OOO3-3146/95/$

U. Yildiz, B. Hazer, I. Capek Introduction It has been recognized that the polymerization of macromonomers is connected with the diffusion- and chemical-controlled kinetic events.

2 U. Yildiz, B. Hazer, I. Capek Introduction It has been recognized that the polymerization of macromonomers is connected with the diffusion- and chemical-controlled kinetic events. The high segment density, the chain dimensions of macromonomers or macroinimers and/or their coils and polymer chain entanglements (crosslinks) differ from those of linear polymer chains. These effects reduce the propagation of a macromonomer relative to that of a small comonomer. In normal dispersion polymerization usually surfactants are used to prepare polymer dispersions. Dispersion polymerization involves the polymerization of monomers dissolved in an organic diluent in the presence of a polymeric stabilizer to produce insoluble polymers dispersed in the continuous phase. The formation of stable dispersions can be achieved by incorporation of surface active groups from the initiator to the surface of polymer particles or by copolymerization with surface active monomers. The mechanism of free-stabilizer dispersion polymerization or copolymerization of a macromonomer or macroinimer is very complex and. poorly understood, because the monomer or comonomer itself acts as a monomer (initiator) as well as a stabilizer. A graft copolymer for example acts as a stabilizer. The graft copolymer formed during the dispersion copolymerization of poly(oxyethy1ene) macromonomer and styrene or MMA acts as a stabilizer'. It consists of hydrophobic and hydrophilic units which associate with each other or with a macromonomer to form organized structures (micelles or polymer particles). Thus, these clusters consist of a hydrophobic core and a hydrophilic shell. This paper reports on the preparation of polymer dispersions where the surfactant is formed during the copolymerization of styrene or MMA with poly(oxyeth1yene) (PE0)-macroinimers. Here the influence of the macroinimer type and concentration of the properties of the graft copolymer and the kinetic parameters of dispersion polymerization of MMA and styrene is discussed. Materials Experimental Analytical grade poly(oxyethy1ene)s (PEO-400 and PEO-I 500, where the number shows the molecular weight of PEO) and 4,4'-azobis(4-cyanovaleryl chloride) were 136

3 Dispersion polymerization initiated by macromonomeric azoinitiators used. Commercially available styrene (St) and methyl methacrylate (MMA) were purified by usual methods. Tivice-distilled water and ethanol were used as polymerization medium. Synthesis of macromonomer initiators (poly(azo-bis-4-vinylbenzyl ether)-400, -1500; MIM-400, MIM-1500) Macroinimers were synthesized by the reaction of 4,4'-azobis(4-cyanovaleryl chloride), PEO-400 (- 1500) and 4-vinylbenzylchloride according to the following pr~cedure~.~. In a typical procedure for MIM-400, 10 mmol of PEO-400 and 25 mmol of powdered NaOH were stirred for 1 h at room temperature. To this solution was added 10 mmol of 4-vinylbenzyl chloride in 10 ml of benzene in the presence of hydroquinone at 10 C. After 2 h of stirring, 5 mmol of 4,4'-azobis(4-cyanovaleryl chloride) in 10 ml of benzene was slowly added at the same temperature. After 4 more hours of stirring, 3 ml of concentrated HCI was added to this solution, which was stirred, and dried over anhydrous sodium sulfate. The mixture was filtered and the product was precipitated in diethyl ether and dried under vacuum for 4 h. This viscous liquid (or solid) was kept in a refrigerator until use. MIM-400 was a viscous liquid, while MIM-1500 was a white solid. Polymerization procedure Batch dispersion polymerizations of MMA and St in the presence of a small amount of macroinimer were carried out at 60 "C. In all runs the recipe contains ethanol/water (4/1 v/v) as the continuous phase and ca mol.dm-3 St or mol * dm-3 MMA. The polymerization technique and conversion determinations have been described in detail elsewhere4, '. Polymer and macroinimer characterization Thermal analysis was carried out on 7-11 mg of sample on a DSC V4.08 DuPont The polymer samples were heated at a rate 20 "C/min from 100 to 200 "C, quickly cooled and then scanned a second time using the same heating rate and temperature range as for the first scan. Data used for T, (the glass transition), T, (the melting transition) and H, (the enthalpy of fusion) values were reported for the first scan. T, was taken as the onset temperature and T, as the peak of the melting endotherm. IR spectra of the compounds were recorded on a Perkin Elmer 1600 FTIR spectrometer. IR spectra: cm-': CH,-etheric bonds of PEG, 1620 cm-': vinyl and benzyl groups, 1750 cm-': carbonyls of ester groups, 2250 cm-': bonds of C-N. 137

4 U. Yildiz, B. Hazer, I. Capek 'H-NMR spectra obtained on deuterated chloroform solutions were recorded at 17 C on a Bruker AC-200 NMR spectrometer. NMR spectra (6 in ppm): 3.6 (CH,CH,O groups in PEO), 4.6 (s, CH, groups in vinylbenzyl chloride), 5.2 and 5.8 (m, CH,=CH--[CH,-] groups in phenyl groups), 6.6 and 6.8 (CH,=CH-[CH-] groups in phenyl groups), 7.2 (s) phenyl groups in vinylbenzyl chloride. The swelling of gel samples was carried out in toluene and in water and the degree of swelling was calculated from the volume ratio of swollen gel to dry sample6. Rate of polymerization Results and discussion The conversion versus time data of the dispersion polymerization of styrene initiated with MIM-400 (macroinimer) are shown in Fig. 1. Two regions can be distinguished on the conversion-time curves. Initially, after a short induction period, the polymerization starts with high rates, and after 50 or 60% conversion gradually decreases. The shapes of the conversion curves do not indicate any acceleration of the polymerization event at medium or high Fig. 1. Time / h Variation of monomer conversion in the dispersion polymerization of St initiated by MIM-400 with reaction time. Recipe: 5 ml ethanol-water (411 v/v), [St] = 1.5 mole dm-3, [MIM-500]. 102 (mol. dm-3): (A), (A). 138

5 Dispersion polymerization initiated by macromonomeric azoinitiators conversions, where the polymer particles are present. The conversion curves take on the shape similar to those for precipitation polymerization or dead-end polymerization. The clear reaction system at low conversion indicates that the reaction begins as a homogeneous polymerization in which graft copolymer molecules with hydrophobic and hydrophilic units are formed. These amphiphilic polymer molecules associate with themselves and precipitate from the medium and form later latex particles. The macromolecular clusters are supposed to consist of a hydrophobic core and a hydrophilic shell. The graft copolymer is formed during polymerization and adsorbed on the polymer surface to stabilize particles against coalescence7, 8. The formation of coagulum even at a high macroinimer concentration results from the interparticle crosslinking eventsg. Thus, the increase of the macroinimer concentration increases both the grafting to get copolymer molecules and the interparticle crosslinking. In the reaction conditions chosen, the ratio of low-molecular-weight monomer (St or MMA) is extremely large in comparison to that of the macroinimer. The molar ratio [macroinimer]/[monomer] varies in the range The concentration of St or MMA is constant ca. 1.5 mol.dm-3 and the concentration of macroinimer varies from ca to 0.1 mole dm h E U - g 2.00 v -. -@ b log {[MIM]/(mol.drn )} Fig. 2. Variation of the rate of polymerization with the macroinimer type (MIM-400 and MIM-1500) and concentration. (0) MMA/MIM-1500, (A) MMA/ MIM-400, (A) St/MIM

U. Yildiz, B. Hazer, I. Capek The conversion-time data for the radical polymerization of MMA or St initiated by macroinimer (MIM) were used to estimate the rate of polymerization.

6 U. Yildiz, B. Hazer, I. Capek The conversion-time data for the radical polymerization of MMA or St initiated by macroinimer (MIM) were used to estimate the rate of polymerization. The rates of polymerization determined in the medium conversion range are summarized as a function of the MIM type and concentration in Fig. 2. These results show that the rate of polymerization increases with increasing macroinimer concentration in all systems. In the range of medium conversion R, is ca mol - dm-3 - s-l which is ca. by one order smaller than that obtained in the dispersion copolymerization of St or MMA with PEO macromonomer (both proceed under the same reaction conditions; temperature, monomer and comonomer concentration and rate of initiati~n)~. The smaller rates in the St/macronimer system may be attributed to immobilization of macroinimer in the crosslinked polymer particles (the decrease of the surface active comonomer and initiator concentration). In the crosslinked polymer particles the monomolecular termination is operative (trapped radicals). The suppressed swelling of crosslinked particles decreases the monomer concentration at the reaction loci. The rate of bulk polymerization of St (ca. 8.7 mole dm-3) or MMA (ca. 9.3 mol.dm-3) initiated by MIM-400 or MIM-1500 (the weight ratio macroinimer/monomer = 0.33, the molar ratio [macroinimer]/[monomer] = 0.002, 60 C) was found to be ca. 5 - mol - dm-3 - s-l which is several times larger than that found in the present dispersion run. This can be ascribed to the different monomer concentration at the reaction loci and crosslink density (crosslinker concentration) of the monorner/polymer phase, the higher monomer/mim ratio and the lower crosslink density of polymer network. The relationship R, oc [MIMIX (from Fig. 2) was used to get the reaction order (x) which varies as follows: R, a [MIM-400]'.7 (for MMA), R, a [MIM-1500]'.6 (for MMA), R, a [MIM-400]2.5 (for St) The value of x > 1.0 indicates that the macroihimer takes part in both propagation and initiation. Thus, the overall value of x is the sum of two or three contributions: i) monomer (x = 1) and ii) initiator (x = bimolecular termination; x > monomolecular termination). The value of x is assumed to be also a function of the surface activity of the graft copolymers (or the number of polymer particles). These results indicate that the macroinimer is more effective in the system with styrene. This behavior may result from the higher degree of grafting of PSt matrix, higher colloidal 140

7 Dispersion polymerization initiated by macromonomeric azoinitiators stability of polymer particles and larger number of particles. Indeed, in the dispersion copolymerization of St or MMA with PEG-macromonomer the grafted PSt was a more effective stabilizer than the grafted PMMA',8. Larger rates of MMA polymerization with MIM-1500 result from the lower crosslink density and the higher surface activity of the graft copolymer which increase both the monomer concentration at the reaction loci and the number of reaction loci8. In all systems the limiting conversion of 80-90'70 was reached. This behavior cannot be ascribed to the decomposition of initiator because the half life time of AIBN is ca. 70 h at 60 C'0. The limiting conversion found in the homogeneous polymerization is ascribed to the glass transition temperature of the monomer-polymer mixture in which the monomer and initiator molecules are immobilized. The glass transition temperature for PSt and PMMA is above the reaction temperature which favors this approach. Thus, the limiting conversion is due to the immobilization of initiator in the polymer network supported by IR spectra (the presence of CN group in the polymer matrix). After the critical time due to the consumption (or immobilization) of a larger part of initiator the polymerization proceeds under dead-end conditions' ' cn m E v Y o (D 0 r Time / h Fig. 3. Variation of the rate of polymerization in the dispersion polymerization of St initiated by MIM-400 (0.038 mole dm-3) with the reaction time; cf. legend to Fig. 1 (the run (A)). 141

8 U. Yildiz, B. Hazer, I. Capek The high value of the reaction order on [macroinitiator] and the formation of crosslinked polymer matrix indicate that the unirnolecular termination (the first radical-loss process) should be operative. The bulky substituents around the propagating macroradical end increase the stability of radicals (trapped) and suppress the mutual penetration of growing radicals. The phase separation, precipitation of polymer chains and the formation of isolated polymer-rich and monomer-rich domains immobilize radicals which may lead to depression of both propagation and termination but in a different way. To determine the order of the decay process with respect to the free-radical concentration, we have plotted both in R, against time (first-order kinetics) and I/R, against time (second-order kinetics) (Fig. 3). The first-order plot was reasonably linear whereas the second-order plot was curved. This would seem to indicate that the free-radical loss process was first order, in agreement with the above discussion [MIM]. 10 / (mol.dm ) Fig. 4. Variation of degree of crosslinking (00) with the macroinimer type (MIM-400 and MIM-1500) and concentration. In water: (+), (0),(A), in toluene: (A), (O),(+). (+, +) MMA/MIM-I500, (0, 0) MMA/MIM-400, (A, A) St/MIM

Dispersion polymerization initiated by macromonomeric azoinitiators Swelling data The graft copolymers obtained in the dispersion polymerization of St or MMA with macroinimer (MIM-400 or MIM-1500)

9 Dispersion polymerization initiated by macromonomeric azoinitiators Swelling data The graft copolymers obtained in the dispersion polymerization of St or MMA with macroinimer (MIM-400 or MIM-1500) were insoluble in organic solvents or water. The swelling properties of these graft copolymers are summarized in Fig. 4. It can be concluded that the degree of swelling decreased with increasing macroinimer concentration. This behavior is ascribed to the variation of degree of crosslinking with the macroinimer (crosslinker) concentration. Thus, the degree of swelling is inversely proportional to the degree of crosslinkingi2. The most intensive decrease in the degree of swelling is observed in the MIM-1500 system (in both toluene and water). This indicates that the EO units are active in chain-transfer and crosslinking reactions, as it was reportedi3. A high swelling degree of these graft copolymers in water supports their hydrophilicity. This results from the presence of hydrophilic domains of PEO. The crosslinking which is characteristic for the homopolymerization or copolymerization of doubly unsaturated monomers, leads to formation of polymer particles (or polymer phase) with a very rigid structure, which does not favour swelling or the polymerization within the particles. The graft copolymers (St/MIM-1500) obtained showed endothermic peaks in the range C. The first heating data led to estimate T, ca. 40 C and H ca. 4 J/g. The second heating cycle showed that the recrystallization rates were very fast. Thus, graft copolymers with MIM-1500 only show a T, around 40 "C, MIM-400 does not show a T, because it stays as an amorphous unit in the graft copolymer structure. Conclusions From the foregoing discussion it appears that the copolymerization of macroinimer with styrene or MMA forms amphiphilic graft copolymers (stabilizers). Association of amphiphilic copolymers forms the micelles or primary particles. The low colloidal stability of particles (the formation of coagulum) was attributed to the interparticle crosslinking. In all runs the rate of polymerization increased with increasing macroinimer concentration. This results from the increase of the radical concentration and the number of reaction loci (polymer particles). The swelling properties in water or toluene of crosslinked polymers were inversely proportional to the macroinimer (crosslinker) concentration. 143

10 U. Yildiz, B. Hazer, I. Capek K. Ito, K. Tanaka, H. Tanaka, G. Imai, S. Kawaguchi, S. Itsuno, Macromolecules 24 (1991) 2348 * B. Hazer, Makromol. Chem. 193 (1992) 1081 B. Hazer, B. Erden, R. W. Lenz, J. Polym. Sci., Part A, Polym. Chem. 32 (1994) 1739 I. Capek, M. Riza, M. Akashi, Polym. J. (Tokyo) 24 (1992) 959 I. Capek, L. Q. man, Makromol. Chem. 187 (1986) Okay, Makromol. Chem. 189 (1988) 2201 I. Capek, M. Riza, M. Akashi, Makromol. Chem. 193 (1992) 2843 * I. Capek, M. Akashi, J. Macromol. Sci., Rev. Macromol. Chem. Phys. C33 (1993) 369 lo I. Capek, W. Funke, Makromol. Chem. 191 (1990) 2549 J. C. Masson, Polym. Handb., 3rd ed. 1989, p. II/1 A. V. Tobolsky, J. Am. Chem. SOC. 80 (1958) 5927 l2 0. Okay, J. Appl. Polym. Sci. 46 (1992) 421 l3 P. Gramain, Y. Frere, Makromol. Chem. 188 (1987)

Terpolymerization of 2-ethoxy ethylmethacrylate, styrene and maleic anhydride: determination of the reactivity ratios

Bull. Mater. Sci., Vol. 27, No. 3, June 2004, pp. 243 249. Indian Academy of Sciences. Terpolymerization of 2-ethoxy ethylmethacrylate, styrene and maleic anhydride: determination of the reactivity ratios