Dispersion polymerization of styrene and methyl methacrylate
initiated by macromonomeric azoinitiator
Ufuk Yıldız1,
*, Baki Hazer2
1Kocaeli University, Department of Chemistry, 41300, Kocaeli, Turkey
2Zonguldak Karaelmas University, Department of Chemistry, 67100, Zonguldak, Turkey, and TUBITAK-Marmara Research Center, P.O. 21 Gebze 41470 Kocaeli, Turkey
(Received 16 July 1998)
SUMMARY: The free radical dispersion polymerization of styrene (St) and methyl methacrylate (MMA) initiated by poly(oxyethylene) (PEO) macroazoinimer (MIM-400) in water/ethanol, was investigated at three different temperatures (50, 60 and 808C) for seven polymerization times (3, 6, 9, 12, 24, 36 and 48 h). PSt-PEO and PMMA-PSt-PEO networks were obtained. In each case, polymer gel fractions depend on the polymeri-zation temperature and polymeripolymeri-zation time. With the same initial concentration of MIM-400, maximum gel fraction was found at 80 wt.-% with St copolymerization while 100 wt.-% in case of copolymerization with MMA at 808C for 48 h.
ZUSAMMENFASSUNG: Die mit einem Poly(oxyethylen)(PEO)-Macroazoinimeren (MIM-400) initierte radikalische Dispersionspolymerisation von Styrol (St) und Methylmethacrylat (MMA) in Wasser/Ethanol wurde bei drei verschiedenen Temperaturen (50, 60 und 808C) fu¨r sieben Polymerisationszeiten (3, 6, 9, 12, 24, 36 und 48 h) untersucht. Dabei wurden PSt-PEO- bzw. PMMA-PEO-Netzwerke erhalten. In jedem Fall ha¨ngt das Ausmaß der Gelbildung von der Polymerisationstemperatur und -zeit ab. Bei gleicher anfa¨nglicher Konzentration an MIM-400 lag der maximale Gelanteil nach 48 h Polymerisationsdauer fu¨r Styrol bei 80 Gew.-%, fu¨r MMA bei 100 Gew.-%.
Introduction
It has been recognized that the polymerization of macro-monomers is connected with the diffusion- and chemically controlled kinetic events. The high segment density, the chain dimensions of macromonomers or macroinimers (macroinimers are macromonomeric initiators which can homopolymerize by themselves or copolymerize with a vinyl monomer) and/or their coils and polymer chain en-tanglements (crosslinks) differ from those of linear poly-mer chains. These effects reduce the chain propagation of a macromonomer relative to that of a small comonomer.
In normal dispersion polymerization usually surfactants are used to prepare polymer dispersions. Dispersion poly-merization involves the polypoly-merization of monomers dis-solved 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 stabilizer-free dispersion polymeri-zation or copolymeripolymeri-zation of a macromonomer or macro-inimer is very complex and hardly understood, because the monomer or comonomer itself acts as the monomer (initia-tor) as well as the stabilizer. The
poly(oxyethylene)-poly-styrene PSt) and poly(methyl methacrylate) (PEO-PMMA) graft copolymers formed during the dispersion copolymerization of PEO-macromonomer and St or MMA act as a phase stabilizer. They consist 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 hydro-phobic core and a hydrophilic shell1).
The dispersion copolymerization of poly(oxyethylene) macromonomers was found to be a useful tool for the pre-paration of stable and monodisperse polymer particles.
Generally, dispersion polymerization is a heteroge-neous process by which polymer particles are formed in the presence of a suitable steric stabilizer from the initi-ally homogeneous reaction medium. The initiation of dis-persion polymerization is a two-step process. It starts in the continuous phase by the primary radicals derived from an initiator. The second step begins in the polymer particles by entering oligomer radicals. In contrast to the bulk or solution polymerization high rates, high conver-sions and high molecular weights can be achieved by the dispersion polymerization2).
The influence of the macroazoinimer type and concen-tration on the properties of the graft copolymer and the kinetic parameters of dispersion polymerization of St and
* Correspondence author.
Die Angewandte Makromolekulare Chemie 265 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 0003-3146/99/0203–0016$17.50+.50/0
Dispersion polymerization of styrene and methyl methacrylate 17 MMA was discussed in our previous paper1)
. Here the influence of the polymerization time and polymerization temperature on the gel fractions of the graft copolymers is discussed.
Experimental
Materials
Analytical grade poly(oxyethylene) (PEO-400) (the number refers to the molecular weight of PEO) and 4,4 9-dicyano-4,49-azovaleric acid (ACPA) were supplied by Fluka. 4-vinylbenzyl chloride (mixture of ortho (3%) and para iso-mers (97%)) was supplied by Aldrich. They were used with-out further purification. Commercially available St and MMA were purified by the usual methods, and other reagents were used without further purification. Twice-distilled water and ethanol were used as polymerization medium.
Synthesis of macroinimer MIM-400
MIM-400 was prepared from PEO-400, 4,4 9-dicyano-4,49-azovaleryl chloride (ACPCL) and 4-vinylbenzyl chloride3, 4) (VBCL) as shown in Scheme 1.
Extraction of the sol fraction
Toluene was chosen as the extraction solvent and used at room temperature. Crude gel samples were placed in excess of toluene and the solvent was replaced every other day over a period of 3 weeks until no further extractable polymer could be dedected. The swollen networks were then washed several times with methanol and dried under vacuum to a constant weight at room temperature. The amount of poly-mer soluble in toluene was determined gravimetrically after evaporation and precipitation in methanol. The weight frac-tion of gel, Wg was calculated as
Wg = g/(g + s)
where g and s are the weights of network after extraction and soluble polymer, respectively5, 6).
Polymerization procedure
Batch dispersion polymerizations of MMA and St in the pre-sence of a small amount of MIM-400 (0,5 g; 59 mM) were
carried out at 50, 60 and 808C for 3, 6, 9, 12, 24, 36 and 48 h. In all runs the recipe contained ethanol/water (4 mL/1 mL) as the continuous phase and 1 g St or MMA. The shaker rate was set at 150 strokes min–1.
Results and discussion
Dispersion copolymerization of MMA and St with MIM-400 was carried out at three different temperatures (50, 60, and 808C) for 3, 6, 9, 12, 24, 36 and 48 h.
Conversion-time data for the dispersion copolymeriza-tion of MIM-400 (Mn= 1496 g mol–1) with MMA and St are shown in Fig. 1 and Fig. 2. Two regions can be distin-guished on the conversion-time curves. Initially, after a short induction period, the polymerizations start with high rate, and after conversions of 60, 80, and 90 wt.-% (MMA) and 40, 50 and 70 wt.-% (St) at 50, 60 and 808C, respectively, the rate gradually decreases. The clear
reac-tion system at low conversion indicates that the reacreac-tion begins as a homogeneous polymerization in which graft copolymer molecules with hydrophobic and hydrophilic units are formed. These amphilic polymer molecules associate with themselves and precipitate from the med-ium and later form latex particles. The macromolecular clusters are supposed to consist of a hydrophobic core and hydrophilic shell. The graft copolymer is formed dur-ing polymerization and adsorbed on the polymer surface to stabilize particles against coalescence7, 8).
The conversion-time data for the radical polymeriza-tion of MMA or St initiated by MIM-400 were used to estimate the rate of polymerization with conversion, which has been shown in Fig. 3 and Fig. 4. The maximum
Scheme 1. Synthesis of MIM-400.
Fig. 1. Plot of conversion vs. polymerization time for the dis-persion copolymerization of MIM-400 with MMA; (0) 508C, (f) 608C, (h) 808C.
18 U. Yıldız, B. Hazer
rates were found as 13.68N 10–5, 18.59N 10–5 and 24.28N 10–5 mol L–1s–1at 50, 60 and 808C, respectively, for MMA, and 8.22N 10–5, 10.05N 10–5 and 13.03N 10–5 mol L–1s–1at 50, 60 and 808C, respectively, for St. These figures show that the maximum rates appear at low con-version and that the rates decrease with concon-version, as reported by Capek2). The decrease of the polymerization rate with increasing conversion may have contributed to the decrease of monomer concentration in particles and the increase of the radical number per particle (or the ter-mination rate). This behavior differs from that of the emulsion (or microemulsion) where the rate of
polymeri-zation versus conversion curve consists of 3 intervals (2 nonstationary intervals and 1 stationary one)2)
.
The smaller rates in the MMA (or St)/MIM-400 disper-sion polymerization system may be attributed to the immobilization of MIM-400 in the crosslinked polymer particles (decrease of the concentrations of surface active comonomer and initiator). In the crosslinked polymer par-ticles the monomolecular termination is operative (trapped radicals).
The dispersion polymerization of MMA and St with MIM-400 gave PSt-PEO and PMMA-PEO crosslinked copolymers which were insoluble in organic solvents or water. The gel fractions of copolymerization of MIM-400 with MMA and St are shown in Tab. 1 and Tab. 2, for equal quantities of MIM-400 (0.5 g) and St (or MMA) (1 g) and three different polymerization temperatures for several polymerization times. The gelation behavior of MIMs was already reported4). It can be explained as shown in Scheme 2.
At the beginning, a small amount of MIM decomposes into macromonomer radicals which initiate the free radi-cal copolymerization of St (or MMA) with undecom-posed MIM, and a crosslinked copolymer is formed. In that case undecomposed MIM behaves as a macrocross-linker. As polymerization proceeds, azo groups in the net-work cleave to produce radical ends, and the netnet-work structure gets denser. The crosslink density of the net-work is also affected by a cage effect.
Tab. 1 and Tab. 2 show the dependence of gel fraction on the conversion of MIM with St and MMA. The limit-ing conversions and gel fractions after 48 h polymeriza-tion at 50, 60 and 808C were 52, 51 and 69 wt.-% for St and 68, 81 and 80 wt.-% for MMA, the gel fractions were 79, 78 and 96 wt.-% for St and 95, 100 and 100 wt.-% for
Fig. 2. Plot of conversion vs. polymerization time for the dis-persion copolymerization of MIM-400 with styrene; (0) 508C, (f) 608C, (h) 808C.
Fig. 3. Variation of the rate of polymerization in the dispersion copolymerization of MIM-400 and MMA with conversion; (0) 508C, (f) 608C, (h) 808C.
Fig. 4. Variation of the rate of polymerization in the dispersion copolymerization of MIM-400 and styrene with conversion; (0) 508C, (f) 608C, (h) 808C.
Dispersion polymerization of styrene and methyl methacrylate 19
MMA. This result can be ascribed to the complete decomposition of initiator in the case of MMA at 808C for 48 h.
Conclusion
From the foregoing discussion it appears that the disper-sion copolymerization of macroazoinimer with St or
MMA yields crosslinked copolymers. The polymerization proceeded up to 81% and 100% conversion with St and MMA, respectively. The polymerization rate-conversion dependence is described by a curve with a maximum at low conversions. The decrease of polymerization rate after the maximum is attributed to the decrease of mono-mer concentration in the polymono-mer particles. The gel frac-tions of crosslinked polymers were inversely proportional to the polymerization temperature.
The macroazoinimer MIM-400 can be used in disper-sion polymerization for obtaining crosslinked copolymers.
The authors gratefully acknowledge the support from the TUBITAK Mu¨nir Birsel Foundation (Ankara) for a fellowship for U.Y.
1)U. Yıldız, B. Hazer, I. Capek, Angew. Makromol. Chem. 231 (1995) 135
2)I. Capek, M. Riza, M. Akashi, J. Polym. Sci., Part A: Polym. Chem. 35 (1997) 3131
3)B. Hazer, B. Erdem, R. W. Lenz, J. Polym. Sci., Part A: Polym. Chem. 32 (1994) 1739
4)U. Yıldız, B. Hazer, Macromol. Chem. Phys. 199 (1998) 163 5)H. J. Nagash, O. Okay, H. Yıldırım, J. Appl. Polym. Sci. 56
(1995) 477
6)O. Okay, Makromol. Chem. 198 (1988) 2201
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Tab. 1. Dispersion copolymerization of methyl methacrylate with MIM-400 at three different polymerization temperatures for several polymerization times; polymerization conditions: MMA 1.0 g, MIM-400 0.5 g, ethanol/water ratio 4 : 1 (v/v).
Run No. Polymerization Total Gel fraction Temp. (8C) Time (h) polymer yield (g) (wt.-%) 50 – 1 50 3 0.135 4.9 50 – 2 50 6 0.439 26 50 – 3 50 9 0.631 40 50 – 4 50 12 0.971 63 50 – 5 50 24 1.104 73 50 – 6 50 36 1.154 76 50 – 7 50 48 1.189 78 60 – 1 60 3 0.224 9.2 60 – 2 60 6 0.623 40 60 – 3 60 9 1.004 65 60 – 4 60 12 1.304 86 60 – 5 60 24 1.371 90 60 – 6 60 36 1.418 93 60 – 7 60 48 1.445 95 80 – 1 80 3 0.435 25 80 – 2 80 6 0.886 57 80 – 3 80 9 1.123 73 80 – 4 80 12 1.382 91 80 – 5 80 24 1.425 94 80 – 6 80 36 1.498 100 80 – 7 80 48 1.500 100
Tab. 2. Dispersion copolymerization of styrene with MIM-400 at three different polymerization temperatures for several poly-merization times; polypoly-merization conditions: St 1.0 g, MIM-400 0.5 g, ethanol/water ratio 4 : 1 (v/v).
Run No. Polymerization Total Gel fraction Temp. (8C) Time (h) polymer yield (g) (wt.-%) 50 – 1 50 3 0.031 0.1 50 – 2 50 6 0.151 6.6 50 – 3 50 9 0.345 20 50 – 4 50 12 0.571 36 50 – 5 50 24 0.633 40 50 – 6 50 36 0.722 47 50 – 7 50 48 0.788 51 60 – 1 60 3 0.148 3.2 60 – 2 60 6 0.321 18 60 – 3 60 9 0.553 34 60 – 4 60 12 0.777 50 60 – 5 60 24 0.876 57 60 – 6 60 36 0.901 59 60 – 7 60 48 1.042 68 80 – 1 80 3 0.195 9.9 80 – 2 80 6 0.432 27 80 – 3 80 9 0.734 48 80 – 4 80 12 1.040 68 80 – 5 80 24 1.109 73 80 – 6 80 36 1.149 76 80 – 7 80 48 1.209 80
