ISSN 15600904, Polymer Science, Ser. B, 2012, Vol. 54, Nos. 3–4, pp. 223–233. © Pleiades Publishing, Ltd., 2012. Original Russian Text © S.A. Kurochkin, M.A. Silant’ev, E.O. Perepelitsina, M.P. Berezin, A.A. Baturina, V.P. Grachev, and G.V. Korolev, 2012, published in Russian in Vysokomolekulyarnye Soedineniya, Ser. B, 2012, Vol. 54, No. 4, pp. 623–634.
POLYMERIZATION
Synthesis of Highly Branched Polymers via ThreeDimensional Radical Polymerization in the Presence of Oxygen1 S. A. Kurochkin*, M. A. Silant’ev, E. O. Perepelitsina, M. P. Berezin, A. A. Baturina, V. P. Grachev, and G. V. Korolev Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow Oblast, 142432 Russia *email:
[email protected] Received October 6, 2011; Revised Manuscript Received November 28, 2011
Abstract—It is shown that branched and highly branched vinyl polymers can be prepared by threedimen sional radical polymerization in the presence of dissolved oxygen, as exemplified by the oxidative copolymer ization of styrene and divinylbenzene. The conditions of synthesis of highly branched polymers with a high yield—the ratio between monovinyl and divinyl comonomers and the rate of oxygen bubbling—are deter mined. The kinetics of formation of branched polystyrenes and the features of their molecularmass distribu tion are studied. Elementalanalysis data show that the polymeric product contains 22–24 wt % oxygen, which, according to the IR data, enters into the composition of carbonyl, hydroxide, and peroxide groups. The thermal decomposition of polymeric products is investigated via the TGA–DSC method. The main exo thermal peak at ~145°C is associated with the decomposition of peroxide groups, which is accompanied by the evolution of formaldehyde. DOI: 10.1134/S1560090412040021 1
The design of macromolecules of different topolo gies (from linear to branched, highly branched, and dendrimers) makes it possible to vary the properties of polymeric materials in a wide range without any change in the chemical nature of monomer units. The controlled synthesis of polymers with desired degrees of branching has been intensively studied in recent years [1–3]. From the chemical viewpoint, both poly condensation [4–9] and polymerization [10–14] methods are used to prepare branched polymers. The threedimensional radical polymerization falls into the latter group. This process can be easily controlled (in accordance with chainreaction laws) and makes it possible to use a wide variety of largetonnage mono mers, such as styrene and its derivatives, (meth)acry lates, vinyl acetate, and other easily accessible chemi cal compounds with vinyl groups. The synthesis of branched polymers via three dimensional radical polymerization presupposes the use of branching comonomers with two or more vinyl groups. However, under conventional conditions of threedimensional radical copolymerization, network 1 This
work was supported by the Russian Foundation for Basic Research, project no. 060332543. † Deceased.
polymers are formed and the critical conversion of gelation is no greater than 1%. To avoid the formation of polymeric networks and to obtain soluble branched polymers with a high yield, it is necessary to vary the parameters of the chain reaction through introduction of additives controlling substantial chain growth. Thus, there are two highly promising approaches: namely, the introduction of chaintransfer agents [11, 15–21] (note the first publication [22], where the method of obtaining branched soluble and fusible thermosetting polymers through the threedimen sional polymerization of poly(ester acrylates) in the presence of triethylene glycol as a chaintransfer agent was described) and the introduction of livingpoly merization agents [12–14, 23–28]. In fact, with the use of these approaches, polymers with different degrees of branching can be prepared without any risk of gelation. However, in order to suppress gelation and to form highly branched polymers, a very large amount of additives controlling the length of primary polymeric chains is required. Their concentration should be commensurable with the concentration of the divinyl monomer, as predicted by the theory of threedimensional radical polymerization [29, 30]. This paper concerns the alternative approach to controlling the length of primary polymer chain that
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was suggested by Korolev in 2005 [3]. This approach relies on the data from [31], where the experiments on the oxidative radical polymerization of oligo(ester acrylates) in a thin layer showed that, in the range of degrees of polymerization of 40–90%, the undercured polymer film contained up to 40–50% soluble poly mer products, a part of which was related to branched polymers. Molecular oxygen О2 manifests itself in radical reactions as a strong acceptor of free radicals R• (reac tionrate constant ky) to yield highly active peroxide radicals RO•2 , which react with hydrocarbons (reac tionrate constant kRH) and split off hydrogen atoms from these compounds [32].
7
8
−1 −1
k y =10 −10 M s R + O 2 ⎯⎯⎯⎯⎯⎯⎯ → ROO •
−1
−1
•
o
k RH =1.5 − 20 M s , T =50 −100 C → ROOH + R '• ROO• + R'H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
During polymerization of vinyl monomers, a grow ing polymeric radical reacts with dissolved oxygen to yield a polymeric peroxide radical, which, as a result of rapid interaction with hydrocarbon (a monomer or a solvent), terminates the growth of a substantial chain, while the kinetic chain persists. At the same time, the polymeric peroxide radical can react also with the double bond of the monomer (reactionrate constant kr), thereby adding it to the polymer chain and, thus, regenerating the chain growth. In this case, the struc ture of the main polymer chain will contain peroxide groups [33]. −1
−1
k r =100 −550 M s , T =50 −100 °C ROO + CH 2 =CH −Ph ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ → RO −OCH 2 −(Ph)C H •
The numerical simulation of the kinetics of three dimensional radical polymerization of styrenelike and methacrylic monomers in the presence of dis solved oxygen made it possible to obtain data on the effect of concentrations of components in the reaction mixture (dissolved oxygen, the comonomers, and the initiator) and of temperature on the kinetics of pro cess, the critical conversion of gelation, and the struc tural parameters of the resulting polymers [34]. In this study, we experimentally verified whether highly branched polymers can be synthesized through the threedimensional radical polymerization in the presence of dissolved oxygen as a regulator of primary polymerchain length for the copolymerization of sty rene and divinyl benzene. EXPERIMENTAL Materials Styrene was treated with a 10% aqueous solution of NaOH to remove hydroquinone, washed with distilled water to neutrality, dried over calcined CaCl2, and dis tilled in vacuum. Divinyl benzene (DVB) of technical grade (Aldrich, 80 wt % DVB isomers in ethylvinyl benzene (EVB)) was used as received. AIBN used as an initiator was recrystallized from ethanol. Solvent (оxylene of highpurity grade) was used without addi tional purification. The used oxygen was of firstclass technical grade (the volume fraction of oxygen was no less than 99.7%). Experimental Procedure Styrene, DVB of the technical grade, and оxylene (a total volume of 150 ml) were charged into a 500ml threeneck roundbottomed flask equipped with a reflux condenser communicating with the atmo sphere. A calculated amount of AIBN was added to the solution of monomers, and the resulting mixture
•
was stirred until complete dissolution of the initiator. The reaction mixture was preliminary saturated with oxygen via its bubbling through a steel capillary (1 mm in diameter) for 5 min. After that, the flask was placed in a Polystat 1210535 thermostat (ColeParmer) con taining silicon oil at a temperature of 95 ± 1°С. During the experiment, oxygen was bubbled at a steadystate rate that was measured with a PMA0.1GUZ rotame ter (calibration against air). The reaction mixture in the flask was continuously stirred with a fluoroplastic twoblade stirrer. The stir rer was driven by an IKA RW14 basic device, which maintained a steadystate rotation rate of 750 rpm during variation in the viscosity of the liquid being stirred. The rate of rotation was measured with an SLine EM2234 digital phototachometer. During the experiments, 2ml samples were taken and dried at room temperature to a constant weight. The conversion of the monomers was determined gravimetrically: m Cm = p , msω0 where mp is the weight of the sample after evaporation of volatile components (g), ms is the weight of the sam ple (g), and ω0 is the weight fraction of the monomers in the initial mixture. To simplify the gravimetric procedure for deter mining the conversion of monomers, the volume of the sample, rather than its weight, was measured, and this value was substituted into Eq. (1): mp , (1) C= V sρ s ω0 where Vs is the volume of the sample (ml) and ρs is the density of the sampled solution at the temperature of synthesis (g/ml). Because the temperature of synthesis is higher than room temperature, the density of a solution at the moment of sampling, ρs, is smaller than the density of
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the initial mixture, ρ0, on the one hand. On the other hand, in the studied process, the polymer forms; therefore, the density of solution increases with con version. The substitution of ρs = ρ0 into Eq. (1) will lead to systematic error in the determination of conversion С. To estimate this error, a series of experiments were per formed where both techniques were used to estimate monomer conversions. Table 1 presents the kinetic data on the oxidative polymerization of styrene and DVB. The difference in the determination of conversion via these two tech niques does not exceed 2%. The molecularmass distribution of the polymers was analyzed via sizeexclusion chromatography on a Waters Alliance GPCV 2000 gelpermeation chro matograph (two PLgel 5μm MIXEDC columns connected in series, THF as an eluent, a flow rate of 1 mL/min, a temperature of 35°С, and Empower soft ware). The dried polymeric product was dissolved in THF, and the solution was filtered through an Anatop25 0.2μm PTFE filter (Whatman). Chro matograms were registered with an RI differential refractometer at 900 nm and a MALLS DAWN HELEOS II Wayatt multiangle laserlightscattering detector (ASTRA v.5.3.2.20 software). Eighteen scat tering angles were taken at a laser wavelength 658 nm. The molecular mass of polymers was determined with both singledetector (RI) (calibration against polysty rene standards) and twodetector (RI + MALLS) [35] techniques (dn/dc was assumed to be 0.185). TGA and DSC experiments were performed on an STA 409C Luxx NETZSCH synchronous thermal analyzer (dT/dt = 5 K/min, argon atmosphere) cou pled with a QMS 403C Aelos quadrupole mass spec trometer. The intensity of the ion current of charged particles with m/ze = 30 was registered on the mass spectra of volatile products of polymer degradation. The contents of carbon and hydrogen in the polymeric product were determined on a Vario cube CHNS ele mental analyzer (Elementar Analysensysteme GmbH). The IR spectra of polymer samples were measured on an αBruker FTIR spectrometer. The content of pendant double bonds in the polymers was determined via ozonolysis on an ADS4M analyzer. RESULTS AND DISCUSSION To gain insight into the detailed mechanism of oxi dative polymerization of styrene and its derivatives, experiments are usually performed at a temperature of ~50°С [33] because, in this case, most side reactions complicating this nonsimple mechanism can be elim inated. However, such low temperatures are not suit able for attaining the full conversion of monomers over a reasonable time. The main goal of this study was to ascertain conditions for the synthesis of highly branched polymers with a marked yield. Therefore, a POLYMER SCIENCE
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Table 1. The conversion of monomers into polymers during the oxidative copolymerization of styrene and DVB in oxy lene solution (41 wt % monomers) at 95°C ([M1]0 : [M2]0 = 100 : 124; the rate of oxygen bubbling is 110 l/h (1 atm); and [AIBN]0 = 0.01 mol/L) Time, min
С*
Сm
ρs, g/mL**
15 30 60 120 180
0.006 0.014 0.130 0.424 0.748
0.006 0.014 0.132 0.417 0.723
0.88 0.88 0.88 0.91 0.93
* Calculation through Eq. (1) at ρs = ρ0 = 0.893 g/mL. ** Calculation through the equation ρs = ms /Vs at Vs = 2 mL.
temperature of 95°С was selected to achieve high monomer conversions. Note that, owing to its consumption during the reaction with propagating radicals, dissolved molecu lar oxygen has a concentration that may be lower than the concentration corresponding to saturation, which is ~1.12 × × 10–2 mol/L in xylene at 100°С [36]. Therefore, it is extremely urgent to monitor the cur rent concentration of oxygen. However, at present, there are no reliable techniques for determining the content of oxygen in organic liquids that could provide rapid and accurate measurements of oxygen concen tration during the reaction. Therefore, in the discus sion of the experimental data, we were forced to lean upon only the measured rate of bubbling of gaseous oxygen. The AIBNinitiated radical polymerization in the presence of dissolved oxygen was conducted at differ ent ratios of monovinyl M1 (styrene and ethylvinyl benzene contained in technicalgrade DVB) and divi nyl M2 monomers (DVB isomers contained in techni calgrade DVB) and at two rates of oxygen bubbling, 90 and 110 l/h (Table 2). In this case, the initial con tent of monomers in the mixture was always 41 wt %. The final degree of conversion (at a time of synthesis of tf = 300 min) relative to that of initial monomers was above unity in most cases. This circumstance was asso ciated with the fact that, in calculations of conversion С (Eq. (1)), only the weight fraction of the monomers in the initial mixture is taken into account, while the polymer being formed contains oxygen atoms along side with monomer units, and, because, in the chain transfer reaction RO•2 + R'H → ROOH + R'•, оxylene mostly acts as an R'H hydrocarbon, the frag ments of oxylene are contained at the ends of primary polymer chains. The kinetic curves of the radical copolymerization of styrene and DVB performed in the presence of dis solved oxygen (Figs. 1a, 1b) show that, after addition of DVB, the rate of polymerization increases. The rate 2012
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Table 2. Initial conditions of synthesis, reduced rate of copo lymerization (W/[M])0 , and final conversions of monomers Cf at time tf ([AIBN]0 = 0.01 mol/L, T = 95°C) Experi [M1]0 : [M2]0 ment 1 2 3 4 5 6 7 8 9 10 11 12
tc , min
C c*
(W/[M])0 × 103, min–1
Flow rate of О2 90 l/h 100 : 0 300 0.78 100 : 56 300 1.21 100 : 87 300 1.33 100 : 104 300 1.53 100 : 131 150 – Flow rate of О2 110 l/h 100 : 0 300 0.68 100 : 61 300 1.22 100 : 78 300 1.25 100 : 104 300 1.36 100 : 124 300 1.41 100 : 180 125 0.72 100 : 180 150 –
3.8 6.6 7.7 7.2 7.6 3.4 9.0 10.4 9.4 9.9 8.5 8.5
* The gel is formed only in experiments 5 and 12.
of radical polymerization in the presence of RO•2 per oxide radicals is
W =−
(
)
d[M] = k p[R•] + kr[RO•2] [M], dt
(2)
where [M] is the monomer concentration (mol/L), R• is the propagating radical with a monomer unit at the chain end, RO•2 is the peroxide radial, kp is the rate constant for the reaction of propagating radical R• with the double bond of the monomer, and kr is the rate constant for the reaction of peroxide radical RO•2 with the double bond of the monomer. Under the quasisteadystate approximation kp[R•] + kr[RO•2] ≈ const, we arrive at − ln
(
)
[M] • • = k p[R ] + kr[RO 2] × t [M]0
Then, taking into account expression (2), we obtain the relationship − ln
[M] = − ln(1 − C) = W × t , [M]0 [M]
(3)
In the rectifying coordinates of this relationship, anamorphoses of the kinetic curves were obtained at the initial portion for the radical copolymerization of styrene and DVB in the presence of dissolved oxygen (Figs. 1b, 1c), while, from the slopes of straight lines passing through experimental points, the initial
⎛ ⎞ reduced rates of polymerization ⎜ W ⎟ were deter ⎝[M]⎠ 0 mined (Table 2). Note that, on the whole, the kinetics of the studied process is determined by many factors that affect the value of the reduced rate of polymerization. One of these factors is associated with the formation low molecularmass and polymeric peroxide and hydrop eroxide compounds, which act as additional sources of radicals initiating polymerization at early stages and as main sources of radicals at later stages of the process, when the introduced AIBN initiator has been com pletely consumed. (The halflife of AIBN is τ1/2 = 15 min at 95°С.) After the addition of DVB, the induction period is observed; in all cases, it is nearly equal to 15–18 min. This effect is most probably related to the presence in technical DVB of an inhibitor, ptertbutylcatechol, which actively interacts with peroxide radicals. After the addition of DVB, the rate of polymerization increases several times. (At bubbling rates of 90 and 110 l/h, the reduced rate of copolymerization increases ~1.9 and 2.8 times, respectively, relative to the rate of styrene homopolymerization.) This effect is associated with an increase in the concentration of double bonds when a part of the styrene is replaced with DVB. When even small amounts of DVB are added during radical polymerization conducted in the absence of oxygen, network polymers form already at the early stages of the process. Under the conditions of experi ments 1–4 and 6–10, after addition of DVB (Table 2), the monomers are fully consumed without gelation. Only at molar ratios of monovinyl and divinyl mono mers of 100 : 131 (experiment 5) and 100 : 180 (exper iments 12) and oxygen bubbling rates of 90 and 110 L/h, respectively, do insoluble network polymers form at a monomer conversion of ~90%. Note that, in experiment 12, gelation is observed at a much higher content of DVB. This result is presumably related to a decrease in the length of primary polymer chains (PPCs) at a higher concentration of dissolved oxygen. The [M1]to[M2] ratios for experiments 5 and 12 are optimum for the production of highly branched poly mers under the given conditions of oxidative polymer ization. Highly branched polymers may be obtained only near the gel point [30]. For this aim, the process should be stopped at a conversion that is slightly lower than the critical conversion of gel formation. In this case, the critical conversion of gelation should be high enough to obtain the polymer at a high yield. Table 3 summarizes the molecularmass character istics of the polymers obtained at the final conversion that are shown in Table 2. If no gelation is observed, the polymers being formed have a low molecular mass and are yellowish honeylike substances. However, such a polymer is obtained only when the time of syn thesis is 300 min. At a shorter time, the polymers are
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−ln(1 − C) 0.4
4
(b)
3 5 1.0
4 2
3 2
5 0.5
1
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1
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100 C 1.5
300 Time, min −ln(1 − C)
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2 0.4
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60 Time, min
5
3 6
1
6
4
0.2
0.5
1
100
200
300 Time, min
20
40
60 Time, min
Fig. 1. (a, c) Kinetic curves for radical copolymerization of styrene and DVB and (b, d) their anamorphoses. Bubbling of oxygen at flow rates of (a, b) 90 and (c, d) 110 l/h. (a, b) [M1]0 : [M2]0 = (1) 100 : 0, (2) 100 : 56, (3) 100 : 87, (4) 100 : 104, and (5) 100 : 131; (c, d) [M1]0 : [M2]0 = (1) 100 : 0, (2) 100 : 61, (3) 100 : 78, (4) 100 : 104, (5) 100 : 124, and (6) 100 : 180.
less colored and more viscous; in some instances, they are solid. The cause of this effect is that during poly merization, the thermooxidative degradation of the formed polymer occurs, as evidenced by the molecu larmass distributions of the samples taken at different times of synthesis (Fig. 2). As the time of synthesis increases, the share of the highmolecularmass frac tion decreases. After addition of DVB, the molecularmass distri butions of the polymers become wider and the chro matographic curve assumes a polymodal pattern (Fig. 3a). In fact, this evidence confirms that, in the presence of DVB, the branched polymers are formed because the length of PPCs should slightly change with an increase in the content of DVB, while the appearance of highmolecularmass fractions may be explained by the crosslinking of PPCs solely. The higher the molecular mass, the greater the branching degree of a macromolecule. The occurrence of highly branched macromolecules in the polymers obtained in the presence of DVB is well demonstrated by the chro matographic curve recorded with the multiangle light POLYMER SCIENCE
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scattering detector (Fig. 3b). If, in the case of linear polymers, the chromatographic curve measured with the lightscattering detector (Fig. 3b, curve 1) almost coincides with the curve obtained with the refracto metric detector (Fig. 3a, curve 1), then, for the poly mers obtained in the presence of DVB, which crosslinks linear PPCs, the chromatographic curve measured with the lightscattering detector is shifted (Figs. 3b, curves 2, 3) relative to the curve registered with the refractometric detector (Fig 3a, curves 2, 3) toward smaller elution times, that is, toward higher molecular masses, and, consequently, toward greater degrees of branching of macromolecules. This shift is especially pronounced in the case of the polymer obtained near the gel point (Fig. 3, curves 3). Polymer sample 9 was fractionated through frac tional precipitation (acetone and nheptane were used as a solvent and a precipitator, respectively) into three fractions. Figure 4 shows the gel chromatograms of these fractions. As is seen from Table 4, the molecular mass characteristics of highmolecularmass fraction 9F3 determined with the singledetector (RI) and 2012
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Table 3. Molecularmass characteristics of the polymers synthesized under the conditions of experiments from Table 2 and de termined via singledetector (RI) and twodetector (RI + MALLS) techniques Mn
Experiment*
Mw
Mw /Mn
Mn
Mw
RI 1 2 3 4 6 7 8 9 10 11
740 1020 830 770 1100 1012 920 920 840 3200
Mw /Mn
RI+MALLS
1250 5800 5250 2300 1800 10350 8700 3400 2500 58900
1.7 5.7 6.3 3.0 1.6 10.2 9.4 3.4 3.0 18.3
780 1800 3000 1500 730 1250 1050 1700 2500 6730
1300 12100 28000 5000 1400 16300 9600 8000 8800 154000
1.7 6.9 9.4 3.3 1.9 13.1 9.2 4.8 3.5 22.9
* In accordance with Table 2.
Table 4. Weight fractions ωF and molecularmass characteristics of the fractions of polymer 9 from Table 2 Mn
ωF , %
Fraction 9F1 9F2 9F3
Mw
Mw /Mn
RI
52.3 25.3 22.4
530 1600 3100
720 3400 11600
1 2 3
16
18 Vr, mL
Fig. 2. Chromatographic curves for polymers prepared in experiment 9 (Table 2) detected via RI. The times of syn thesis are (1) 180, (2) 240, and (3) 300 min. The areas below the curves are normalized to unity.
Mw
Mw /Mn
RI+MALLS 1.4 2.1 3.7
twodetector (RI + MALLS) techniques differ signif icantly. The twodetector technique makes it possible to determine absolute molecular masses, whereas the singledetector technique gives underestimated molecular masses for the branched polymers. The larger the difference between the values determined
14
Mn
480 1900 10200
740 4900 26800
1.5 2.6 2.6
with these two techniques, the greater the degree of branching of the polymers. For lowmolecularmass fraction 9F1, these differences are absent. This frac tion largely contains linear chains identical to PPCs from which higher molecular mass branched and highly branched macromolecules are composed (frac tions 9F2 and 9F3). The IR spectra of the polymers (Fig. 5) signifi cantly differ from the spectra of conventional PS owing to the emergence of additional absorption bands in the region of stretching vibrations of C=O and O–H groups. A wide absorption band with a max imum at 3430 cm–1 is associated with the presence of polyassociates of hydroxyl groups. Two overlapping absorption bands with maxima at 1700 and 1720 cm–1 are attributed to the vibrations of C=O ketone and aldehyde groups, respectively [37]. A wide absorption band with a maximum at 1100 cm–1 is due to vibra tions of peroxide groups in the polymer chain [37, 38]. The elemental analysis of the polymers obtained via oxidative polymerization at different times of syn thesis (Table 5) shows that, already at the very onset of the process, there are on average two oxygen atoms per styrene unit with the gross formula C8H8, regardless of the oxygen bubbling rate. This gross formula may cor respond to the gross formula of styrene polyperoxide with the structural formula [–C8H8–O–O–]n. The
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fractions of polymers with different molecular masses are almost the same in terms of elemental analysis (Table 6). The data on the content of pendant double bonds outlined below for the polymers prepared and sepa rated at the end of synthesis (300 min) testify that the residual unsaturation of the polymeric products is low. [C=C] ×
mol/g
6
7
8
9
10
2.7
7.5
6.0
12.0
7.0
This situation is expected because polymerization occurs almost to full monomer conversion, including the conversion of pendant double bonds. Neverthe less, it is not indicative of a high degree of cyclization, as in the case of synthesis of branched polymers based on ABn monomers, where the absence of branching functional B groups in the polymer is evidence for a significant contribution of the intramolecular cycliza tion reaction. In the threedimensional radical polymerization, pendant double bonds are consumed through their interaction with propagating radicals based on alien POLYMER SCIENCE
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Fig. 4. Chromatographic curves (1) of the polymer pre pared in experiment 9 (Table 2) and (2–4) of its fractions (2) 9F1, (3) 9F2, and (4) 9F3 (Table 4) detected via (a) RI and (b) MALLS. The areas under the curves are nor malized (a) to the shares of fractions and (b) to unity.
Fig. 3. Chromatographic curves for polymers prepared in experiments (1) 6, (2) 9, and (3) 11 (Table 2) detected via (a) RI and (b) MALLS. The areas under the curves are normalized to unity.
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macromolecules or its own macromolecules. If pen dant double bonds interact with propagating radicals of the alien macromolecule, a junction is formed via crosslinking of two PPCs of different macromolecules and the joining of these macromolecules leads to the formation of a higher molecular mass macromolecule. This consumption of pendant double bonds is con firmed by the presence of highmolecularmass frac tions on the chromatographic curves of the polymers, whose molecular mass is dozens times greater than the molecular mass of PPC (Figs. 3–5). If the process is carried out under conditions when gelation occurs, the weightaverage functionality of macromolecules (the quantity of pendant double bonds) should grow with conversion and, at the time of gelation, should turn toward infinity. (A network polymer with an infi nite molecular mass is formed.) However, when the conditions of the process are such that gelation is absent until full conversion of the monomer, the weightaverage functionality of branched macromole cules passes through a maximum and, at the full con version of the monomer, is equal to zero, even if 2012
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should remain unchanged; that is, the weightaverage functionality of macromolecules should decrease, whereas the moment of gelation must shift toward greater monomer conversions. In experiment 12 (Table 2), gelation is observed; that is, under these conditions of synthesis, even if the reactions of intramolecular cyclization occur, the weightaverage functionality of macromolecules turns toward infinity during polymerization. In this case, the moment of gelation is preceded by a state of polymer ization system (Table 2, experiment 11) that is charac terized by the presence of highly branched macromol ecules with a high weightaverage functionality of pen dant double bonds. In other words, the intramolecular cyclization reactions are not dominant. As the initial concentration of DVB is reduced (Table 2, experiment 10), no gelation is observed up to the limiting conver sion, and the resulting polymer is practically free of pendant double bonds, as seen from the above given data. Nevertheless, it is difficult to perceive that, in this case, the probability of intramolecular cyclization reactions must significantly increase relative to that in experiments 11 and 12 (Table 2). The absence of pen dant double bonds in the polymers obtained in exper iments 8–10 is associated to a greater extent with the circumstance that the concentration of double bonds at high conversions is low. The newly generated radi cals react with the remaining pendant double bonds, and, owing to their small concentration, the number average length of PPCs, ((Pn)PPC), formed at later
Absorption C=O
–O–O– –O–H
8
12
16
28 36 ν × 10−2, сm−1
Fig. 5. Typical IR spectrum of branched PS prepared via the threedimensional radical copolymerization of styrene and DVB in the presence of dissolved oxygen.
intramolecular cyclization reactions are eliminated [39]. If pendant double bonds are involved in the inter action with propagating radicals of its own macromol ecule, cyclic structures should be formed. As a result of this interaction, pendant double bonds should disap pear, while the average molecular mass of the polymer Table 5. Elemental composition of the polymers Experiment* 1
3
5 6
8
10
Found, %
C8HxOy
Time of synthesis, min
С
H
O
x
y
60 120 300 60 120 300 60 120 60 180 300 60 120 300 60 120 300
69.0 69.9 70.6 69.3 71.2 71.4 68.9 70.1 70.6 70.8 70.7 70.5 69.7 69.4 70.5 70.6 70.0
6.5 6.7 6.4 6.0 6.7 6.4 5.0 4.9 8.0 7.9 7.9 6.7 7.1 7.5 7.6 7.6 7.6
24.2 23.2 22.9 24.7 22.0 22.2 25.9 25.0 21.1 21.1 21.4 22.2 22.7 23.0 21.7 21.7 22.2
9.0 9.2 8.7 8.3 9.0 8.6 7.0 6.7 10.9 10.7 10.7 9.1 9.8 10.4 10.3 10.3 10.4
2.1 2.0 1.9 2.1 1.9 1.9 2.3 2.1 1.8 1.8 1.8 1.9 2.0 2.0 1.8 1.8 1.9
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Table 6. Elemental composition of fractions of the polymer synthesized under the conditions of experiment 9 from Table 2 C8HxOy
Found, % Fraction С
H
O
x
y
9F1
69.3
6.6
24
9.1
2.1
9F2
68.4
6.6
25
9.3
2.2
9F3
69.1
6.4
24.4
8.9
2.1
stages of polymerization is no more than one unit, in accordance with the equation ( Pn ) PPC = kr[M] kRH[R'H] As a result, the content of pendant double bonds in the polymer decreases and cyclic structures practically cannot be formed. In this case, only slightly branched polymers with low molecular masses may form, as fol lows from the data of Table 3 (experiments 2–4 and 7⎯10). The content of highly branched macromole cules with high molecular masses does not exceed 20% (Fig. 4, curve 2; Table 4, fraction 9F3). Owing to the presence of peroxide groups in mac romolecules, the polymeric product features thermal instability, which was evaluated via TGA (Figs. 6a, 6b).
All polymers synthesized via oxidative copolymeriza tion of styrene and DVB are characterized by low ther mal stability relative to common PS. The temperature at which the weight loss is mL = 5% is Т5% = 125– 130оС. The differential TGA curve shows a peak due to degradation at ~145оС, after which the degradation of the polymer continues, but not so intensely. The indicated peak on the differential TGA curve corre sponds to the exothermal peak on the DSC curve (Fig. 6a) and the peak on the ion current curve corre sponding to ions with m/ze = 30 (Fig. 6d), which char acterizes the quantity of formaldehyde evolving owing to degradation of the polymers in accordance with the following reaction [37].
CH2 CH O O CH2 CH O O CH2 CH O
CH2
CH O
O CH2
CH O
CH2 +
With an increase in the time of synthesis, the areas under the exothermal peak on the DSC curve and under the peak on the curve of the evolution of form aldehyde decrease. This finding implies that, at the beginning of the process, the polymeric product con tains a larger amount of polymeric peroxides, a cir cumstance that is responsible for a decrease in the molecular mass of the polymers during synthesis. Table 7 lists the values of heat evolved owing to deg radation of polymers synthesized via the oxidative copolymerization of styrene and DVB in the presence of dissolved oxygen. In accordance with [40], the the oretical and experimental enthalpies for the decompo POLYMER SCIENCE
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O CH2
O CH2 CH
. .
CH O + O CH2
CH
CH + nCH2O + nHC O
sition of styrene polyperoxide are ΔHtheor = ⎯218 kJ/mol and ΔHexp = –209 kJ/mol, respectively. Using the latter value, the quantity of peroxide groups contained in the polymeric products synthesized in this study was estimated (Table 7). The theoretical quantity of peroxide groups in poly(styrene peroxide) [–C8H8–O–O–]n is 7.4 × 10–3 mol/g. Thus, the poly mers formed under our experimental conditions con tain a significant amount of peroxide groups. Owing to the presence of peroxide groups, such branched and highly branched polymers may find application as components of curing compositions, where they will play the role of not only initiators with 2012
232
KUROCHKIN et al. m, %
dm/dt, %/min (a)
100
(b) 2
m = 5%
1
2 3 80
3 1 2
1
200
400 T, °C
60
dq/dt, W/g 2 1
I × 1012, A 12
(c)
400 T, °C
200
(d)
1 1
8
2
2 4
3 3
0 400 T, °C
200
400 T, °C
200
Fig. 6. (a) Integral and (b) differential TGA curves, (c) DSC curves, and (d) the value of ion current I for ions with m/ze 30 mea sured in the scanning mode for the polymer prepared under the conditions of experiment 9 (Table 2). The times of synthesis are (1) 60, (2) 120, and (3) 300 min; the heating rate is 5 K/min.
a high density of initiating groups but also viscosity regulators. Moreover, these polymers may be used for the design of new macromolecular structures. During decomposition of peroxide groups of branched Table 7. Values of heat evolved in the region of the exothermic peak on the DSC curve and the content of hydroperoxide groups calculated from these values for polymers prepared via the threedimensional radical copolymerization of styrene and DVB in the presence of oxygen Experi ment*
Time of synthe sis, min
Q, kJ/mol
[О–О] × 103, mol/g
2
180 300 60 180 300 120 300 60 120 300 60 120
0.64 0.62 1.02 0.48 0.39 0.83 0.40 0.84 0.73 0.25 1.07 0.88
3.0 3.0 4.9 2.3 1.9 4.0 1.9 4.0 3.5 1.2 5.1 4.2
4
7 9
11
* In accordance with Table 2.
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