Copolymerization of isoprene with polar vinyl monomers: Reactivity ratios, characterization and thermal properties

European Polymer Journal 49 (2013) 1760–1772

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Copolymerization of isoprene with polar vinyl monomers: Reactivity ratios, characterization and thermal properties David Contreras-López a, Enrique Saldívar-Guerra a,⇑, Gabriel Luna-Bárcenas b a b

Centro de Investigación en Química Aplicada (CIQA), Blvd Enrique Reyna Hermosillo 140, Saltillo, Coah. 25294, Mexico CINVESTAV, Unidad Querétaro, Libramiento Norponiente 2000, Fracc Real de Juriquilla Queretaro, Qro. 76230, Mexico

a r t i c l e

i n f o

Article history: Received 10 November 2012 Received in revised form 18 March 2013 Accepted 22 March 2013 Available online 10 April 2013 Keywords: Isoprene Maleic anhydride Glycidyl methacrylate Polar monomer Reactivity ratios Copolymer

a b s t r a c t Reactivity ratios for the copolymers isoprene (IP)-co-maleic anhydride (MAH) and isoprene (IP)-co-glycidyl methacrylate (GMA) are reported. Copolymers were prepared by free radical polymerization using benzoyl peroxide (BPO) as initiator at 70 °C. These copolymers were characterized by FTIR and 1H NMR. The monomer compositions in the copolymer were determined by 1H NMR and the reactivity ratios (ri) were calculated applying diverse linear methods, namely Finemann–Ross (FR), Inverted Finemann–Ross (IFR), Kelen–Tüdös (KT), Extended Kelen–Tüdös (EKT) and the nonlinear method of Tidwell–Mortimer (TM). By using the latter procedure, the values of the reactivity ratios were estimated as 0.119 and 0.248 for the system IP (1) and GMA (2) respectively; whereas for the IP and MAH system were 0.057 and 0.078 respectively. These values suggest the formation of nearly-alternating copolymers in both systems. Molecular weights were determined by gel permeation chromatography (GPC). Glass transition temperatures (Tg) of the copolymers were obtained by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Good agreement is obtained between experimental Tg values and the model of Couchman. Tg increases when the concentration of polar monomer in the copolymer increases. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Plastics industry is one of the most active sectors in the world’s economy for the sustained demand of its products in many technological applications; this includes polymers, copolymers and their blends [1]. Polymerization of 1,3-dienes, such as isoprene, remains an active research area in both industry and academia [2–7]. Derivatives of this type of diene monomers are of great commercial importance. Polymerizations of these monomers are usually carried out by anionic [8,9], cationic [10,11] and coordination [5,12–14] techniques. However, these schemes are highly sensitive to impurities and to the presence of many functional groups.

⇑ Corresponding author. Tel.: +52 844 438 9830; fax: +52 844 438 9463. E-mail address: [email protected] (E. Saldívar-Guerra). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.03.030

On the other hand, the inherent robustness of free radical polymerizations can be advantageously applied to the polymerization of 1,3-dienes [15–17] in order to diversify the variety and applications of this family of polymers [2,18,19]. The polymeric derivatives belonging to this family can be used as compatibilizers in blends containing natural rubber [20,21]. They can also be used as impact modifiers of rigid thermoplastics containing polar groups, such as acrylic polymers for medical applications [22,23]. Isoprene is a conjugated diene that is industrially attractive because it is available at low cost; yet it poses many challenges due to the presence of a pendant methyl group. This methyl group leads to various possible configurations in the polymer backbone depending upon the polymerization mechanism. Polymerization of isoprene generates various microstructures that exhibit different mechanical and physicochemical properties [24,25] (Scheme 1). For instance,

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2. Theory The structure of a copolymer depends on the time of reaction and the relative concentrations of the comonomer and its reactivity (polarity, resonance and steric factors) [30]. Mayo and Lewis [31] introduced the first kinetic model of a free radical copolymerization, the terminal kinetic model, in which the reactivity of a radical depends only on the last monomeric unit in the chain. According to this model, the instantaneous relative consumption of the two monomers is given by:

dm1 m1 M 1 r1 M1 þ M2 ¼ ¼ dm2 m2 M 2 M1 þ r 2 M2

Scheme 1. Polymerization microstructures.

of

isoprene

generating

several

trans-1,4- and 3,4-configurations are semicrystalline polymers with a Tg higher than that of the cis-1,4-isomer. Considering the limited abundance of natural rubber and the increasing demand for various high performance synthetic rubbers, isoprene-based elastomers are therefore of great importance in the market. Literature reports on the free radical copolymerization of isoprene with polar monomers are very limited. Particularly in the case of maleic anhydride, it seems that the formation of a Diels–Alder cycloadduct [26–28], that competes with the polymerization reaction, has precluded further development of these copolymers. Hence, it is important to deepen the understanding of the monomer incorporation into the copolymer growing chain as this will greatly influence the thermal and mechanical properties of the final copolymer. Indeed, there is only one report of the reactivity ratios for the copolymerization system IP– GMA [29] and none found for the IP–MAH system. The spirit of our study is to further the knowledge of the free radical copolymerization of isoprene with polar monomers to create more advanced and controlled structures, including random or tapered blocks in the polymer chain. These could be achieved by using controlled free radical polymerization (CRP) techniques, in particular nitroxide mediated polymerization and reversible addition fragmentation chain transfer (RAFT) polymerization. However, the effective applications of these synthesis methods requires prior knowledge of the reactivity ratios which are known to remain fairly constant when going from conventional free radical polymerizations to CRPs. At present, our group is focusing on this type of approach. The objectives of this work are: (1) to find the proper synthesis conditions to copolymerize isoprene with two readily available polar monomers: maleic anhydride and glycidyl methacrylate; (2) to estimate the reactivity ratios in these copolymerizations, and (3) to study the thermal properties of the final copolymers. To accomplish the above objectives, FTIR, NMR, GPC, DMA and DSC analyses are performed.

ð1Þ

where m1 and m2 are the mole fractions of the two copolymerized monomers, M1 and M2 are the mole fractions of the monomers in the feed, r1 and r2 are the reactivity ratios of the species involved. The reactivity ratios are assumed constant independent of the conversion; however, they are usually determined at low conversions [30]. Numerous methods exist to determine the reactivity ratios, such as the methods of Finemann–Ross (FR) [32], Inverted Finemann–Ross (IFR) [32], Kelen–Tüdös (KT) [33], Extended Kelen–Tüdös (EKT) [34] and Tidwell–Mortimer (TM) [35], which have been used for determining ri at low conversions. The first four use linear estimation techniques and only the last one (TM) uses a non-linear estimation technique. 3. Experimental 3.1. Materials Isoprene (IP, Sigma–Aldrich) and glycidyl methacrylate (GMA, Sigma–Aldrich) monomers were washed twice with a solution of 5% NaOH, twice with distilled water, and dried on Na2SO4. Such washed monomers were distilled prior to a polymerization reaction; isoprene was distilled at atmospheric pressure and GMA was distilled under vacuum. Maleic anhydride (MAH, Fluka) and benzoyl peroxide (BPO, Sigma–Aldrich) were both recrystallized from chloroform solutions. The solvents N, N0 -dimethyl formamide (DMF), chloroform, hexane, methanol and acetone (reagent grade) were used as received. 3.2. Instrumentation 1 H NMR spectra were carried out on an Eclipse 300 Jeol Instrument at 300 MHz using deuterated chloroform (CDCl3) for IP–GMA copolymers and deuterated acetone (Ac-d6) for IP–MAH copolymers at room temperature. The Delta NMR Processing software Version 4.3.6 [Windows NT] by JEOL was used to analyze the spectra. In both cases 16 scans were used with samples of 25 mg in 0.35 mL of deuterated solvent. FTIR spectra were obtained on a Nexus 470 Spectrometer in the 4000–400 cm1 range using 32 scans and 4 cm1 resolution. Molecular weights relative to polystyrene standards were determined by gel permeation chromatography (GPC) using UltrastyragelR

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columns in tetrahydrofuran (THF) at 40 °C and a solvent flow rate of 1 mL/min in a Waters 410 apparatus. Thermal analysis of polymeric materials was carried out by differential scanning calorimetry (DSC) in a Perkin Elmer 7 series instrument. The heating rate was set to 10 °C/min and a temperature range of 100 to 100 °C. Dynamic mechanical analysis (DMA) was performed on a TA Instrument DMAQ800 with a clamp voltage at a frequency of 1 Hz and strain amplitude of 20 lm. The heating rate was 5 °C/min and the temperature range was from 100 to 90 °C. 3.3. General procedure for the copolymerization kinetics of IP– GMA Copolymerization reactions were carried out in glass vials (8.5 cm length, 23 mm outside diameter) sealed with aluminum caps and a rubber septum. The monomers and initiator were mixed and split into several 2 mL aliquots. These aliquots were kept in sealed glass vials under nitrogen atmosphere. Vials were immersed into an ice bath for 20 min to promote degassing. At this stage, more nitrogen was bubbled for 20 min in the reaction mixture in order to remove oxygen. This operation was repeated thrice. Care was taken to ensure that essentially no monomer loss occurred by venting during this stage. Vials were immersed in an oil bath at 70 ± 0.5 °C to start the copolymerization. Vials were taken out the oil bath at regular intervals to quench the reaction by adding a 2% hydroquinone solution in an ice bath. Once the desired conversion (<15%) was achieved, 15 mL of chloroform were added to each of the remaining vials to dilute the solution. This solution was slowly poured into 60 mL of methanol to precipitate the copolymer. This dissolution– precipitation procedure was repeated five times. The fibrous copolymer precipitated was finally filtered and washed with hexane. All samples were dried in a vacuum oven at 40 °C for 24 h and analyzed by FTIR, 1H NMR, GPC and DSC. The dried product had a whitish appearance. 3.4. General procedure for the copolymerization and kinetics of IP–MAH Copolymerization reactions were carried out in a fume hood and performed at different compositions of the system IP/MAH. A jacketed two-necked glass flask equipped with magnetic stirring and a condenser was charged with a solution of recrystallized MAH and DMF using a ratio of 5% by weight relative to the MAH. DMF was used to fully dissolve the MAH in the reaction medium. It is noteworthy that in the absence of DMF, the solubility of MAH in the isoprene is limited to ca. 3–5% by weight similar to that of MAH in styrene [36]. Since the amount of DMF used in the experiments is very small, the polymerization can be considered, in practical terms, a bulk polymerization. The solution was deoxygenated with nitrogen (UP grade) for 20 min. Another solution containing distilled isoprene and BPO (1.5 mol% with respect to IP) was deoxygenated with nitrogen (UP grade) for 20 min to remove impurities such as oxygen or moisture. During this operation the solution

container was immersed in an ice bath to prevent loss of monomer. The MAH/DMF mixture was heated to 70 ± 0.5 °C by means of a recirculating bath oil under magnetic stirring. Once the temperature reached 70 °C, the IP/BPO mixture was carefully added through an addition funnel that was previously connected to the jacketed two-necked glass flask over a period varying in the range 5–20 min, depending on the monomer composition used. Reaction time was recorded from the initial addition of the IP/BPO mixture. This procedure allows one to favor polymerization over Diels–Alder cycloaddition [26]. Samples were taken at regular intervals (typically every 1 h) to measure the conversion gravimetrically. After reaching the desired overall conversion (<15%), 20 mL of acetone were added to the reacting mixture to dissolve the copolymer. This mixture was slowly poured into 100 mL of hexane under stirring to precipitate the copolymer. The excess of solvent was removed by decantation. This dissolution–decantation procedure was repeated five times. The copolymer was recovered as a fibrous bundle. All samples were finally filtered and washed one more time with hexane and then dried in a vacuum oven at 40 °C for 24 h. The dried product had a yellowish-tobrownish appearance depending upon the copolymer composition. Samples were analyzed by FTIR, 1H NMR, GPC and DSC. 4. Results and discussion 4.1. IP–GMA copolymer The copolymerization of IP with GMA in bulk (Scheme 2) was studied by varying the mole fractions of the diene from 0.1 to 0.9 in the feed. FT-IR. Characteristic bands for the IP–GMA copolymer are observed in Fig. 1. The copolymers exhibit similar characteristic absorption bands as those observed in the homopolymers; the bands at 2992 and 2926 cm1 are associated to the stretching of ACH2A and ACH3 of the copolymers with different IP/GMA compositions. The bands at 742, 1130, 1315 and 1325 cm1 are associated with vibrations of the AC(CH3)@CHA group. Out of these, the 1130 cm1 band appears to be due to a vibration of the AC(CH3) in this group, while the 1315 cm1 band corresponds to the vibration of the CAH group in cis conformation, while the band in 1325 cm1 corresponds to the vibration of the same group in trans conformation. Additionally, the band at 1666 cm1 is for 1,4-addition and that at 1645 cm1 for 3,4- addition of the isoprene units. The bands at 889 and

Scheme 2. Copolymerization reaction of isoprene–GMA.

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Fig. 1. FT-IR spectra of IP–GMA copolymers in different ratios in mol% in the feed. (A): PGMA, (B): IP–GMA 20:80 copolymer, (C): IP–GMA 50:50 copolymer, (D): IP–GMA 80:20 copolymer, (E): PIP.

837 cm1 are due to stretching vibrations of the unsaturation present [37]. On the other hand, the strong absorption at 1733 cm1 is due to stretching of the carbonyl group of GMA and the band at 911 cm1 corresponds to the epoxy group of GMA. 1 H NMR (Copolymer composition). The 1H NMR spectrum of the IP–GMA copolymer is presented in Fig. 2. The figure shows a comparison of spectra for different compositions of the copolymer (mol% in the feed), including those of the homopolymers at the extremes. The 1H NMR spectrum of PGMA (Fig. 2, protons h and i) shows signals at 3.1 and 2.6 ppm due to methylene and methyne protons of the epoxy group, respectively, and a peak of multiplets at 4.3 and 3.8 assigned to the ACH2A group (g) between

the ester and the epoxy protons. The signal of the methyl group (f) appears at 1.3 ppm. The 1H NMR spectra of the IP–GMA copolymer (Fig. 2B– D) show peaks around 4.9 ppm assigned to the proton (b) from the isoprene unsaturation. Peaks at 2.0–2.2 ppm are assigned to the methylene protons (d) of the main chain. The protons from the CH3A (c) of the isoprene units are assigned to the peak at 1.7 ppm. In order to determine the amount of each comonomer incorporated into the copolymer formed, the integration of selected peaks from the 1H NMR spectra was used for this purpose. This technique is widely used for estimating composition in both the industrial and the academic fields [38,39]. The peak assignments were done according to ta-

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Fig. 2. 1H NMR spectrum for the IP–GMA system. Left: Copolymer of IP–GMA in different ratios in mol% in the feed. (A): PGMA, (B): IP–GMA 20:80 copolymer, (C): IP–GMA 50:50 copolymer, (D): IP–GMA 80:20 copolymer, (E): PIP. Upper right: Amplified 1H NMR spectrum of IP–GMA 50:50 copolymer. Lower right insert: Assignments of characteristic peaks of each unit involved (the numbers in the groups denote the chemical shifts (ppm)).

bles available in the literature [40]; some of them were also confirmed via the software ACD/ChemSketch Version 4.55. Reactivity ratios. In order to illustrate the calculations involved in the determination of the copolymer composition, let us take as an example the 1H NMR spectrum of the IP–GMA copolymer with a mole ratio of 50:50. It is possible to identify three regions in the spectrum (Fig. 2, upper right): (i) the peaks corresponding to the protons in ACH and ACH2ACH3 groups in the IP–GMA and isoprene units, lying in the region of d = 0.9–2.3 ppm; (ii) the peaks assigned to protons in the glycidyl groups of GMA units, located in the region of d = 2.4–4.5 ppm and; (iii) the peaks of protons adjacent to double bonds, AHC@and @CH2, of isoprene units in the region of d = 4.6–5.3 ppm.Following Rusakova et al.[29], we define the mole fractions of GMA and isoprene present in the copolymer as m2 and (1m2) respectively and, given that the GMA unit contains ten protons and the isoprene unit eight protons, the number of protons which is proportional to the total area of the spectrum (ST) is:

ST a10m2 þ 8ð1  m2 Þ ¼ 8 þ 2m2

ð2Þ

On the other hand, the peaks in the range of 2.4–4.5 ppm correspond to five protons of the glycidyl group, whose

area is denoted by SG and is proportional to 5m2, resulting in:

SG 5m2 ¼ ; ST 8 þ 2m2

m2 ¼

8SG 5ST  2SG

ð3Þ

Table 1 shows the copolymer molar compositions obtained by the above procedure for various isoprene–GMA copolymerizations at low conversion, in which the monomer molar ratio in the feed was varied. Average molecular weights by number (Mn) and weight (Mw) of the copolymers (determined by GPC) are also shown in Table 1. From Table 1 it is apparent that the average molecular weight decreases as the content of IP is increased in the copolymerization. This seems to be related to two factors: (i) the lower average propagation constant as the content of IP in the feed increases and (ii) a possible higher rate of chain transfer to monomer as the IP content increases. Although there is not enough kinetic parameter data available in the literature for these monomers (IP and GMA), some rough estimations for the values of the relevant kinetic constants can be obtained from published data for these or similar monomers. The only value that we found reported for the propagation constant (kp) of isoprene is 2.8 L/(mol s) at 5 °C, while that for butadiene (similar diene without the methyl group of isoprene) at the same temperature is 5.69 L/(mol s), that is, double the value for iso-

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% mol GMAa

Composition of the initial monomer (M1)a,c

Conversion (%)

Composition of the copolymer (m1)b,c

Mn

Mw

Ð

10 20 30 40 50 60 70 80 90

90 80 70 60 50 40 30 20 10

0.11 0.21 0.29 0.38 0.50 0.61 0.72 0.80 0.93

9.9 11.1 8.0 11.5 8.7 13.3 12.1 14.1 14.7

0.23 0.32 0.43 0.45 0.52 0.52/0.54d 0.54 0.58 0.72

93,000 79,000 57,000 39,000 10,200 8900 9900 3400 5200

2,30,000 2,10,000 1,70,000 76,000 28,000 26,000 33,000 12,000 18,000

2.5 2.7 3.0 1.9 2.7 2.9 3.3 3.5 3.5

Temperature: 70 °C, initiator: BPO. 1: IP, 2: GMA The values of initial feed composition (mol %) of the first two columns are nominal values. Actual values in column 3 (only monomer 1 shown) slightly differ from the nominal ones due to the experimental error at weighing. a Feed composition. b Determined by 1H NMR. c Molar composition. d This experiment was replicated.

prene, both values from the same old Ref. [41]. A more recent kp data for butadiene at 70 °C is 295 L/(mol s) [42]. The kp value for GMA at 70 °C is reported as 1939 L/(mol s) [43], around 6–7 times larger than that of butadiene at the same temperature, and presumably some 12–14 times larger than that of IP if we use all of the above data and make simple extrapolations. These values are consistent with the trend shown by our own data if we do simple linear calculations based on the conversion–time data from Table 1; roughly, the polymerization rate for the 10/90 IP/GMA copolymerization is three times larger than that for the 90/10 IP/GMA reaction. A plot not shown (available in Supporting Information) of the estimated polymerization rate with varying monomer composition in the feed, using all the data in Table 1, shows a consistently descending polymerization rate as the IP content is increased in the feed. The change in kp and polymerization rate with monomer composition will definitely contribute to generate lower Mn copolymer as the content of IP increases in the copolymerization, but does not seem to be enough to explain the dramatic change on the molecular weight with composition. On the other hand, for transfer to monomer rate constants (ktr) we found the value of 1.32 L/(mol s) for butadiene at 70 °C [42], while no value was found for GMA; however, for another methacrylate (methyl, MMA) the ktr value reported at 50 °C is 0.0334 L/(mol s) [44]. Based on the kp of MMA at 50 °C, which can be estimated as 651 or 1,157 L/(mol s) depending on the source [43,45], the transfer constant CM (ktr/kp) for MMA at 50 °C is 5.13  105 or 2.9  105 L/(mol s), respectively. Similarly, the value estimated for CM of butadiene at 70 °C would be 4.5  103 L/(mol s), around two orders of magnitude larger than that of MMA. This suggests that the value of CM for isoprene could also be significantly larger than that of GMA, also contributing to lower the Mn of the copolymers for increasing contents of IP in the feed. These polymeric materials were soluble in DMF, THF, DMSO, acetone, toluene and chloroform, but insoluble in methanol, ethanol, water and hexane. Fig. 3 shows the curve of instantaneous copolymer composition (mole fraction of isoprene) obtained for a given feed monomer composition for the IP–GMA system. Iso-

prene (IP) and glycidyl methacrylate (GMA) are designated as monomers 1 and 2, respectively. The graph shows that this pair of monomers exhibits an azeotropic point (where the instantaneous composition of the copolymer is equal to the composition of the monomer feed) at 50.7 mol % of IP. The reactivity ratios and the compositions of glycidyl methacrylate copolymer and isoprene were determined by the linearized methods of Finemann–Ross (FR) [32], Inverted Finemann–Ross (IFR) [32], Kelen–Tüdös (KT) [33], Extended Kelen–Tüdös (KTE) [34] and also by the non-linear method of Tidwell–Mortimer (TM) [35]. Some of the older methods (e.g. FR and KT) are nowadays mostly used to get initial estimates for the non-linear estimation techniques. The linear techniques use some form of linear regression or fitting to a linear model. Both the ‘‘independent’’ and ‘‘dependent’’ variables involve the copolymer composition, which is actually a response measured with an inherent error. Statistically, this violates the assumptions required by the least squares method, so no valid inference can be made about the fitted parameters. Strictly speaking, a method of nonlinear least squares (NLLS) should be used in order to avoid these problems. Tidwell and Mortimer (TM) were among the first to use such a technique to estimate reactivity ratios [35]. It must be pointed out that the TM method still suffers from a shortcoming due to the fact that the independent variable is not measured without error (a requirement for the application of NLLS), especially when the data are taken at relatively high conversions. The experimental error was calculated using a pool-variance estimate. On the one hand, a measure of the experimental error was obtained by the sum of squared residuals of the model (assuming the model is a correct one). In order to have a second source of estimation for the experimental error, two experiments (one for each copolymerization system) were replicated. The two sources were combined to provide an estimation of the experimental error (standard deviation) which, for the system IP–GMA, was r = 0.028 with 8 degree of freedom. As a result, the estimated 95% confidence intervals for the reactivity ratios (TM technique) are r1 = 0.119 ± 0.048 and r2 = 0.248 ± 0.161

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Fig. 3. Copolymer composition vs. feed composition for the copolymerization system IP–GMA. Experimental data points are compared with the prediction using the Mayo–Lewis equation (solid line) with the reactivity ratios of the TM technique. Table 2 Results of reactivity ratios calculated using various methods for the IP/GMA system. Method

r1

r2

r1  r2

FR IFR KT EKT TM

0.115 0.238 0.121 0.124 0.119

0.206 0.316 0.224 0.198 0.248

0.0236 0.0754 0.0270 0.0246 0.0295

r1: Reactivity ratio of IP. r2: Reactivity ratio of GMA.

The reactivity ratios of the monomers obtained by various methods are summarized in Table 2. According to the values in Table 2, except for the IFR method, the product r1r2 is less than 0.03, indicating that the IP/GMA copolymerization exhibits a strong tendency to alternate [46]. For this system the only other values of reactivity ratios found in the literature were reported by Rusakova et al. [29] as r1 = 0.135 and r2 = 0.195, confirming that the copolymer is nearly alternate; however, they do not report details of the method used for their estimation. These values are in good agreement with those calculated in this work by the FR method. It is also interesting to note that reported values of reactivity ratios for the copolymerization of isoprene with another methacrylate (IP = 1, methyl methacrylate, MMA, = 2), r1 = 0.65 and r2 = 0.26, significantly differ from our system in the value of r1. This seems to be related to a relatively greater thermodynamic affinity of GMA than MMA to polyisoprene, as measured by the square difference of the appropriate solubility parameters: 0.49 cal/ cm3 for GMA-polyisoprene compared to 1.69 cal/cm3 for MMA-polyisoprene, estimations based on [47].1 1 The solubility parameters were estimated using the group contribution formula (qR Gi/M), where Gi is the molar attraction constant of group i, qGi is the sum for all the atoms and groupings in the molecules, q is the polymer density and M is the polymer molecular weight.

Notice that for the calculation of the reactivity ratios it is desirable to use samples at conversions as low as possible so the error introduced by the composition drift is minimized. In this regard, Penlidis et al. [48] discuss that if conversions are higher than about 15–20% the reactivity ratios should be obtained by integrating the copolymerization equation to take into account the composition drift. DSC and DMA. Fig. 4 shows the glass transition temperatures (Tg) obtained by differential scanning calorimetry (DSC) as a function of comonomer concentration. We tested several popular Tg models for copolymers which include those of Fox, Pochan,Wood and Couchman. The best correlation with our experimental data was obtained by a model based on the Couchman theory [49,50]:

  %wt2 C p2 Ln

T g2

T g1 %wt1 C p1 þ%wt2 C p2

T g ¼ T g1 e

ð4Þ

where Tgi is the glass temperature of the homopolymer i, % wti is the weight fraction of monomer i in the copolymer and Cpi is the heat capacity of the homopolymer i. The better fit obtained with the Couchman model is attributed mainly to the fact that the this model follows rigorously the thermodynamic theory of glass transition, including the effect of the heat capacities; other models are less rigorous than this one. It is noteworthy that the experimental data are in excellent agreement with the Couchman’s model. One can observe that the higher the concentration of isoprene, the lower the Tg. One can also note that all thermograms showed a single glass transition temperature, which implies that a copolymer of homogeneous composition was formed. Tgs of the copolymer were also determined by dynamic mechanical analysis (DMA). These independent measurements confirm the trend shown by DSC. DMA results for the 90:10 IP-co-GMA are shown in Fig. 5. Table 3 shows the comparison of Tg values obtained by the DSC and the DMA techniques. It is noteworthy that increasing the concentration of GMA in the copolymer

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Fig. 4. Tg behavior of the IP–GMA copolymer as a function of weight concentration of IP. The continuous line represents the prediction of the Couchman model.

Scheme 3. Copolymerization reaction of IP with MAH.

Fig. 5. DMA of IP–GMA 90:10 copolymer. Table 3 Summary of DSC/DMA Tg values for the IP/GMA copolymer system. % w/w IP

DSC Tg (°C)

DMA Tg (°C)

12 40 55

74.4 48.7 30.3

71.3 52.6 33.7

the value of its Tg increases. This effect is mainly due to the presence of the polar comonomer GMA. 4.2. IP–MAH copolymer The copolymerization of MAH-IP (Scheme 3) was studied by varying the mole fractions of the diene from 0.4 to

0.9 in the initial feed. This composition range was chosen such that the formation of the cycloadduct could be minimized. This cycloadduct is the main reaction product when the composition of the initial feed is below 0.4 mol fraction. To satisfy the Mayo–Lewis model [31], conversions below 15 wt.% were experimentally studied. The copolymerization reaction was carried out in bulk and BPO, the free radical initiator, was used in relatively large amount (1.5 mol % with respect to the diene) to favor the polymerization with respect to the corresponding cycloaddition Diels–Alder reaction [26]. The compositions in the monomer feed and in the copolymers are presented in Table 4. The copolymers were soluble in DMF, THF, DMSO and acetone, but were insoluble in methanol, ethanol, chloroform, toluene, water and hexane. FTIR. IR spectra of the IP–MAH copolymer were qualitatively analyzed and they are shown in Fig. 6. It can be observed that the copolymers have similar characteristic absorption bands that are related to the homopolymeric units; the broad bands at 3077, 1666, 1645, 889 and 837 cm1 are due to stretching of isoprene with different microstructures. The strong absorption bands at 1854 and 1780 cm1 confirm the presence of two carbonyl

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Table 4 Results of the copolymerization of IP with MAH. mol% IPa

mol % MAHa

Composition of the initial monomer (M1)a,c

Conversion (%)

Composition of the copolymer (m1)b,c

Mn

Mw

PDI

90 80 70 60 50 40

10 20 30 40 50 60

0.91 0.80 0.71 0.61 0.50 0.40

13.3 8.7 9.8 8.0 11.1 6.6

0.61 0.58 0.54/0.52d 0.52 0.49 0.48

5900 5600 4500 4700 3500 2900

17000 8800 7400 7500 6800 6300

2.9 1.6 1.6 1.6 1.9 2.2

Temperature: 70 °C, initiator: BPO; 1: IP, 2: MAH. The values of initial feed composition (mol %) of the first two columns are nominal values. Actual values in column 3 (only monomer 1 shown) slightly differ from the nominal ones due to the experimental error at weighing. a Feed composition. b Determined by 1H NMR. c Molar composition. d This experiment was replicated.

Fig. 6. FT-IR spectra of IP–MAH copolymers in different ratios in mol% in the feed. (A): MAH, (B): IP–MAH 40:60 copolymer, (C): IP–MAH 50:50 copolymer, (D): IP–MAH 80:20 copolymer, (E): PIP.

groups in the MAH unit and the bands at 1242 and 1059 cm1 are indicative of the presence of the CAOAC group, which is associated to the MAH ring.

1 H NMR (Copolymer composition). Fig. 7 shows a comparison of the 1H NMR spectra of IP/MAH copolymers of varying compositions, including the corresponding homo-

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Fig. 7. 1H NMR spectrum for the IP–MAH system. Left: Copolymer of IP/MAH in different ratios in mol % in the feed. (A): IP–MAH 40:60 copolymer, (B): IP– MAH 50:50 copolymer, (C): IP–MAH 80:20 copolymer, (D): PIP. Upper right: Amplified 1H NMR spectrum of the IP–MAH 50:50 copolymer. Lower right insert: Assignments of characteristic peaks of each unit involved (the numbers in the groups denote the chemical shifts (ppm)).

polymers at the extremes. The spectra of the copolymers (Fig. 7A, B and C) exhibit peaks at 5.6 ppm attributed to proton b from the isoprene units. The signals at 3.15– 3.0 ppm are due to the protons e and f of the ACH group in MAH and those at 2.6–2.8 ppm are assigned to protons a and d in the ACH2 group. It is considered that the protons c of the ACH3 group in the IP units correspond to the peaks at 1.7 ppm. As in the case of the spectrum for the IP–GMA copolymer, in order to estimate the copolymer composition, it is possible to divide the 1H NMR spectrum of the IP–MAH copolymer in three regions (Fig. 7): (i) the proton signals in the region of d = 1.0–2.8 ppm, belonging to groups ACH, ACH2A and ACH3 of the isoprene units; (ii) the proton peaks in the region of d = 3.0–3.3 ppm of the methyne groups in the MAH units and; (iii) peaks in the region of d = 4.5–5.9 ppm from the ACH@ and @CH2 groups in isoprene units. If the molar fractions of MAH and IP in the copolymer are designated as m2 and (1m2) respectively, and since the number of protons in the monomer units are 2 and 8 for MAH and isoprene respectively, the total area of the peaks in the spectrum (ST) is in this case proportional to:

ST a2m2 þ 8ð1  m2 Þ ¼ 8  6m2

ð5Þ

The area (SM) corresponding to the protons of the anhydride group of MAH in the range of 3.0–3.3 ppm is proportional to 2m2; therefore:

SM 2m2 8SM ¼ ; m2 ¼ ST 8  6m2 2ST þ 6SM

ð6Þ

From Eq. (6) it is possible to estimate the copolymer composition. Table 4 shows the values of the mole fractions of MAH in the copolymer estimated in this way from the 1H NMR spectra. Average molecular weights by number (Mn) and weight (Mw) of the copolymers are also shown in Table 1 (determined by GPC). Some of the polydispersity values are rather low, but these are generally associated with conversions below 10%. The largest polydispersity value was obtained for the experiment with 90% IP which also proceeded up to 13% conversion. It seems that at lower conversions the polydispersity is close to the ideal one (1.5); however, as the content of isoprene and the conversion increases, some initial branching may occur due to the residual double bonds in the dead polymer chains, leading to increased polydispersity Fig. 8 shows the copolymer composition curve generated by the corresponding comonomer feed compositions. This graph shows that this pair of monomers forms an azeotropic mixture containing 49 mol%. Reactivity ratios. The reactivity ratios were determined by the same techniques used for the IP–GMA system. The reactivity ratios of MAH and IP obtained by various methods are presented in Table 5. Values of the product r1r2 < 0.03 in column four indicate that the IP/MAH system copolymerizes in an alternate fashion. The donor–acceptor

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Fig. 8. Curve feed composition. vs copolymer composition in the polymer system for IP-co-MAH. Experimental data points are compared with the prediction using the Mayo–Lewis equation with the reactivity ratios of the TM technique.

Table 5 Results of reactivity ratios calculated using various methods for the IP/MAH system. Method

r1

r2

r1  r2

FR FRI KT KTE TM

0.053 0.067 0.067 0.063 0.057

0.061 0.038 0.050 0.043 0.078

0.0032 0.0025 0.0034 0.0027 0.0044

r1: Reactivity ratios for the IP, r2: Reactivity ratios for the MAH.

character of the monomers could play a role in the alternating nature of these two copolymerization systems (especially in the case of the IP–MAH system). The work of Hall and Padias has shed light on the mechanisms occurring in this kind of copolymerization systems [51]. Although the slow addition of IP to the reaction mixture could in principle have some influence on the determination of the reactivity ratios, the time to complete the addition was kept to a minimum (5–20 min) with respect to the total reaction time necessary to reach the desired conversion in order to minimize this effect. This procedure was necessary to avoid the formation of the Diels Alder adduct of isoprene and MAH. The experimental error (standard deviation) estimated for this system was r = 0.019 with 5 degrees of freedom. The estimated 95 % confidence intervals for the reactivity ratios (TM technique) are r1 = 0.057 ± 0.071 and r2 = 0.078 ± 0.046. Notice that zero is included in the confidence interval for r1, which is due to the alternating nature of this system. DSC and DMA. Fig. 9 shows the glass transition temperatures (Tg) obtained by DSC in which a tendency is observed with respect to the concentration of comonomer present in the polymeric material. One can see that the higher the concentration of isoprene, the lower the Tg. It is noteworthy that this family of copolymers exhibits a sin-

gle Tg. which clearly indicates the random nature of the copolymers formed. The values obtained for the glass transition temperatures by DSC were confirmed using DMA studies. Fig. 10 illustrates the behavior exhibited by the IP–MAH 90:10 copolymer. Table 6 shows a comparative summary of the glass transition temperatures obtained by means of the DSC and DMA techniques. It shows that by decreasing the concentration of isoprene in the copolymer, the Tg increases due to the presence of the polar maleic anhydride monomer.

5. Conclusions In this work, reactivity ratios for a commercially-important diene, isoprene (IP), with two polar monomers, glycidyl methacrylate (GMA) and maleic anhydride (MAH) were reported. For the case of IP–GMA copolymer, there is only one study in the literature that dates back to 1974 in which the reactivity ratio is reported [29]. In the case of IP–MAH, experimental details are only given for a 50:50 feed composition ratio and their reactivity ratios are not provided at all [26]. In our study, we report the reactivity ratios for the systems IP–GMA and IP–MAH that were estimated using different methods of calculation, namely Finemann–Ross (FR), Inverted Finemann–Ross (IFR), Kelen–Tüdös (KT), Extended Kelen–Tüdös (EKT) and the nonlinear method of Tidwell–Mortimer (TM). By using the latter procedure, the values of the reactivity ratios were estimated as 0.119 and 0.248 for the system IP and GMA respectively; whereas for the IP and MAH system were 0.057 and 0.078 respectively. These values suggest the formation of nearly-alternating copolymers in both systems. Increased polar monomer content in the copolymer (MAH or GMA) increases the Tg of the polymeric material. The Couchman´s model for predicting Tg values is in

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Fig. 9. Tg behavior with respect to the weight concentration of IP in IP–MAH copolymers. The continuous line represents the prediction of the Couchman model.

edges the financial support of this research by CONACYT Grant 101670. References

Fig. 10. DMA of the IP–MAH 90:10 copolymer.

Table 6 Summary of DSC/DMA Tg values for the IP–MAH copolymer system. % w/w IP

DSC Tg (°C)

DMA Tg (°C)

40 49 52

99.2 72.2 65.9

100.9 75.0 69.8

good agreement with experimental data using DSC and DMA techniques for both IP–GMA and IP–MAH systems.

Acknowledgements The authors are thankful to Dr. Román Torres Lubián for his valuable help in the interpretation of the NMR spectra. We also thank Prof. Scott Parent of Queen´s University for helpful comments. Enrique Saldívar-Guerra acknowl-

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