Polymer Sciences Open Access

Keywords

Copolymerization; Coplolymer; Polymerization; α-methylstyrene; Catalyst system

Introduction

The use of α-methylstyrene as a monomer has been limitedly applied to low temperature polymerization system due to the ceiling temperature of α-methylstyrene [1-4]. The restriction to low temperatures results from the thermodynamic equilibrium that favors a dimer over higher oligomers as the ceiling temperature of a-methylstyrene is approached [6-11]. The copolymerization of α-methylstyrene and conjugated dienes in hydrocarbon solvent at 65-75°C was studied using a novel new catalyst system based on organometallic compound of group I (n-butyl lithium) or group II (dibutyl magnesium) in combination with the alkoxides potassium or cesium of 2-etylhexyl alcohol in the presence of chelating diamine such as, N,N,N’,N’-tetramethylethylene diamine [TMEDA, 9]. The catalyst system based on the cesium alkoxide/TMEDA/ dibutyl magnesium in molar ratio 2:2:1 has previously proven that a comonomer mixture of α-methylstyrene and butadiene can be copolymerized at 65°C without the homopolymerization of α-methylstyrene [9]. The monomer of α-methylstyrene should be incorporated into the polymer chain as singlet units with the possibility of a small fraction of diads, since the rate of selfpropagation for α-methylstyrene is near zero in high temperature [10,11], the rate of cross-propagation must be higher. Therefore, α-methylstyrene can be fully converted into a polymer chain, restricting the negative rate effect of depropagation.

Determining the reactivity ratios for the copolymerization [12] of a-methylstyrene and 1,3-butadiene should provide further insight to the incorporation order outlined above. Differential methods and the integrated method of determining r₁ and r₂ based on the copolymer composition equation will be used to model the poly (α-methylstyrene-co-butadiene) rubber (α-MeSBR) copolymerization system.

Experimental

The anionic solution polymerization of α-methylstyrene (α-MeS) and 1,3-butadiene (Bd) was polymerized from 15wt% monomer concentration (315g monomer in 2100 total g hexane solution) in one-gallon reactor equipped with air driven motor stirrer nitrogen inlet and a cooling coil. Feed ratios of 10, 20, 30, 40, and 50 wt% α-MeS were used to polymerize the five α-MeSBRs used in the study. The reactions were run at 10°C to slow the kinetics enough to provide accurate conversion data. The catalyst consists of Cesium salt of 2-ethyl hexyl alcohol (6.3 mmol) reduced with n-butyllithium or dialkyl magnesium (3.15 mmol) in the presence of a chelating diamine (N, N, N’, N’ tetramethylethylene diamine, 6.3 mmol) to give final target molecular weight near 100,000 g/ mol. The reaction kinetics was followed by taken samples during the polymerization into a sealed glass vial containing undecane/ ethanol mixture using the undecane as an internal standard and analyzed for monomer consumption by a gas chromatography. At full conversion, the copolymer was terminated with ethanol and transferred to a vessel from the reactor to be stabilized with an antioxidant. The polymer was then dried in a vacuum oven. Reactivity ratios were calculated from experimental data. The monomer composition in copolymers was determined by ¹³C and ¹H NMR.

Results and Discussion

The Kinetics of α-methylstyrene (α-MeS) copolymerizing with 1,3-butadiene (Bd) is unknown in the anionic polymerization system above the ceiling temperature of α-MeS. However, we have followed the assumption (r₁=k₁₁/k₁₂) and (r₂=k₂₂/k₂₁) whereas M1+ M2 form a chain end terminated with M2. Results of the calculations for determining the reactivity ratios r₁ (Bd) and r₂ (α-MeS) using both the integrated method and differential methods are shown in Table 1.

Table 1 Reactivity ratios of butadiene an α-methylstyrene.

There are considerable differences within each set of r₁ and r₂ values as calculated by the integrated method. As the weight % of α-MeS in the feed is increased, r₁ and r₂ appears to decrease with a value of 0.4 and 1.5, respectively. We believe that this phenomenon is a direct result of the original monomer feed ratio. At 10wt% α-MeS in feed, this value only corresponds to 6% molar quantity α-MeS, as seen in Table 2. It has been found that only a certain range of monomer feed ratios (molar) leads to the most reliable data suitable for calculating reactivity ratios; the range is 30-70% M2 [12]. Only the 50/50 wt% feed ratio falls in this range, equal to 35/65 molar ratio α-MeS/Bd. Therefore, only the r₁, r₂ values for this feed ratio can be considered valid. It is interesting to note that as the wt% of α-MeS in feed increases from 10% to 50%, both reactivity ratio values decrease. At lower molar amounts of α-MeS in system, the value of r1(Bd) may inflated due to a lower value for k₁₂, the cross-propagation term, since there are low amounts of M2 (α-MeS) in system. The value of r₂ (α-MeS) may be higher since lower concentrations of the monomer capable of depropagation may in turn artificially inflate its rate constant for forward propagation.

Table 2 Incorporated mol% of α-MeS and Bd in α-MeSBR analyzed by GC.

Using the differential methods developed to calculate r₁, r₂ values, different results were seen. The r1 values agree with those calculated from the integrated form of the copolymer composition equation for 50/50 wt%, but the values for r₂ were not in agreement. The value of -0.10 can be considered as zero when considering errors inherent to sampling and calculations. The value of 1.5-1.6 for r1 (Bd) can be considered valid with respect to the mechanism of incorporation outlined in the background section of this publication. This value indicates that the butadienyl-Cs live end will show a slight preference to selfpropagation over cross-propagation to α-MeS monomer. The r₂ value of 0.0-0.4 indicates either a very small tendency to selfpropagate or, in the case of r₂=0.0, no self-propagation.

The resultant polymer would display a highly random incorporation tendency with small runs of Bd units dispersed by single α-MeS units. The polymer composition vs. total conversion data in Figure 1 for 50/50 w% α-MeSBR supports this conclusion, showing almost constant composition of α-MeS in the polymer chain above roughly 10% conversion. In addition, following the above proposed mechanism of comonomer incorporation, feed ratios of 50 wt% α-MeS and above do not incorporate higher than 45-48 w% α-MeS into chain at total conversion. This follows the assumption that the incorporation of the monomer capable of depropagation is dependent on the cross-propagation addition of the diene monomer. For example, in the extreme case of assuming a perfectly alternating copolymerization of higher than 50 mol% feed ratio α-MeS, the total amount of α-MeS polymerized will only equal that of the diene comonomer.

Figure 1: Time (minutes) vs. conversion (%) of α-MeSBR (30/70) via 2/2/1 of CsOR/TMEDA/MgBu₂.

As the % of α-MeS in the initial feed ratio increases from 10% to 40% by weight (Table 3), the Tg of the resultant polymer increases due the proposed relatively random incorporation of α-MeS units into the chain. As shown in Figure 2 the polymers show relatively constant composition with conversion after approximately 10% conversion. The 50/50 α-MeSBR displays a Tg lower than what would be expected, following the above trend. In addition, NMR results show no peaks in the spectra that can be associated with hydrogen absorbance due to block α-MeS in the polymer [data not shown]. Preliminary results from ozonolysis and reduction analysis of the 40/60 α-MeSBR sample polymer indicate no large α-MeS blocks [9,10].

Figure 2: Total conversion (%) vs. polymer composition (%) of α-MeSBR (50/50) via 2/2/1 of CsOR/TMEDA/MgBu₂.

Table 3 Monomer composition at 90% conversion characterized by NMR and its glass transition temperature.

The reactivity ratio values reported above are those calculated from equations derived under the assumption that all propagation steps are irreversible and that no depropagation at the live chain end exists. In this study, even though the reaction temperature is below the ceiling temperature for the neat α-MeS monomer, depopulation still exists because our amps monomer concentration is not high enough to eliminate depolymerization at the actual reaction temperature of 10°C [8]. In order to accurately calculate reactivity ratios from copolymerization’s involving one or more monomers that are capable of depolymerization at the chain end, the copolymer composition equation must be modified to include terms that account for propagation-depropagation equilibrium and contributions of the depropagation step to the forward rate constant [13]. The current methods utilized for calculating reactivity ratios in this analysis do not account for this phenomenon. It is possible for a depropagation step if an α-methylstyrenyl-Cs chain end is present and α-MeS monomer adds to this active center before a cross-propagation step can take place, creating a butadienyl-Cs center, then the reactivity ratios presented above may not accurately describe the incorporation of co-monomers into polymer.

Conclusion

Reactivity ratios of the poly(α-methylstyrene-co-butadiene) polymerization was studied in cesium-based catalyst system. At 50/50 wt% (35/65 molar ratio) α-MeS/Bd, the reactivity ratio for butadiene calculated by both the integrated method and differential methods agree at approximately 1.50. At the same feed ratio, the reactivity ratios for α-MeS calculated by both the integrated and differential methods are not in agreement, being 0.40 and -0.1, expecting to be zero for r₂ (α-MeS). The differences in the above r₂ values may stem from the complications involved with modeling a system exhibiting reversible propagation steps with equations based on solely irreversible propagation at the chain end.

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