Modeling of Ionic Polymerization Processes: Styrene and Butadiene

Transcription

1 resented at AIChE Spring 999 Meeting, Houston Modeling of Ionic olymerization rocesses: Styrene and Butadiene Abstract Ashuraj Sirohi and K. Ravindranath olymer Technology Group Aspen Technology Inc. Canal ark, Cambridge, MA 24 A comprehensive mathematical model for ionic polymerization processes has been developed in olymers lus framework. The model is based on the method of moments and predicts the polymer number, weight and z-average molecular weights. The model takes into account all the important reactions such as reversible initiator association, chain initiation, propagation, association of growing polymer chains, exchange and coupling reactions, equilibrium between free ions and ion pairs, etc. The ionic kinetic scheme in olymers lus was used to model copolymerization of styrene and butadiene in a semi-batch reactor. Simulation results for monomer conversion, polymer molecular weight averages, fraction of live polymer, copolymer composition, etc. were compared with the available literature data. The effect of coupling agent, iodine, on the coupling efficiency was examined for styrene-butadiene thermoplastic elastomers. Introduction A number of commercially important polymers such as styrene-butadiene copolymers, polybutadiene, polyisobutylene are produced by ionic polymerization. The ionic mechanism is the best tool to produce polymers of controlled microstructure. In ionic polymerization, the negatively and positively charged free ions and ion pairs, which are often in equilibrium, are the active propagating species. These processes are complicated due to the presence of multiple propagating species and other complex reactions such as reversible association of the growing polymer species and the initiator, exchange and coupling reactions. In addition, solvent and poisons influence not only the reaction rate but also the molecular weight distribution (MWD). As a result, modeling of ionic polymerization is a challenging task. We have developed a comprehensive ionic kinetic scheme in olymers lus which takes into account all the above mentioned phenomena. This kinetic scheme can model anionic, cationic and group transfer polymerization. In this work, we focus on modeling styrene-butadiene copolymers produced by anionic polymerization. Styrene and butadiene can be polymerized over a wide range of temperatures from room temperature to elevated temperatures ( C) using n-, sec- or ter-butyllithium initiators. There was three different types of copolymers produced by anionic polymerization: tapered block, di/tri-block and random.

2 resented at AIChE Spring 999 Meeting, Houston Tapered or graded block copolymer: When alkyllithium, styrene and butadiene are mixed in a batch reactor, a tapered block copolymer is formed. One block would be a butadiene-styrene (B/S) copolymer with only small amounts of styrene and the other a polystyrene (S) block with an overall B/S-S structure. Di/tri-block copolymer: These polymers are known as thermoplastic elastomers (TE). The polymerization is carried out in a semi-batch reactor by sequential addition of monomers. Styrene-butadiene-styrene (SBS) block polymers are produced by first polymerizing styrene to form the S block followed by addition of half of butadiene to form the half B block. Then a di-functional coupling agent such as iodine is added to link the living polymer chains and form the tri-block polymer. Random copolymer: olymerization in a continuous reactor would produce a random copolymer. The copolymer composition can be varied by manipulating styrene and butadiene feeds. Random copolymers are also produced in a batch reactor using alkali metal oxides and alkyl lithium. In the following sections, the kinetic scheme used to model copolymerization of styrene and butadiene is described. The unknown kinetic rate constants are estimated using the data published by Hsieh et. al. [,2]. Wherever possible, the simulation results are compared with the experimental data. Finally, we demonstrate the application of this set of kinetics to study the effect of coupling agent concentration on coupling efficiency. Kinetics of Styrene-Butadiene olymerization The ionic kinetic scheme in olymers lus can handle anionic, cationic and group transfer polymerization. A subset of the ionic kinetic scheme shown below is used to model semi-batch copolymerization of styrene and butadiene using butyllithium as an initiator. Initiator Dissociation : Chain Initiation : ropagation : Association : Coupling : : (BuLi) 6 BuLi + M j, 6 BuLi j n, k + M j n + δ j, j 2 n, k + m, k n + m, k 3 n, k + I2 n,k 3 4 n + m n + m i n,k is a growing polymer with an index i. The index i keeps track of different types of active species (e.g., free-ions, ion-pairs, dormant esters, coupled polymer, associated polymer). The subscript n and k represent the chain length and the end segment, respectively. The built-in kinetic scheme in olymers lus has several other reactions, e.g., activation and initiation reactions, exchange reactions, chain transfer reactions and termination reactions that are not shown in the above kinetic scheme. Initiator n-buli exists as hexamer [] aggregate in 2

3 resented at AIChE Spring 999 Meeting, Houston cyclohexane solvent. We assume that the associated polymeric species exist as dimers when either styrene or butadiene end segments are attached to the lithium atom of the growing polymer chain. The superscript represents the growing polymer, 2 the associated polymer, 3 the polymer with coupling agent (I 2 ) at the end and 4 the coupled (tri-block) polymer. In this work, it is assumed that there is only one type of propagating species, i.e., only ion-pairs are active species. The model is capable of handling any number of growing species. Hsieh et.al. [,2] has extensively studied the polymerization of styrene and butadiene in hydrocarbon solvents using different initiators and solvents. They have reported monomer reactivity ratios in various solvents. Homopolymerization data [] is used to estimate the intrinsic propagation rate constants for styrene and butadiene. In ionic polymerization, the reactivity ratios (r s = k ss /k sb, r b = k bb /k bs ) are a strong function of solvent. For styrene (s) and butadiene (b), the reactivity ratio r b is about 5 times larger than r s in cyclohexane solvent. The cross propagation rate constants can be obtained from the reactivity ratios once homopolymerization rate constants have been determined. It has been seen in experiments that the propagation rate is ½ order in living polystyrene indicating the dimeric nature of the associated polymer [2]. Almost 8-9 % of the polymer chains are associated in the dimeric form for initiator concentrations as low as -5 M. The percentage of live polymer chains in the associated form is determined by the association equilibrium constant. Our simulations predict that 85 % of total polymer is in the associated form. The forward and reversible association rate constants do not effect the results as long the equilibrium constant is fixed. The initiator dissociation equilibrium constant, chain initiation, propagation, polymer association equilibrium constant have been determined for styrenebutadiene copolymerization in cyclohexane solvent using n-buli initiator and compared with the experimental data in Figures -3. Simulation Results Simulations were carried out for homopolymerization of styrene at different initial concentrations of n- BuLi. Figures and 2 show the percent of remaining n-buli as a function of monomer conversion. The amount of unreacted n-buli depends on the chain initiation and the propagation rate. As the initiator concentration increases the overall polymerization rate goes up and the time required for percent conversion goes down. Therefore, the total time for chain initiation decreases which in turn leads to lower consumption of n-buli Unreacted BuLi, % x-2 8.7x-3 2.6x-3.9x Conversion, % Figure : n-buli remaining as a function of styrene (. mol/l) conversion in cyclohexane at 5 C at various n- BuLi concentrations (mol/l). (Solid lines: simulation results; Symbols: data of Hsieh et. al. [,2] ) 3

4 resented at AIChE Spring 999 Meeting, Houston initiator. For an initiator concentration of 2.6x -2 mole/l, 8 % of BuLi is remaining at % monomer conversion. However, for an initiator concentration of 2.6x -3 mole/l, n-buli is completely consumed before reaching % monomer conversion. It is not desirable to have initiator left at the end of polymerization. If Unreacted BuLi, % Conversion, % Styrene,. mol/l Butadiene,.7 mol/l /5 25/75 Conversion, % Figure 2: n-buli remaining as a function of styrene and butadiene conversion in cyclohexane at 5 C at an initial n-buli concentration of 8.7 mol/l. (Solid lines: simulation results; Symbols: data of Hsieh et. al. [,2] ) Time, minutes 5/5 arts Monomer : Cyclohexane : n-buli :.3 Figure 3: Copolymerization of butadiene and styrene in cyclohexane at 5 C (Solid lines: simulation results; Symbols: data of Hsieh et. al. [,2] ) the initiator remains at the end, terminating agents are used to kill the initiator so that no further polymerization occurs during down stream processing stages. Figure 2 compares the unreacted BuLi as a function of monomer conversion for styrene and butadiene. At a given conversion, there is more unreacted BuLi in styrene polymerization than is in butadiene. If the propagation rate is faster than the initiation rate for a monomer then the initiator will have less time to react and hence more initiator would be remaining at a given monomer conversion. Since styrene homopolymerizes faster than butadiene, enough styrene is not available for longer time to initiate new chains, leading to lower consumption of n-buli for styrene. Figure 3 compares the tapered block copolymerization simulation results with the experimental data [,2] for different compositions of butadiene and styrene. In all simulations, concentrations of solvent and initiator were kept the same, and only the mass percentage of each monomer was varied from to. The simulation results match the experimental data pretty well. It is well known that styrene homopolymerizes more rapidly than butadiene with alkyllithiums in hydrocarbon solution. However, styrene does not polymerize first in the mixture. Butadiene is first incorporated with small amounts of styrene into the copolymer, and only when butadiene is completely reacted the residual styrene polymerizes to form styrene blocks by homopolymerization onto the polybutadienyl lithium end group. The kink in the plots indicates the completion of butadiene polymerization. 4

5 resented at AIChE Spring 999 Meeting, Houston Because of the stability of living nature of the allylic lithium end group, butadiene-styrene copolymers of widely different structures and properties can be prepared. Using the estimated kinetic parameters, the model is used to simulate tri-block (SBS) polymer synthesis. In SBS polymer, the rubbery soft block (polybutadiene) is between the two hard polystyrene blocks. The arrangement of hard and soft blocks yields commercially useful properties. These.9 copolymers have two phases, two glass Aggregate.8 transition temperatures and are.7 characterized by high raw strength,.6 complete solubility and reversible.5 Coupled thermoplasticity. The general strategy to Fraction Live + Dead Time, minutes Figure 4: Fraction of live, dead, aggregate, coupled polymer after the coupling agent is added at 26 min. The coupling agent, I 2, concentration is high. Fraction Aggregate Live + Dead Coupled Time, minutes Figure 5: Fraction of live, dead, aggregate, coupled polymer after the coupling agent is added at 26 min. The coupling agent, I 2, concentration is low. [,3] produce tri-block polymers is as follows: ) charge the batch reactor with styrene and solvent; 2) add the sec-buli initiator and allow styrene to polymerize for an hour; 3) add butadiene and let it also polymerize for an hour; 4) add a difunctional coupling agent (I 2 ) and allow 4 minutes for coupling; and 5) add a short stopper to kill the remaining initiator and live polymer chains. To produce di/tri block copolymers it is essential that all the polymer molecules have the same structure and no new polymer chains are formed during coupling. As seen in Figures and 2, a fraction of n- BuLi is present through out the simulation. Since polymer chains are initiated over a period of time when n-buli initiator is used, the polydispersity index of the polymer would be greater than one. The polydispersity undesirably affects the sequence lengths in SBS. It is also not desirable to have any initiator left at the end of polystyrene block (i.e. step 2) since it will form butadiene homopolymer when butadiene is added. To meet these stringent criteria sec-buli is normally used as an initiator for making di/tri block copolymers. sec-buli is a fast initiator compared to n-buli and initiates polymer chains almost instantaneously. In our simulations with sec-buli, we assume that the chain initiation is fast and the initiation rate constant is comparable to the chain propagation rate constant for both the monomers. The effect of iodine coupling agent concentration on the fraction of live, dead, aggregate and coupled can be seen in Figures 4 and 5. Coupling efficiency is defined as the fraction of the total polymer that is 5

6 resented at AIChE Spring 999 Meeting, Houston di-functionally (tri-block copolymer) coupled for a bi-functional coupling agent. The coupled polymer is a tri-block polymer. The coupling efficiency is one of the important quality control parameter in the production TEs and is controlled by manipulating the type and concentration of coupling agent. The concentration of coupling agent, I 2, in Figure 5 is % of I 2 concentration in Figure 4. Lowering the concentration decreases the coupling efficiency from 84 % to 2 %. Concluding Remarks: A set of intrinsic ionic kinetic constants for styrene-butadiene copolymerization has been estimated using the experimental data reported in the literature. The model was used to match the initiator and conversion levels for a batch copolymerization of styrene and butadiene in cyclohexane using n-buli initiator. This model was then used to study the effect of coupling agent (Iodine) concentration on the coupling efficiency in SBS tri-block (thermoplastic elastomers) copolymerization. Using this model, it is feasible to examine styrene-butadiene block copolymer production via continuous operation with different reactor configuration and feeding strategies. For example, a series of CSTRs with a different feed at each CSTR can be optimized to obtain block copolymers. Design engineers can use the validated kinetic model to optimize the production of thermoplastic elastomers. The model can also be used to simulate production of star polymers using multi-functional coupling/linking agents. An improved fundamental understanding of the kinetics helps process engineers improve existing processes and design new products faster. roducing polymer with controlled molecular weight, MWD and microstructure is the key to success in this competitive market. A general mathematical model presented here helps achieve these goals. The authors would like to thank Aspen Technology, Inc. for the support of this work. References: [] Hsieh, Henry L.; Roderic. Quirk, Anionic olymerization rinciples and ractical Applications, Marcel Dekker, Inc. NewYork. 996 [2] Hsieh, H.L.; W.H. Glaze, Kinetics of Alkyllithium Intiated olymerizations, Rubber Chem. Tech., 43, 22, 97 [3] Hsieh, H.L. Synthesis of Radial Thermoplastic Elastomers, Rubber Chem. Tech., 49, No. 5, 35-3, 976 6