Styrene anionic solution polymerization

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

K-Resin SBC synthesis is a batch anionic solution polymerization of styrene and 1,3-butadiene using an n-butyllithium (NBL) initiator in a process referred to as living polymerization . Although often referred to as a catalyst, each NBL gives rise to a distinct polymer chain. Polymer chains grow by adding monomer... 

Manufacture Styrene-diene copolymers are produced by anionic solution polymerization, typically using i-butyl lithium in cyclohexane at 60-120°C. The system may also be promoted by small amounts of an amine such as tetramethylethylene diamine or an ether, such as tetrahydrofuran. Polymerization solids are in the range of 20-25%. 

SBR may also be produced by anionic solution polymerization of styrene and butadiene with alky-llithium initiator (e.g., butyllithium) in a hydrocarbon solvent, usually hexane or cyclohexane. In contrast to emulsion SBR, which may have an emulsifier (soap) content of up to 5% and nonrubber materials sometimes in excess of 10%, solution SBR seldom has more than 2% nonrubber materials in its finished form. Solution SBR has a narrower molecular weight distribution, higher molecular weight, and higher cis-1,4-polybutadiene content than emulsion polymerization SBR. 

These thermoplastic elastomers are prepared by anionic solution polymerization with or-ganometallic catalysts. A typical example of such preparation is polymerization of a 75/25 mixture of butadiene/styrene in the presence of jec-butyllithium in a hydrocarbon-ether solvent blend. At these reaction conditions butadiene blocks form first, and when all the butadiene is consumed styrene blocks form. In other preparations, monomers are added sequentially, taking advantage of the living nature of these anionic polymerizations. 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

Currently, more SBR is produced by copolymerizing the two monomers with anionic or coordination catalysts. The formed copolymer has better mechanical properties and a narrower molecular weight distribution. A random copolymer with ordered sequence can also be made in solution using butyllithium, provided that the two monomers are charged slowly. Block copolymers of butadiene and styrene may be produced in solution using coordination or anionic catalysts. Butadiene polymerizes first until it is consumed, then styrene starts to polymerize. SBR produced by coordinaton catalysts has better tensile strength than that produced by free radical initiators. 

Continuous solution Anionic Pure styrene monomer Much recycled solvent Anionic initiators Polymerize to completion Low residual monomer High polymerization rate Good for spec, copolymer Sensitivity to impurities Initiator cost Color of product Cannot produce HIPS Not proven for high-volume GP... 

Rabagliati et al. (14) studied the polymerization of styrene in a three phase system containing an anionic-nonionic surfactant mixture and brine. Both AIBN and potassium persulfate initiators were used. The system was reported to be microemulsion continuous and even multicontinuous. (14). No autoacceleration was observed and the authors concluded that the polymerization exhibits an inverse dependence of the degree of polymerization on initiator concentration, similar to bulk solution polymerization. 

The yield of the absorption at 320 m/x was found to rise with concentration in a way suggesting that it is formed by anionic process—e.g., by protonation of a styrene anion by its geminate partner (31). If it is a polymerizing radical, then the rate constant for addition of the benzyl-type radical to styrene must be at least KP-lOW-1 sec.-1. The yield of the observable anion seemed much less dependent on concentration, which is consistent with the view that it is formed by those electrons (G.— 0.2) which escape from the spur. Absorptions with peaks close to 320 m/x are seen for all the other solvents above, but the component at the longer wavelength is seen only with the aliphatic hydrocarbons. For methanol solutions this may be because of rapid protonation. 

Over 200 references describing spontaneous, and chemically initiated styrene polymerization chemistry are reviewed with special emphasis on advances taking place in the past decade. The review is limited to chemistry useful for making amorphous high molecular weight polystyrene in solution polymerization processes. Chemical initiators have been categorized into three basic groups as follws 1) anionic 2) mono-radical and 3) diradical. Analytical techniques used for determination of free radical polymerization kinetics and mechanisms are also discussed. 

Styrene is one of the oldest and most studied monomers. It spontaneously generates free radials upon heating above 100 °C and polymerizes yielding amorphous polystyrene (PS). Styrene can also be polymerized by other mechanisms (anionic, cationic, or Zeigler-Natta) with the aid of chemical initiators. Commercially, over twenty billion pounds of PS are produced annually worldwide. All of this polystyrene is produced via free radical (FR) chemistry, and mostly via continuous solution polymerization processes. The commercial preference for the continuous solution process is due mainly to economic factors. Non-solution polymerization processes (suspension and emulsion) have lower reactor efficiency (product/reactor volume) due to reactor volume occupied by the water which adds to the manufacturing cost. 

Evidence clearly shows that the thermal stability of PS is dependent on the polymerization mechanism. Upon heating at 285 C for 2.5 h under different vacuum levels, anionic PS loses less molecular weight (Fig. 14) and generates l s styrene monomer (Fig, 15) than FR PS both produced using continuous solution polymerization processes [73]. 

Ionic polymerization systems of commercial importance employ mostly batch and continuous solution polymerization processes. Suitable monomers for ionic polymerization include conjugated dienes and vinyl aromatic. Among these, the anionic polymerization of styrene-butadiene (SB) and styrene-isoprene (SI) copolymers and the cationic polymerization of styrene are the most commercially important systems. 

SOLUTION POLYMERIZATION Solution SBR typically made in hydrocarbon solution with alkyl lithium-based inihator. In this stereo-specific catalyst system, in principle, every polymer molecule remains live until a deactivator or some other agent capable of reacting with the anion intervenes. Able to control molecular weight, molecular weight distribution, and branching. Able to make random and block copolymers with designed chain sequence. Able to make copolymer with controlled styrene content. Able to control the butadiene structure of vinyl/ ds/ trans. Higher purity due to no addition of soap. 

Block Copolymerization. A polymerization with long chain lives can be used to make block copolsrmers (qv). An important commercial example is styrene/butadiene blocks produced by anionic polymerization (qv). A solution polymerization is done in a batch reactor, starting with one of the two monomers. That monomer is reacted to completion and the second monomer is added while the catalytic sites on the chains remain active. This produces a block copolymer of the AB form. Early addition of the second monomer produces a tapered block. If the second monomer is reacted to completion and replaced by the first monomer, an ABA triblock is obtained. This process is not easily converted to continuous operation because polsrmerizations inside tubes rarely approach the piston-flow environment that is needed to react one monomer to completion before adding the second monomer. Designs using static mixers (also known as motionless mixers) are a possibility. 

The solution polymerization system with anionic catalysts is best suited for the preparation of rubbers of controlled structure. The well-known structural control that is attainable in styrene/butadiene rubbers in such systems is given in Table 2. Of particular interest is... 

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