Initiation, of styrene polymerization

In initiation by an alkali-metal alkyl, the alkyl links up with the monomer to form a carbanion, leaving the metal as a cation to compensate the negative charge. An example is the initiation of styrene polymerization by butyl lithium [70-72] ... 

Higashimura and his co-workers have examined trifluoroacetic acid as an initiator of styrene polymerizations, and using n.m.r. spectra clearly identified a polystyryl or styryl ester (4). Equally clear from the kinetic data was that this species is not capable of propagating. 

A few examples are in place. Carbanions are good initiators of polymerization of many vinyl and diene monomers. Their reactivities decrease along the series primary, secondary, tertiary. Thus, benzyl carbanion is a poor initiator of styrene polymerization, whereas the oligomeric a-methyl styrene anions or cumyl carbanions are very efficient110. ... 

Fig. 23. Typical conversion curves recorded spectrophotometrically for initiation of styrene polymerization by n-Buli in benzene.

Fig. 24. Dependence of the initial rate of initiation of styrene polymerization in benzene on the concentration of n-butyl lithium. The slope of the solid line = 1/6...

The initiation of styrene polymerization by r-butyl lithium in benzene is an apparent exception. The rates were found to be proportional to the first power concentration of the lithium compound and independent of the monomer concentration which was varied from 0.1 to 10-3 M172). This strange behavior is illustrated by Fig. 26. The authors proposed that the dissociation of r-butyl lithium into smaller, active aggregates, dimers or monomers, is the rate determining step of this process. However, the rate of initiation of isoprene by r-butyllithium in benzene shows a more conventional behavior it increases with increasing concentration of the monomer, although again it is proportional to first power concentration of r-butyl lithium. 

The proposed explanation of the strange behavior of r-BuLi in the initiation of styrene polymerization in benzene is questionable because the initiation of polymerization of isoprene is faster than of styrene (compare the data shown in Fig. 26 after correcting them for the difference in the concentrations of r-BuLi). Therefore, an alternative explanation is advocated, namely a complexation of r-BuIi with the monomer followed by the initation,... 

The effect of impurities appears to be of a similar nature. For example, meticulously purified /-BuLi is a fast initiator of styrene polymerization in benzene, whereas the commercial product is a slow one. On the other hand, some impurities alleviated the induction period observed in polymerizations taking place in aliphatic hydrocarbons. 

In the presence of an excess of tetrahydrofuran, say at 0.15 M or more, the initiation of styrene polymerization by n-butyl lithium is virtually instantaneous210). The optical density at 335 nm (Xmax of the living polymers) reaches its maximum before any measurements can be taken. The aggregates of butyl lithium either disintegrate under the influence of the ether or become extremely reactive. The latter suggestion seems more plausible since Waack was observing still the undissociated tetrameric methyl lithium in diethyl ether. 

Scheme 1. Flory nrechanism for spontaneous initiation of styrene polymerization...

Initiation of styrene polymerization using a wide variety of chemically generated FR has been studied. The stability, size, and electronics of a FR greatly affects the rate at which it adds to vinyl monomers. 

Scheme 8. Initiation of styrene polymerization using benzopinacole...

The usefulness of l-alkoxy- and t-butyldimethylsiloxy-functionalized alkyllithium initiators for anionic polymerization have been evaluated using five experimental criteria. All of these criteria must be satisfied for an initiator to be generally useful. These protected hydroxyl-flmctionalized initiators are all useful for the polymerization of butadiene and styrene monomers. Inefficient initiation of styrene polymerization was observed in cyclohexane, but not in benzene. 

Ultrasonic initiation of styrene polymerization was investigated by Qiu [182, 183]. Although hybrid particles could be obtained, plain polymer nanoparticles were found in the system as well. 

Solutions of potassium metal in ethers, however, form ion radicals through additions of electrons to the monomers. It should be noted that fresh solutions of potassium metal in various ethers like tetrahydrofuran or dimethoxyethane are blue in color. This blue color is attributed to the presence of spin-coupled electron pairs (ea). The initiation of styrene polymerization that takes place between 0 and -78 °C is therefore pictured as follows ... 

Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

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