Polystyryl carbanion

Anionic polymerization offers fast polymerization rates on account of the long life-time of polystyryl carbanions. Early studies have focused on this attribute, most of which were conducted at short reactor residence times (< 1 h), at relatively low temperatures (10—50°C), and in low chain-transfer solvents (typically benzene) to ensure that premature termination did not take place. Also, relatively low degrees of polymerization (DP) were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high temperatures (>100° C), at long residence times (>1.5 h), utilize a chain-transfer solvent (ethylbenzene), and produce polymer in the range of 1000—1500 DP. 

Addition of styrene to a green solution of naphthalene" Na+ in tetrahydrofuran leads to an instantaneous change of color from green to red. Styrene polymerizes rapidly and quantitatively within a few seconds, and when the reaction is completed, addition of water converts the red solution of polystyryl carbanions into colorless solution of polystyrene. After precipitation of the polymer it was shown spectroscopically25 that the residual solution contains an amount of naphthalene equal to that used in the preparation of the initiating catalyst. This observation confirms the proposed mechanism of initiation of the polymerization. 

The red solution of polystyryl carbanions can be kept for days without change in color or viscosity. No changes are observed on addition of further amounts of naphthalene to the red solution. These observations raise some questions. An electron transfer, say for example, between naphthalene" and phenathrene, is a reversible process and it leads eventually to an equilibrium between naphthalene , naphthalene, phenathrene-, and phenanthrene. Is the reaction involving styrene irreversible Now, the initial process of electron transfer from naphthalene to styrene that produces... 

The sodium hydride eliminated in Reaction 5-73 may also participate in hydrogen abstraction from XXI. The order of stability of polystyryl carbanions depends on the alkali metal in the order K > Na > Li. 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

Thus, the synthesis of a styrene-methyl methacrylate block polymer requires that styrene be the first monomer. Further, it is useful to decrease the nucleophilicity of polystyryl carbanions by adding a small amount of 1,1-diphenylethene to minimize attack at the ester function of MMA [Quirk et al., 2000]. Block copolymers of styrene with isoprene or 1,3-butadiene require no specific sequencing since crossover occurs either way. Block copolymers of MMA with isoprene or 1,3-butadiene require that the diene be the first monomer. The length of each segment in a block copolymer is controlled by the ratio of each monomer to initiator. The properties of the block copolymer vary with the block lengths of the different monomers. 

Coupling of polymers to form graft copolymers is accomplished by nucleophilic reaction between living polystyryl carbanion and various chlorine-containing polymers such as... 

Preparation of desired molecular weight polystyryl carbanion ( Living Polystyrene ) by anionic polymerization (Fig. 2). Anionic polymerization has been used extensively to provide control over molecular weight with narrow molecular weight distribution. 

Reaction of the mesylated lignin prepared in step 1 (Fig. 1) with the polystyryl carbanion (living polystyrene) from step 2 (Fig. 2). The carbanion displaces the mesylate groups on the lignin in a nucleophilic displacement reaction with the formation of the polystyrene-lignin graft copolymer (Fig. 3). 

Thus polystyryl carbanions and polyacrylonitrile carbanions prepared by anionic polymerization were reacted with cellulose acetate or tosylated cellulose acetate in tetrahydrofuran under homogenous reaction conditions. The carbanions displaced the acetate groups or the tosylate groups in a S v2-type nucleophilic displacement reaction to give CA-g-PS and CA-g-PAN. Mild hydrolysis to remove the acetate/tosylate groups furnishes the pure cellulose-g-polystyrene (Figure 3). 

Living polymers do not live forever and even in the absence of terminating agents decays with time [2]. The most stable of all living anionic systems are polystyryl carbanions, as they are stable for weeks in hydrocarbon solvents. The mechanism for the decay of polystyryl carbanions on aging, referred to as spontaneous termination, is not completely established. The generally accepted mechanism consists of hydride elimination ... 

As mentioned earlier, living polymers do not, in reality, have infinite life times even in the complete absence of terminating agents as they undergo decay on aging, a process known as spontaneous termination. Polystyryl carbanions, known to be the most stable of all anionic chains as they can survive for weeks in hydrocarbon solvents, undergo spontaneous termination by a mechanism known as hydride elimination, as shown by the equation (Odian, 1991) ... 

TABLE 7.1 Equilibrium Dissociation Constants of Organoalkali Metal Salts of Polystyryl Carbanions and the Propagation Rate Constants for the Corresponding Ion Pairs and Eree Ions in THE at 25 °C [48, 97-99]... 

The validity of this assumption has been demonstrated (1) by adding more styrene to the living polystyryl carbanions, and (2) by adding another monomer such as isoprene, to form a block copolymer. 

As mentioned above, the polystyryl carbanions are particularly stable and persist for weeks in nonpolar solvents. Yet, even in the absence of terminating agents, the concentration of the carbanion active centers decreases with time. The mechanism of decay is not fully understood. Based on spectral evidence, it is believed to consist of a hydride-ion elimination ... 

Polystyryl carbanions are much less stable in polar solvents. They decay within a few days at room temperature. At lower temperatures, however, the stability is considerably better. The termination in polar solvent occurs by a mechanism of abstracting a-hydrogens and/or by a nucleophilic attack on the carbon-oxygen bonds. 

The polystyryl monocarbanion 4 was prepared in THF at -78 C using sec-butyl lithium as the initiator while the polystyryl dicarbanion (Scheme 2) was prepared using sodium naphthalene as the initiator. The initiator was added first followed by the styrene. A bright, reddish orange-colored solution was obtained confirming the presence of the polystyryl carbanions. 

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