Anionic polymerization temperature effects

Effects of solvent polarity, counter-anion nucleophilidty, temperature, and monomer concentration on the carbenium ion polymerization chemistry have been extensively studied29,36 38,49. Based on previous knowledge26"29 Me3Al was chosen because with this coinitiator undesired chain transfer to monomer processes are absent. Preliminary experiments showed that Et3Al coinitiator did not yield PaMeSt, possibly because the nuc-leophilicity of the counter-anion Et3AlQe is too high and thus termination by hydrida-tion is faster than propagation36. ... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

The low temperature peroxyborane system is very effective for converting thiocarbonyl fluoride to homopolymer. The product is comparable to those formed by anionic polymerization. Since polymerization of thiocarbonyl fluoride is substantially slower than that of the chlorofiuoride, this monomer copolymerizes with exceptional ease with a large number of vinyl compounds to give products that appear to be random copolymers. 

Table I shows the effect of visible light illumination before raising the temperature of the irradiated glasses. It appears that polymerization is not initiated at the polymerization temperature in the absence of anion radicals in the glasses. The small but not zero values of conversion for the illuminated glasses may result from incomplete bleaching of the anion radicals, the diameter of the polymerization vessels (20 mm) being much larger than that of the ESR sample tubes (4 mm). The effect of pre-illumination on the conversion indicates that the anion radicals are involved in the initiation process of the radiation-induced polymerization of nitroethylene.

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

Catalysts of the Ziegler type have been used widely in the anionic polymerization of 1-olefins, diolefins, and a few polar monomers which can proceed by an anionic mechanism. Polar monomers normally deactivate the system and cannot be copolymerized with olefins. However, it has been found that the living chains from an anionic polymerization can be converted to free radicals in the presence of peroxides to form block polymers with vinyl and acrylic monomers. Vinylpyridines, acrylic esters, acrylonitrile, and styrene are converted to block polymers in good yield. Binary and ternary mixtures of 4-vinylpyridine, acrylonitrile, and styrene, are particularly effective. Peroxides are effective at temperatures well below those normally required for free radical polymerizations. A tentative mechanism for the reaction is given. 

Anionic polymerization, however, can be used to produce high molecular weight narrow distribution polystyrene. If all the chains are initiated at the same time and the temperature is kept low to minimize chain transfer, molecular weight distributions very close to monodisperse can be produced. The commercial uses of these polymers seem to be limited to instrument calibrations and laboratory studies of the effects of molecular weight on rheology and physical properties. However, anionic polymerization as a potential commercial method for producing polystyrene has been extensively studied by Dow and others. The potential for high polymerization rates, complete conversion of... 

Under suitable conditions, anionic polymerization is faster than free-radical polymerization and so can be conducted at lower temperatures. The main reasons are fast initiation by an ionic reaction and absence of an effective termination mechanism. However, the sensitivity to impurities is much greater and choice and control of reaction conditions are more delicate. Water, oxygen, carbon dioxide, and other substances able to react with carbanion chain carriers must be strictly excluded. 

The temperature has a decisive effect on the anionic polymerization initiated by DPHLi (10) or benzyllithium in THF. As soon as the temperature is increased from —78 °C up to —40°C, the livingness is lost. Compared to MMA and other methacrylates, tBuMA is an exception for which the anionic polymerization remains under control at temperatures as high as 25 °C. This remarkable behavior is accounted for by the steric hindrance of the ester group, which prevents side reactions from occurring at an appreciable rate. 

The direction of temperature effects in anionic polymerizations is conventional, with increased temperature resulting in increased reaction rates. Observed activation energies are usually low and positive. This apparent simplicity disguises complex effects, however, and the different ion pairs and free ions do not respond equally to temperature changes. Overall activation energies for polymerization will be influenced indirectly by the reaction medium because the choice of solvent shifts the equilibria of Eq. (9-1). 

Overview Anionic Initiation Anionic Propagation Termination Reactions Temperature Effects Anionic Copolymerization Reactions Stereochemistry of Anionic Diene Polymerization... 

Fig. 17. Effect of the initial ratio of activator/initiator ([A]o/[I]o) on the initial rate of anionic polymerization of caprolactam [169]. Concentration of sodium caprolactam [I]o = 0.0044 mole kg [A]q, the initial concentration of activator (tetraacetylhexamethylenediamine), expressed in moles of diacylamine groups per kg temperature, 150°C.

Figure 8 Continuous injection method of contacting the catalytic species fa) heat evolution versus time recorded during successive and continuous injection of the Cl for the anionic polymerization ofL6 at 105 °C (b) effect of temperature on the overall heat evolution (the Cl was delivered 7 h at a rate of 60 pi h )...

Figure 13 Effect of concomitant scanning of temperature and injection rate of Cl on the anionic polymerization of L6...

Polymerization of styrene in liquid ammonia at low temperatures catalyzed by potassium metal represents a good example of a base-initiated anionic polymerization. Styrene, being relatively nonpolar, requires the strongly basic amide ion for effective anionic polymerization. The catalyst is made, in situ, usually before the styrene is added, by the addition of small pieces of potassium metal to liquid ammonia kept at dry ice (solid carbon dioxide) temperatures (Eq. 22.30). 

These are chemically homogeneous multiarm star polymers (usually homopolymers) with only excluded volume interactions [21,22], Due to their synthesis procedure (high vacuum anionic polymerization), they are stable and nearly monodisperse [23]. Their softness can be tuned at the synthesis level (number and size of arms) [23,24] and/or by varying the temperature in different solvents [25,26], Moreover, these systems can be functionalized in various ways [27]. What made these systems truly ideal soft colloids were the breakthroughs in both theoretical description and synthesis. The former refers to the ability to describe analytically their internal structure [28] and their softness in terms of an effective interaction potential [24,29]. 

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