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TABLE 5-13 Ceiling Temperaturesa Monomer Formaldehyde Tri uoroacetaldehyde Trichloroacetaldehyde Propanal Acetaldehyde Pentanal
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Tc ( C) 119b 85 11 31 39 42
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Data from Brandup and Immergut [1989], Kubisa et al. [1980], Vogl [1976, 2000]. b Monomer concentration 1 atm (gaseous); all other data are for neat liquid monomer.
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polymerization reactions were carried out at temperatures below the ceiling temperatures of the monomers. Both anionic and cationic initiators, of the types used in alkene polymerizations, can be used to initiate the polymerization of the carbonyl double bond. These polymerizations are similar in their general characteristics to the ionic polymerizations of alkene monomers. The number of detailed mechanistic studies of carbonyl polymerizations is, however, very small. A characteristic of aldehyde polymerization is the precipitation, often with crystallization, of the polymer during polymerization. Depending on the solvent used, polymerization rate, state of agitation, and other reaction conditions, the polymerization can slow down or even stop because of occlusion of the propagating centers in the precipitated polymer. The physical state and surface area of the precipitated polymer in uence polymerization by their effect on the availability of propagating centers and the diffusion of monomer to those centers. 5-6a Anionic Polymerization
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The carbonyl group is polymerized by a variety of anionic initiators. The strength of the base required to initiate polymerization depends on the substituent(s) attached to the carbonyl group. 5-6a-1 Formaldehyde
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The carbonyl group of formaldehyde is highly susceptible to nucleophilic attack and this monomer can be polymerized with almost any base. Metal alkyls, alkoxides, phenolates, and carboxylates, hydrated alumina, amines, phosphines, and pyridine are effective in polymerizing formaldehyde. The polymerization proceeds as follows: Initiation barcode pdf417 integrationin .net
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A (G+ ) + CH2 O A CH2 O (G+ )
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CH2 O (G+) + CH2
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CH2 O (G+)
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CH2 O (G+) + ZH
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CH2 OH + Z (G+)
with initiation by anionic species A to form an alkoxide anion with nearby counterion G . Propagation proceeds in a like manner and termination occurs by transfer of a proton from ZH. The chain-transfer agent ZH may be any of a variety of compounds that can transfer a proton to the propagating alkoxide anion, such as water or an alcohol. The chain-transfer agent can have an effect on the polymerization rate if Z is not as effective as A in reinitiating polymerization. 5-6a-2 Other Carbonyl Monomers
The polymerization of carbonyl monomers other than formaldehyde follows in a similar manner, although the basicity of the initiator required may be quite different. Strong bases are required to initiate the polymerization of aliphatic aldehydes such as acetaldehyde and higher aldehydes [Starr and Vogl, 1978, 1979]. The inductive effect of an alkyl substituent destabilizes the propagating anion XLIII by increasing the negative charge density on oxygen. The alkyl group also decreases reactivity for steric reasons. (Steric considerations are
probably also responsible for the lower Tc values relative to formaldehyde.) Thus weak bases such as amines cannot be used to polymerize higher aldehydes than formaldehyde. Alkali metal alkyls and alkoxides are required. The presence of adventious water is deterimental, since the initiator reacts to form hydroxide ion, which is too weak to initiate polymerization. Ketones are unreactive toward polymerization because of the steric and inductive effects of two alkyl groups. Exceptions to this generalization are some copolymerizations of ketones with formaldehyde and the polymerization of thiocarbonyl monomers [Colomb et al., 1978]. Aromatic aldehydes are also unreactive. A side reaction occurring with acetaldehyde and higher aldehydes containing a-hydrogens is aldol condensation [Hashimoto et al., 1976, 1978; Yamamoto et al., 1978]. Aldol reaction can be extensive at ambient temperatures and higher but is avoided by polymerization at low temperature. The substitution of halogens on the alkyl group of an aliphatic aldehyde greatly enhances its polymerizability (both from the kinetic and thermodynamic view points [Kubisa and Vogl, 1980]. Trichloroacetaldehyde (chloral) is easily polymerized by such weak bases as pyridine, alkali thiocyanates, and even chloride ion. Further, the polymerization of chloral by butyllithium at 78 C is complete in less than a second [Bus eld and Whalley, 1965]. The electron-withdrawing inductive effect of the halogens acts to stabilize the propagating anion XLIV by decreasing the charge density on the negative oxygen. The effect of halogens on reactivity is also seen for uorothiocarbonyl monomers. Thiocarbonyl uoride is polymerized at 78 C by a trace of mild base such as dimethylformamide. The polymerization of hexa uorothioacetone is an extreme example of the effect of