Captodative, acrylates

Table 6. Initiated radical homopolymerization of captodative acrylates CTI, = C(rf)COOR... 

Table 14. Copolymerization parameters of captodative acrylates H2C=C(d)COOR (MJ in their radical copolymerization with monomers (M2) other than styrene ...

Spontaneous thermal copolymerizations of captodative acrylates with styrene lead to a copolymer with higher molecular weight than the homopolymer. Copolymerization parameters are summarized in Table 17 [70], Both parameters r and r2 in the spontaneous copolymerizations are in agreement with those in the AIBN-initiated copolymerizations within experimental error, supporting a radical mechanism for the spontaneous copolymerizations. 

There have heen many studies on the polymerizability of a-substituted acrylic monomers.3jU35 33S It is established that the ceiling temperature for a-alkoxyacrylates decreases with the size of the alkoxy group. 35 However, it is of interest that polymerizations of a-(alkoxymethyl)acrylates (67)3 15 and a-(acyloxymethyl)acrylates (68)and captodative substituted monomers (69, 70) 39 appear to have much higher ceiling temperatures than the corresponding a-alkylacrylates methyl ethacrylate, MEA). For example, methyl a-... 

In a systematic study of the addition of cyclohexyl radicals to a-substi-tuted methyl acrylates, Giese (1983) has shown that the captodative-substituted example fits the linear correlation line of log with o-values as perfectly as the other cases studied. Thus, no special character of the captodative-substituted olefin is displayed. More recently, arylthiyl radicals have been added to disubstituted olefins in order to uncover a captodative effect in the rate data (Ito et aL, 1988). Even though a-A, A -dimethyl-aminoacrylonitrile reacts fastest in these additions, this observation cannot per se be interpreted as the manifestation of a captodative effect. Owing to the lack of rate data for the corresponding dicaptor- and didonor-substituted olefins, it is not possible to postulate a special captodative effect. The result confirms only that the A, A -dimethylamino-group, as expected from its a, -value, enhances the addition rate. In the sequence a-alkoxy-, a-chloro-, a-acetoxy- and a-methyl-substituted acrylonitriles, it reacts fastest. 

Tanaka, H. Yoshida, S. Kinetic study of the radical homopolymerization of captodative substituted methyl a-(acyloxy)acrylates. Macromolecules 1995, 28, 8117-8121. 

The radical polymerization behavior of captodative olefins such as acrylonitriles, acrylates, and acrylamides a-substituted by an electron-donating substituent is reviewed, including the initiated and spontaneous radical homo- and copolymerizations and the radical polymerizations in the presence of Lewis acids. The formation of low-molecular weight products under some experimental conditions is also reviewed. The reactivity of these olefins is analyzed in the context of the captodative theory. In spite of the unusual stabilization of the captodative radical, the reactivity pattern of these olefins in polymerization does not differ significantly from the pattern observed for other 1,1-disubstituted olefins. Classical explanations such as steric effects and aggregation of monomers are sufficient to rationalize the observations described in the literature. The spontaneous polymerization of acrylates a-substituted by an ether, a thioether, or an acylamido group can be rationalized by the Bond-Forming Initiation theory. 

However, a careful HPLC analysis of the mixture arising from the photo-stimulated reaction between a-methoxydeoxybenzoin and methyl ot-methoxy-acrylate at room temperature demonstrated that a considerable number of products are formed during this reaction [62]. Besides the expected products, some were found to result from the addition of a captodative (methoxy)-(methoxycarb-onyl)alkyl radical to the captodative olefin itself, i.e. a propagation step. The HPLC analysis was not quantitative, but the presence of these products even at high olefin/initiator ratio (2/1) implies that a propagation step cannot be ruled out for all the above experiments only on the basis of the isolation of low-molecular weight products in good yields. As we will show the polymerizability of captodative olefins is confirmed by an analysis of the literature. 

Because of the general lack of quantitative thermodynamic (ceiling temperature Tc) and kinetic (kp, kp/k[J 5) data for the polymerization of the captodative olefins, it is impossible to draw firm conclusions about the importance of electronic factors on their polymerizability. If we compare them with other 1,1-disubstituted olefins by replacing the heteroatom O, S, or N by a CH2 and check the polymerizability of the resulting olefins, we find that the latter are in fact also difficult to polymerize as shown in Table 8 [77], Only methyl acrylates and methacrylates give high polymers easily. The polymerizability decreases rapidly with the steric hindrance of the substituent. 

A conventional explanation for the difficult radical polymerization of 1,1-disubstituted olefins is the known lowering action of bulky substituents on Tc. The classical example of this effect is a-methylstyrene [78]. Reported Tc values for substituted acrylates are collected in Table 9. Unfortunately, no Tc values have ever been reported for any captodative olefins. 

Significant spontaneous thermal polymerizability is one of the most remarkable characteristics of captodative olefins. As mentioned in Sect. 4, methyl a-acylamido-acrylate with a C9 alkyl substituent spontaneously homopolymizes to high molecular weight, even at relatively low temperature (0-20 °C) [73]. 

Table 15. Spontaneous polymerizations of captodative substituted acrylates CH2 = C(r/)COOR in bulk at 60 °C...

Table 17. Copolymerization parameters for the copolymerization of captodative substituted acrylates CH2=C(rf)COOR (Mt) and styrene (M2) at 60 °C...

Density functional theory (DFT) calculations have been used to investigate and rationahze the regio- and stereochemical outcome of 1,3-DC of (C-hetaryl)nitrones with methyl acrylate and vinyl acetate <07T1448>, diphenyl nitrone with captodative olefins 1-acetylvinyl carboxylates <07EJO2352> and diphenyl nitrone with acrolein in the presence of a Lewis acid catalyst <07T4464>. 

Captodative alkenes 67 can be dialkylated, for example, by addition of iso-butyronitrile radical derived from thermal decomposition of AIBN under the same conditions as those which lead to polymerization of other acrylic alkenes. For example, a-morpholino-acrylonitrile (67, c = CN, d = N(CH2CH2)20) leads to 69, in 71% yield (Scheme 12) [4a]. With a-/-butylthio-acrylonitrile (67, c = CN, d = SC(CHj)3), the same process leads to 70 in 88% yield [7]. The adduct radical 68 is highly stabilized, and is in equilibrium with dimer 70. The reaction is quite general, and has been applied to other captodative alkenes (c = CN, COR, CO2R and d = NR2, OR, SR) together with various sorts of radical partners, derived from alkanes, alcohols, thiols, thioethers, amines, amides, ketones, aldehydes, acetals and thioacetals [44, 45]. 

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