Electrophilic monomers

The polymerization of ethyleneimine (16,354—357) is started by a catalyticaHy active reagent (H or a Lewis acid), which converts the ethyleneimine into a highly electrophilic initiator molecule. The initiator then reacts with nitrogen nucleophiles, such as the ethyleneimine monomer and the subsequendy formed oligomers, to produce a branched polymer, which contains primary, secondary, and tertiary nitrogen atoms in random ratios. Termination takes place by intramolecular macrocycle formation. 

StericaHy hindered or very electrophilic substituted ketenes, such as diphenylketene, di-Z rZ-butylketene [19824-34-17, and bis(trifluoromethyl)ketene, are quite stable as monomers. Ketenimines tend to polymerize. The dimerization of thioketenes results in 1,3-dithiacyclobutanones (6) (45), a type of dimer not observed with ketenes. 

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

If the initiation reaction is much faster than the propagation reaction, then all chains start to grow at the same time. Because there is no inherent termination step, the statistical distribution of chain lengths is very narrow. The average molecular weight is calculated from the mole ratio of monomer-to-initiator sites. Chain termination is usually accompHshed by adding proton donors, eg, water or alcohols, or electrophiles such as carbon dioxide. 

Recently, the above mentioned model reaction has been extended to polycondensation reactions for synthesis of polyethers and polysulfides [7,81]. In recent reports crown ether catalysts have mostly been used in the reaction of a bifunctional nucleophile with a bifunctional electrophile, as well as in the monomer species carrying both types of functional groups [7]. Table 5 describes the syntheses of aromatic polyethers by the nucleophilic displacement polymerization using PTC. 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

The transient radicals produced in reactions of hydroxy radicals with vinyl monomers in aqueous solution have been detected directly by EPR43 439 or UV spectroscopy,440-441 These studies indicate that hydroxy radicals react with monomers and other species at or near the diffusion-controlled limit ( Table 3.7). However, high reactivity does not mean a complete lack of specificity. Hydroxy radicals are electrophilic and trends in the relative reactivity of the hydroxy radicals toward monomers can be explained on this basis/97... 

Most studies have concerned the kinetics of arenethiyl radicals with monomers including S and its derivatives468 47" and MMA.469,473 The radicals have electrophilic character and add more rapidly to electron-rich systems (Tabic 3.10). Relative reactivities of the monomers towards the ben/.oy Ithiyl radical have also been examined.453... 

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

A consequence of the selectivity for electrophilic radicals is that not all products are trapped with equal efficiency. With electron-rich monomers (e.g. S) oligomerization may complicate analysis. Other possible complications in the utilization of this method have been discussed by Russell.491... 

The fraction of head-to-head linkages in the poly(fluoro-olefms) increases in the series PVF2 < PVF PVF3 (Tabic 4.2). This can be rationalized in terms of the propensity of electrophilic radicals to add preferentially to the more electron rich end of monomers (i.e, that with the lowest number of fluorines). This trend is also seen in the reactions of trifluoromethyl radicals wilh the fluoro-olefins (see 2.3). 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

The halocarbons react more rapidly with nucleophilic radicals than with electrophilic radicals. Thus, values of Cir with S and VAc are substantially higher than those with acrylic monomers ( fable 6.4) where the transfer constant is close to ideal (Clr=l.0). The haloalkyl radicals formed have electrophilic character (Section 2.3,2). 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

Compared with the aromatic electrophilic substitution approach, the SNAr approach general requires higher reaction temperatures. The polymers generally have well-defined structures. Therefore, it is more facile to control the structures of die products. In addition, it is more tolerable to some reactive functional groups, which makes it possible to synthesize reactive-group end-capped prepolymers and functional copolymers using functional monomers. 

To be eligible to living anionic polymerization a vinylic monomer should carry an electron attracting substituent to induce polarization of the unsaturation. But it should contain neither acidic hydrogen, nor strongly electrophilic function which could induce deactivation or side reactions. Typical examples of such monomers are p-aminostyrene, acrylic esters, chloroprene, hydroxyethyl methacrylate (HEMA), phenylacetylene, and many others. 

Another important consequence of the limitations concerning cross-addition is that anionic polymerization is not suited for the synthesis of random copolymers. If a mixture of two anionically polymerizable monomers is reacted with an initiator, the most electrophilic monomer will polymerize while the other is left almost untouched 30). In other words, a general feature of anionic binary copolymerization is that one of the reactivity ratios is extremely high while the other is close to zero. 

Sequential addition of monomers 6 7-26-27-114) is the most obvious procedure. Once the first monomer has been polymerized, the resulting living species is used as a polymeric initiator for the polymerization of the second one. The monomers are to be added in the order of increasing electron affinity to provide efficient and fast initiation 26 U4). This condition is rather restrictive, and the number of monomer systems that can be used is limited (Table 5). Moreover, when the second monomer contains an electrophilic function (e.g. ester) which could lead to side reactions, it is necessary to first lower the nucleophilicity of the living site. This is best done by intermediate addition of 1.1-diphenylethylene25). The stabilized diphenylmethyl anions do not get involved in side reactions with ester functions, while initiation is still quantitative and fast. 

The validity of this statement is confirmed by the rates of IC1 additions (see Table 12). Because for these additions the formation of a cationic intermediate by direct attack of the electrophile on the double bond is rate determining, their order of rates is comparable to those of polymerizations. It is therefore understandable that the polymerization rates correlate much better with the reactivities of the monomers during an electrophilic addition of cationogenic agents (such as IC1) than with the relatively unspecific EDA complex formation. 

The electrophilicity and therefore the stability of the cationic chain ends are relatively limited, because, on the one hand, the electrophilicity must be large enough to aid a nucleophilic attack by the monomer, but on the other hand, not so large that a chain termination occurs due to recombination with the counterion. For this reason the stability of the cationic chain ends is a function of ... 

The primary attack of an electrophile takes place during both the electrophilic addition to olefines and the cationic polymerization resulting in the formation of a car-benium ion R—C+H—CH3 as a reactive intermediate from the olefine or monomer R—CH=CH2 72) (Eq. 16). In the simplest of cases, the electrophile is a proton. 

Elastic chains 163 Elastomers 27, 32, 33, 63, 65 Electron affinity, monomers 150 Electrophiles 155-157, 162 —, plurifunctional 162 Electrophilicity 149... 

The easy homolysis of C-Br bond in CBr4 allowed us to conduct the radical chain reaction of CBr4 with 3,3,3-trifluoropropene under common conditions (benzoyl peroxide), although in this case the strong electrophiles are used as reagents (an addend and a monomer), i.e. a very unfavorable combination of polar factors for proceeding the process takes place (ref. 6). 

Random copolymerization occnrs between butadiene and styrene [15]. There are no appreciable differences in the nncleophilic and electrophilic abilities between the radical centers with the vinyl and phenyl groups at the end of the growing polymer chain or in the donor/acceptor properties between the monomers. 

Chain initiation might conceivably be brought about by polarization of the monomer by an electrophilic catalyst as follows ... 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

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