Nucleophilic monomers

It seems to us however that propagation, proceeding via tetrahedral intermediate should involve activated monomer. Nucleophilic attack of the amine on positively charged carbon atom in activated monomer molecule is followed by proton transfer either to oxygen or nitrogen atoms. The deavage of C—O bond (termination) in the intermediate is accelerated by two antiperiplanar orbitak at N atoms 188b). 

H2)2 Usually the b"asicity of a heteroc lic compound is affected in the expected order by the inductive effects, conjugation, steric effects, and ring size ( ). The order of basicities is the only estimation of the monomer nucleophilicities, and it reflects fairly well the overall reactivity observed in ring-opening cationic polymerization. 

Covalent Initiators. The initiation with alkylating compounds depends both on the ability of the initiator to form a cation and on the monomer nucleophilicity. Strong alkylating agents such as esters of superacids (CF SOoR, FSO2R, and CISO2R) are able to initiate directly without sides reactions both strong and weak nucleophilic monomers (44. 45). 

In the preceding section we have disscussed the influence of monomer nucleophilicity on copolymerization behaviour. Only monomers of similar nucleophilicities can be randomly copolymerized. Attempted simultaneous copolymerization of e.g. amines with ethers, and sulfides or oxazolines with ethers gives homopolymers of the more nucleophilic monomer. However, stepwise addition of a more basic comonomer to a living polymer derived from the less nucleophilic heterocycle yields block copolymers (cf. Sect. 15.3.1.2). 

The type of propagating species is mainly determined by the nucleophilicity of the monomer and basicity of the leaving group of the initiator, whereby covalent propagating species are present when the basicity of the counterion is higher than the nucleophilicity of the monomer and cationic propagating species are present if the basicity of the counterion is lower than the monomer nucleophilicity. The most nucleophilic monomer, 2-methyl-2-oxazoline, polymerizes via cationic propagating species with all counterions except... 

One monomer is relatively more electrophilic and the other more nucleophilic in a mixture of two monomers. Nucleophilic monomers are electron donors and electrophilic monomers are, in contrast, electron acceptors. On mixing such monomers reactions leading to spontaneous polymerization without the addition of initiator can occur according to the relative donor-acceptor strengths. 

Optimum combination of carbocation stability and monomer nucleophilicity is necessary for effective polymerization. Indeed, isobutylene positioned in the middle of both the carbocation stability and monomer nucleophilicity scales can be polymerized effectively to yield high molecular weight polymers, while the other monomers listed above mostly yield low molecular weight products by carboca-tionic polymerization. 

In addition to carbocation stability and monomer nucleophilicity, steric factors are also important. For example 1,1-diphenylethylene, a very reactive monomer can only be dimerized, because of the formation of a sterically buried carbenium ion (27). Other monomers that cannot be polymerized to high polymers because of steric congestion in the transition state are 2,4,4-trimethyl-l-pentene, 1,2-diphenylethylene (stilbene), 3-methylindene, and 2-methylenenorbomane. 

Another general feature of the cahonic process is that kp(= kp = kp) with a given onium cation increase with increasing nucleophilicity of the monomer, whereas for a given monomer kj, there is a decrease with increasing nucleophilicity of the heteroatom in the onium active species. However, the observed net result is that kp depends on the cyclic monomer nucleophilicity-that is, it increases in the order amines < oxazolines < sulfides < ethers < acetals [107]. 

Acrylamide, C H NO, is an interesting difiinctional monomer containing a reactive electron-deficient double bond and an amide group, and it undergoes reactions typical of those two functionalities. It exhibits both weak acidic and basic properties. The electron withdrawing carboxamide group activates the double bond, which consequendy reacts readily with nucleophilic reagents, eg, by addition. 

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. 

Frcc-Radical Reactions. Eree-radical reactions of maleic anhydride are important in polymeri2ations and monomer synthesis. Nucleophilic radicals such as the one from cyclohexane [110-82-7] serve as hydrogen donors that add to maleic anhydride at the double bond to form cyclohexylsuccinic anhydride [5962-96-9] (20) (63). 

I itro-DisplacementPolymerization. The facile nucleophilic displacement of a nitro group on a phthalimide by an oxyanion has been used to prepare polyetherimides by heating bisphenoxides with bisnitrophthalimides (91). For example with 4,4 -dinitro monomers, a polymer with the Ultem backbone is prepared as follows (92). Because of the high reactivity of the nitro phthalimides, the polymerkation can be carried out at temperatures below 75°C. Relative reactivities are nitro compounds over halogens, Ai-aryl imides over A/-alkyl imides, and 3-substituents over 4-substituents. Solvents are usually dipolar aprotic Hquids such as dimethyl sulfoxide, and sometimes an aromatic Hquid is used, in addition. 

Propa.ga.tlon, The tertiary THF oxonium ion undergoes propagation by an S. mechanism as a result of a bimolecular colHsion with THF monomer. Only colHsions at the ring a-carbon atoms of the oxonium ion result in chain growth. Depropagation results from an intramolecular nucleophilic attack of the penultimate chain oxygen atom at the exocycHc a-carbon atom of the oxonium ion, followed by expulsion of a monomer molecule. 

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. 

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethyl sil oxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the siHcone (eqs. 10 and 11). 

The chemistry of this cure system has been the subject of several studies (44—47). It is now generally accepted that the cure mechanism involves dehydrofluorination adjacent to hexafluoropropylene monomer units. The subsequent fluoroolefin is highly reactive toward nucleophilic attack by a variety of curatives (eg, diamines, diphenols). 

Care must also be taken in the choice of rubber to insure that the rubber, or one of its additives, does not initiate the premature polymerization of the monomer. Even very low concentrations of a basic or nucleophilic material in the rubber or elastomer will cause the premature polymerization of an alkyl cyanoacrylate adhesive formulation. 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

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. 

The 0-silylated bisphenol monomer is stable at high temperature, and can act as a good nucleophile beyond certain temperature. 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by basic (anionic) catalysts. The chain-carrying step is conjugate nucleophilic addition of an anion to the unsaturated monomer (Section 19.13). 

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