Initiation reaction with monomer

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

Several studies have demonstrated the successful incoriDoration of [60]fullerene into polymeric stmctures by following two general concepts (i) in-chain addition, so called pearl necklace type polymers or (ii) on-chain addition pendant polymers. Pendant copolymers emerge predominantly from the controlled mono- and multiple functionalization of the fullerene core with different amine-, azide-, ethylene propylene terjDolymer, polystyrene, poly(oxyethylene) and poly(oxypropylene) precursors [63,64,65,66,62 and 66]. On the other hand, (-CggPd-) polymers of the pearl necklace type were fonned via the periodic linkage of [60]fullerene and Pd monomer units after their initial reaction with thep-xy y ene diradical [69,70 and 71]. 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

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. 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in these polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine-end with an ester side chain, attack of DMF by the amine-end, or chain transfer to monomer) [11, 12], this conclusion does not seem to be well justified. It is likely that the role of impurities (e.g., water) in these polymerizations is very complex. A possible explanation for the polymerization control observed under high vacuum is that the impurities act to catalyze side reactions with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain-end and thus influence their reactivity. 

Initiation consists of protonation of monomer followed by subsequent reaction with monomer to form the tertiary oxonium ion LXXXI (Eq. 7-109). Propagation for the ring-opening... 

The initiation of tetrahydrofuran polymerization by direct addition of oxonium salts is of interest because it reveals a good deal about the mechanism, but for practical purposes the salts may be formed in the reaction mixture. The obvious method is, of course, to add a little epichlorohydrin to the mixture of monomer and Friedel Crafts reagent for only antimony pentachloride is sufficiently active to start the reaction with monomer alone, but other reactions which accomplish the same purpose are ... 

Barson, Bevington and Eaves have looked into the initiation reaction with azobisisobutyronitrile and with benzoyl peroxide, both in dimethylformamide (23). With AIBN they reported normal initiation. With the peroxide there appeared to be complications, due they thought to hydrogen abstraction from monomer or polymer. By using peroxide labelled in the ring or in the carbonyl group with C-14 they measured the relative importance of the two steps ... 

Aboubakar et al. [47] studied the physico-chemical characterization of insulin-loaded poly(isobutyl cyanoacrylate) nanocapsules obtained by interfacial polymerization. They claimed that the large amount of ethanol used in the preparation of the nanocapsules initiated the polymerization of isobutyl cyanoacrylate and preserved the peptide from a reaction with monomer, resulting in a high encapsulation rate of insulin. From their investigations, it appears that insulin was located inside the core of the nanocapsules and not simply adsorbed onto their surface. 

The counter radical process is based on the reversible capture of the carbon centered radicals formed by reaction of an initiating radical with monomer before propagation can occur. Upon heating or UV exposure, the resulting product can be homolytically cleaved in such a way that another monomer unit, or a number of monomer units, add before the macroradical is again captured. The process is repeated until no monomer remains or irreversible termination occurs. 

The high-energy radiation forms macrocellulosic radicals that are stable in the crystalline areas of cellulose. These radicals can initiate reactions with vinyl monomers to yield grafted polyvinyl-cellulosic fibers with desired properties [522-524]. 

The rate constants of nearly all of the elementary reactions in trityl-initi-ated polymerizations of cyclopentadiene [216], p-methoxystyrene [186], vinyl ethers [217], and a-methylstyrene [218] were determined by kinetic measurements, sometimes combined with conductometric measurements. Monomer conversion was followed by either dilatometry, spectroscopy, or calorimetry. Initiation was followed by the decrease in the 410-nm absorption of the trityl carbenium ions (e = 36,000 mol- L em-1), caused by their reaction with monomer by either direct addition or hydride abstraction. The initiator was assumed not to be consumed in any other reactions. The reaction orders (usually first order in each reagent) and rate constants of initiation were then determined by plotting the rate of initiation versus the initial monomer and initiator concentrations according to Eq. (52). 

Zero order kinetics in monomer was reported [95,97] for the polymerization of vinyl ethers initiated by HI/I2 in hexane and was ascribed to the formation of a complex between iodine (Lewis acid) and monomer. It has been proposed that monomer reversibly forms a complex with the growing chain (.. . -CH2CH(OR)I,l2) which then slowly inserts monomer in the rate-determining step [74]. Another possibility is that the ionization of the dormant species the rate-determining step where the cation subsequently undergoes the rapid reaction with monomer and then soon col-... 

Styrene monomer will spontaneously or auto-polymerize and must be inhibited to prevent reaction during transport and storage. Polymerization is initiated by the generation of free radicals either by the reaction of the styrene with itself ( auto-initiation ) or by means of a peroxide initiator ( chemical initiation ). Radicals rapidly propagate by reaction with monomer and ultimately terminate by coupling with another growing radical or by transferring the radical to a small molecule to start a new chain (chain transfer). 

Some information can be obtained about the rate coefficients of ion-pair dissociation from measurements of molecular weight distribution. If the lifetime in any of the ionic states is short compared with its rate of reaction with monomer, the molecular weight distribution will be the simple narrow Poisson type expected for rapid initiation, propagation via a single species and no termination. A slower ion-pair dissociation process... 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

The process of polymerization consists in general of three steps initiation, propagation, and termination. In radical polymerization, a catalyst is usually employed as a source of free radicals, the primary radicals. A fraction of these initiate a rapid sequence of reactions with monomer molecules, the primary radical thus growing into a polymer radical. Radical activity is destroyed by reaction of two radicals to form one or two molecules. This termination reaction is called mutual recombination, if only one molecule is formed. Termination by disproportionation results in two molecules. For many common monomers, recombination is the normal mode of termination and the kinetic treatment here is based on this termination reaction. Only slight modifications are required for polymerizations in which termination occurs by disproportionation. If both termination processes occur, another variable must be introduced to describe the kinetics of the system fully. 

Both the reaction of an initiator (In) with a monomer (M) and the reactions of propagating polymer (Pn) with a monomer (M) are very fast (Scheme 9.8). Therefore, we have to consider the possibility of disguised chemical selectivity, which is observed for Friedel-Crafts reactions of reactive aromatic compounds and a cation pool. If mixing is slow, the consecutive propagation reactions take place before all of the initiators react with monomers, even if the consecutive propagation reactions are slower than the initiation reaction. This is also an example of disguised... 

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