Mechanism acid catalyzed polymerization

Problem 14.12 Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene. 

Problem 31,11 Propose a mechanism to account for the formation of Bakelite from acid-catalyzed polymerization of phenol and formaldehyde. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

C. Discussion of Possible Mechanisms for the Acid-Catalyzed Polymerization of Pyrroles... 

The mechanism of acid catalyzed polymerization is postulated to be that schematically illustrated in equations 16 and 17. However, according to the recent text by Odian (54), there is no evidence for the rearrangement of the siliconium ion shown in these equations. Alternatively, one could propose that the propagating species is the tertiary oxonium ion. 

Dopico et al. [73] prepared 5- and 6-based furan resins which, after acid-catalyzed polymerization, were subjected to epoxidation with thiokol in different proportions. In a second series of experiments, 6-7% of 2 was added to the epoxidized resins. They showed that all these resins presented a lower flexure resistance compared to unmodilied totally furanic binder. Moreover, the addition of 2 was found to induce negative effects on the mechanical properties of metal-to-metal assemblies. 

In polymerization-induced colloidal aggregation (PICA) processes, a reactive monomer, generally urea formaldehyde, is mixed with a stable, submicrometer diameter metal oxide sol and undergoes an acid-catalyzed polymerization that results in porous, uniformly sized polymer-oxide composite microspheres [24,25], PICA has been applied to a variety of metal oxide systems, primarily silica, but also alumina, titania, zirconia, ferric oxide, and antimony pentoxide [24,25]. The process is affected strongly by solution acidity [26]. At lower pH, polymerization is more rapid and a more porous but mechanically weaker particle is formed. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. The mechanism of the initiation of cationic polymerization and polycondensation has been extensively studied. Trivalent carbenium ions play the key role, not only in acid-catalyzed polymerization of alkenes, but also in poly condensation of arenes (n -bonded monomers), as well as in cationic polymerization of ethers, sulfides, and nitrogen compounds (nonbonded electron-pair donor monomers). Pentacoordinated carbonium ions, on the other hand, play the key role in the electrophilic reactions of a-bonds (single bonds), including the oligocondensation of alkanes and the cocondensation of alkanes and alkenes. 

Since the gelation times decrease with decreasing pH below 2, the polymerization rate is proportional to [H" ]. The acid-catalyzed polymerization mechanism is not clear. Her and others propose a mechanism that involves a siliconium ion intermediate (= Si ) ... 

Write a mechanism for the acid-catalyzed polymerization of 1,4-benzenediisocya-nate and ethylene glycol. To simplify your mechanism, consider only the reaction of one — NCO group with one —OH group. 

Cyclization by backbiting cannot be avoided if active species of propagation are tertiary oxonium ions located at the chain end. Several years ago, we proposed that if acid-catalyzed polymerization of oxiranes is conducted in the presence of hydroxyl-group-containing compounds, the other mechanism competes with the ACE mechanism. This mechanism is based on the established mechanism of acid-catalyzed hydrolysis of oxiranes.In analogy to the hydrolysis mechanism, if an excess of oxirane (e.g., EO) is present, successive reactions of proto-nated monomer molecules with hydroxyl groups lead to extension of the chain as shown in Scheme 17... 

Nijenhuis, A. J., Grijpma, D. W., and Pennings, A. J., 1992, Lewis acid catalyzed polymerization of L-lactide. Kinetics and mechanism of the bulk polymerization. Macromolecules 25 6419-6424. 

Acid-catalyzed polymerizations, such as that described for poly(2-methylpropene), are carried out with H2SO4, HF, and BF3 as the initiators. Because they proceed through carbocation intermediates, they are also called cationic polymerizations. Other mechanisms of polymerizations are radical, anionic, and metal catalyzed. 

The spontaneous polymerization of furan adsorbed on carbon black with or without SnCl4 vapours35 has been explained by a similar cationic mechanism. Also, the polymerization of gaseous furan on liquid acidic surfaces35 has the same origin, but in these systems the polymers suffer an acid-catalyzed hydrolysis of their tetrahydrofuran rings which produces a considerable proportion of hydroxyl and carbonyl groups. 

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