Propagation of Cationic Chain

The initiator ion pair (consisting of carbocation and its negative counterion) produced in the initiation step [Eq. (8.112)] proceeds to propagate by successive addition of monomer molecules. Considering isobutylene polymerization [cf. Eq. (8.108)], for example, this can be represented by 

The addition proceeds by insertion of monomer between the carbocation and its negative counterion. 

Consider, for example, the polymerization of 3-methyl-l,2-butene. Isomerization occurs by a 1,2-hydride ion shift in the first-formed carbocation 

The polymer thus contains mostly the rearranged repeating unit (XIX) but also some of the first-formed repeat units (XX) as some normal propagation occurs at higher temperatures. It is observed that the product contains about 70 and 100% of (XIX) at polymerization temperatures of -130 and —100°C, respectively. 

Propylene, 1-butene, and higher 1-alkenes yield only oligomers (DP no higher than 10-20) with highly irregular structures due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer and elimination, and chain transfer. For example, protonation and ethylation of ethylene are rapidly followed by energetically favorable isomerization  

In some cases, the propagation reaction is accompanied by intramolecular rearrangements due to hydride ion (H ) or methide ion (CH3 ) shifts. Such polymerizations are referred to as isomerization polymerizations. Consider, for example, the polymerization of 3-methyl-1,2-butene in which the carbocation (XV), formed initially, isomeiizes by a 1,2-hydride shift. The resulting ion (XVI), being a tertiary carbocation, is more stable than (XV) which is a secondary carbocation. 

These transformations are facilitated by the fayorable enthalpy differences between primary to secondary (-92 kJ/mol) and secondary to tertiary ( -138 kJ/mol) carbocations. 

Problem 8.16 Write equations to show the different structural units that may result from intramolecular hydride and methide shifts involving only the end unit in the cationic polymerization of 4-methyl-l-pentene. Which of the resulting repeating units would be the most abundant  

---

Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically — 100°C. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

Lewis acids initiate cationic chain-growth polymerizations. There are several possible chain propagation reactions, and the mechanism of cationic chain growth is still open to... 

These are excellent initiators and centres of the cationic propagation of polysiloxane chains. 

Here we can draw an analogy with the equilibrium dissociation reaction, when the association rate constant in equilibrium is not limited by diffusion, regardless of the viscosity of the medium. In our opinion, this question requires at present a theoretical and experimental investigation. It is customary to assume that radical polymerization is characterized by a rather intensive chain termination reaction and a short time for the propagation of one chain, as compared to the time of polymerization. The existence of continuous processes ( living polymers) has been ascertained for anionic9 and cationic polymerization10, where there is no bimolecular interaction of active centers with one another. Let us now examine certain radical polymerization processes in which the chain termination reactions are considerably inhibited or almost excluded. 

The kinetic picture of cationic chain polymerization varies considerably. Much depends upon the mode of termination in any oarticular system. A general scheme for initiation, propagation, and termination is presented below. ° By representing the coinitiator as A, the initiator as RH, and the monomer as M, we can write ... 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

The term Cationic Polymerization covers a wide field of polymerization processes which are characterized by the propagation of the polymer via cationic chain end according to Eq. (1). 

The competing reactions are isomerization of the cationic chain end, transfer reactions to monomer, counterion and solvent, and also termination reactions. The actual process of propagation depends on the concrete interactions between the reactants present in the polymerizing system. A synopsis of interactions expected is given in Table 7. For the most important of them quantum chemical model calculations were carried out. 

The formation of high molecular products during the cationic polymerization depends on whether the propagation reaction, consisting of the interaction of the cationic chain end as a reactive intermediate with the monomer, reproduces the reactive intermediate (see Eq. (1)). For this reason the monomer functions as the agent and as the substrate when in the form of the cation. This means, however, the interaction between the monomer and the cationic chain end is a function of the monomer structure itself when all other conditiones remain the same. 

The propagation of the cationic chain end can only occur if the nucleophilicity of the counterion is reduced sufficiently that recombination with the cation is prevented. The counterion Br, which recombines rapidly with the cationic chain end, can be stabilized by the interaction with the Lewis acid, e.g. SbBr5. An increase in stability, resulting from increasing complexation of the counterion, can be seen by means of... 

The propagation reactions of the growing cationic chain end with the monomer ethene have already been discussed in part 4.3. The reaction enthalpies of the corresponding propagation steps show different tendencies for the gas phase and solution, when the cationic chain end is lengthened. However, as the monomer is increased in size and the cationic chain end remains the same, then the tendencies for the gas phase and solution correspond to each other. This is an indication that the solvent influence on the cationic propagation reaction is determined by the nature of the cations in question and their solvation. 

The localization of the HOMO is also important for another reason. Since it describes the distribution of a hole in a radical cation, it relates to the hindrance that a positive charge will encounter as it propagates along the chain. There is indeed experimental evidence (9) that the hole states of the polysilane chain are localized and that they move by a hopping mechanism. 

Unlike radical chain polymerisation, initiation in cationic polymerisation uses a true catalyst that is recovered at the end of the polymerisation and is not incorporated at one end of the growing chain. Catalysts for cationic chain polymerisation are molecules able to withdraw electrons, mainly Bronsted (H2SC>4, H3PO4) and Lewis acids (BF3, A1C13, SnCh). The choice of solvent for cationic polymerisation is also important because it plays a major role in the association between cation and counter ion. A too tight association will prevent monomer insertion during the propagation step. However, the use of "stabilized"... 

Cationic alkyl metallocene complexes are now considered the catalytically active species in metallocene/MAO systems. Spectroscopic observation has confirmed the presence of cationic catalytic centers. X-ray photoelectron spectroscopy (XPS) on the binding energy of Zr(3d5/2) has suggested the presence of cationic species, and cationic hydride species such as ZrHCp2 that are generated by /1-hydride elimination of the propagating chain end... 

Isobutene is one of the very small number of aliphatic hydrocarbons which form linear high polymers by cationic catalysis (see Section 5). The reason for this is that only in these few among the lower aliphatic olefins is there found the right balance of those factors which determine the path of a cationic polymerisation. For the formation of linear high polymers it is necessary that the propagation reaction should be much faster than all alternative reactions of the growing end of the chain and for any appreciable numbers of chains to be formed at all, the initiation must be fast. ... 

Gandini and Plesch concluded that in these systems the chain-carriers are not ionic. Since they are certainly highly polar and in many respects behave as if they were ionic, we called the polymerizations propagated by them pseudo-cationic. Admittedly, in retrospect our original evidence for the non-ionic nature of the chain-carriers looks less convincing, but since that time many other phenomena have been found which support our view very forcibly [18] the case for the reality of pseudo-cationic polymerizations has been presented in detail [7], and therefore the argument need not be repeated here. 

---