Polymerizations, cationic

Cationic polymerization is the direct analogue of anionic polymerization but with a cation at the terminus of the growing polymer chain. Initiators are either strong acids such as sulfuric, perchloric, or hydrochloric acids, or Lewis acids such as BF3 or T1CI4. Usually the Lewis acids require water or methanol as a cocatalyst , suggesting that the reactive initiator is actually a protic acid. The list of viable monomers is somewhat limited, including isobutene, methyl vinyl ether, and butadiene—all structures that can give stabilized cations. Cationic polymerizations are the least common of the types discussed here. 

In cationic polymerization the end of the growing chain bears a positive charge. Previously, we have seen situations with free-radical chain ends or anionic chain ends (Fig. 2.8). 

Similar to the situation found in anionic polymerization, all types of vinyl monomers cannot be polymerized by the cationic polymerization. Specifically the monomer should have substituents that can stabilize the car-bocation. This means that the substituents should be electron releasing. Some of the monomers that can be polymerized by the cationic polymerization method are shown below (Fig. 2.9). 

Industrially the polymerization of isobutylene by cationic initiators is of importance. 

The initiators for cationic polymerization can be protic acids (sulfuric acid, perchloric acid, hydrochloric acid) or Lewis acids (BF3, BCI3, TiCU, AICI3 etc.,). Protons are the cationic initiators that are generated from protic acids (see Eq. 2.19). 

With Lewis acids it is noticed that a small amount of protic solvents (such as water or methanol) are required which will result in a Lewis acid-Lewis base adduct (see Eq. 2.20). 

Since cationic polymerization is a chain reaction it also can be considered in terms of initiation, propagation and termination stages. 

Cationic active centres are created by reaction of monomer with electrophiles (e.g. R ). Protonic acids such as sulphuric acid (H2SO4) and perchloric acid (HCIO4) are of use as initiators and involve addition of a proton (H ) to monomer. However, hydrogen halide acids (e.g. HCl) are not suitable as initiators because the halide counter-ion rapidly combines with the carbocationic active centre to form a stable covalent bond. Lewis 

For the same reasons as described for free-radical polymerization, propagation proceeds predominantly via successive head-to-tail additions of monomer to the active centre. 

Growth of individual chains is terminated most commonly either by unimolecular rearrangement of the ion pair, e.g. 

Additionally, chain transfer to solvent, reactive impurities (e.g. H2O) and polymer can occur, the latter resulting in the formation of branched polymers. 

As mentioned, in cationic polymerizations the reactive portions of the chain ends carry positive charges during the process of chain growth. These active centers can be either unpaired cations or they can be cations that are paired and associated closely with anions (counterions). 

however, initiations take place by one electron transposition, they occur as a direct result of oxidation of free radicals. They can also take place through electron transfer interactions involving electron donor mmiomers. The carbon cations can form from olefins in a variety of ways. One way is through direct additions of cations, like protons, or other positively charged species to the olefins. The products are secondary or tertiary carlxMi cations  

When the cations add to craijugated dienes, the charges can be distributed over several centers in the products 

The charge may also be distributed in polar monomers like, for instance, in vinyl ethers  

Chemical considerations indicate that the more diffuse the charges the more stable are the ions. Cationic polymerizations are not affected by common inhibitors of free-radical polymerizations. They can, however be greatly influenced by impurities that can act as ion scavengers. These can be water, ammonia, amines, or any other compounds that can be basic in character, affecting rates and molecular weights of the products. Typical cationic polymerizations proceed at high rates even at low temperatures, as low as — 100°C [8]. In the literature one can find many reports of cationic polymerizations of many different monomers with many different initiators. Often, however, such initiators are quite specific for individual monomers and their activities are strongly influenced by the solvents. 

The catalysts for cationic polymerization are Lewis acids and Friedel-Crafts catalysts such as BF3, AICI3, and SnCU and strong acids such as H2SO4. The cocatalysts are, for example, water and isobutene. An example of cationic polymerization is the synthesis of isobutene  

In both anionic and cationic polymerization it is possible to create living polymers . In this process, ve starve the reacting species of monomer. Once the monomer is exhausted, the terminal groups of the chains are still activated. If we add more monomer to the reaction vessel, chain groMh will restart. This technique provides us with a uniquely controllable system in which we can add different monomers to living chains to create block copolymers. 

1 Basic Principles of Cationic Polymerization of Vinyl Monomers 

The initiation usually involves the addition of a cationic species (A ) to a vinyl monomer to produce a carbocationic intermediate associated with a counter anion (X ), which is derived from the initiator. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions [93-95], are 

B externally added Lewis base X weakly nucleophile counteranions 

In both methods the positive charge of the carbocationic center is reduced and thereby the acidity of the p-proton is reduced to suppress the chain transfer. As a result, good molecular weight control and molecular weight distribution control are attained. On the basis of the principles, a number of initiating systems have been developed for living cationic polymerization [99]. 

There are many monomers that can be polymerized via a cationic mechanism (see Table 7.1), but the most important polymers from an industrial point of view are homo- and copolymers from isobutene and from some heterocyclic monomers such as trioxane, tetrahydrofuran, and epoxides. There is a detailed discussion on the mechanistic features in Refs. 1-5, 7, and 181-183. 

As we saw in Section 8.2, the active center in cationic polymerization is a cation and the monomer must therefore behave as a nucleophile (electron donor) in the propagation reaction. Suitability of monomers for cationic polymerization was also discussed in that section and compared in Table 8.1. In short, olefinic monomers with an electron-releasing or electron-donating substituent on the a-carbon can undergo cationic polymerization, while the possibility of resonance stabilization of the carbocationic species increases the reactivity of the monomer (see Problem 8.15). 

Problem 8.15 Compare the cationic polymerizability of (a) ethylene, propylene, and isobutylene (b) styrene, a-methylstyrene, p-methoxystyrene, and p-chlorostyrene. 

By similar reasoning, electron-releasing substituents, such as RO-, RS-, and aryl at ortho or para position, increase the monomer reactivity for cationic pol)mierization. 

Which of each of the following pairs of monomers undergoes cationic polymerization more readily  

The question asked here is about the relative stability of the carbocations that are produced on addition of an electrophile to the monomer, which depends on the ability of the substituent to stabilize or destabilize the adjacent carbocation. 

In a similar way, the grafting-from technique has been applied to the synthesis of poly(chloroethylvinylether) chains by grafted PCL segments, i.e., po-ly(CEVE-g-CL) graft copolymers. Purposely cationically prepared PCEVE were partially modified by the introduction of 5-10% hydroxyl groups [79]. An equimolar reaction of the pendant hydroxyl functions with HAI/BU2 provided diisobutyl aluminum monoalkoxides dispersed along the polyether backbone. 

Strong Lewis adds, that is, electron acceptors, are often capable of initiating addition polymerization of monomers with dectron-rich substituents adjacent to the double bond. Cationic catlysts are most commonly metal trihalides such as AICI3 or BF3. These compounds, although electrically neutral, are two electrons short of having a complete valence shell of eight electrons. They were found to require traces of a cocatalyst, usually water, to initiate polymerization, first by grabbing a pair of electrons from the cocatalyst 

The leftover proton is thought to be the actual initiating spedes, abstracting a pair of electrons from the monomer and leaving a cationic chain end which reacts with additional monomer molecules. 

An important point here is that the gegen or counter ion is electrostatically held near the growing chain end and so can exert a steric influence on the addition of monomer units. Termination is thought to occur by a disproportionation-like reaction which regenerates the catalyst complex. The complex, therefore, is 

The kinetics of these reactions is not well understood, but they proceed very rapidly at low temperatures. For example, the polymeriaation of isobutylene illustrated above is carried out commercially at — 150 F. The average chain length increases as the temperature is lowered. 

Cationic initiation is successful only with monomers like isobutylene having electron-rich substitutents adjacent to the double bond, such as 

Fundamental Principles of Polymeric Materials, Third Edition. Christopher S. Brazel and Stephen L. Rosen. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

The most important initiators are the Lewis adds MX , but they are not particularly active alone and require a cocatalyst SH to act as a proton donor. In general, first we have an ionization process 

The type of cocatalyst also influences the polymerization rate because the activity of the initiator complex depends on how readily it can transfer a proton to the monomer. If the polymerization of isobutylene is initiated by SnQ4, the acid stimgth of the cocatalyst governs the rate, which decreases in the cocatalyst ordw acetic acid nitroethane phenol water. 

Other types of initiator are less important thus, strong adds protonate the double bond of a vinyl monomer 

High-energy radiation is also thought to produce cationic initiation, but this may lead to fragmentation, and a mixture of free-radical and cationic centers. 

As with other carbocations, NMR spectroscopy has been utilized to determine the structure of the 2-norbomyl cation. The NMR spectrum at -70 °C showed 

The Cg-Cj bond is situated at the rear side of the ionizable group and the a electrons attacks the carbon bearing the ionizable group, thus facilitating the ionization. This leads to a nonclassical carbenium ion, which reacts with acetic acid to yield the racemic mixture of acetates. No such anchimeric assistance is available to the endo-isomer, which undergoes slow acetolysis through classical carbenium ions. 

These results were interpreted as implying that the reaction of the exo-substrate occurred solely via a nonclassical carbocation, while the endo-substrate reacted by initial formation of a classical carbenium ion, which then rearranged to the nonclassical carbocation, but not before a small amount had reacted with solvent (attack being sterically directed to the exo-face). 

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy based on the difference of the total chemical shift of a carbocation and the corresponding alkane. Differences in total chemical shift of 350 ppm or more suggest classical carbocations, while differences 

The conclusion that the 2-norbornyl cation is a nonclassical carbocation has been strengthened by the experimental determination of its infrared spectmm when the cation is generated in a cryogenic Sbp5 matrix. The experimental IR spectra agree with those calculated for a nonclassical structure. 

Ethylene oxide Aziridine Tetrahydrofuran Trioxane e-Caprolactone 

FIGURE 3.14 Typical initiation reaction of isobutylene via protonation and the two main types of products with unsaturated end groups formed. 

Similarly, the biodegradable polyesters polyglycolic acid and poly(f-caprolactone), also used for implantable devices (degradation even slower than that of PLA), drug delivery, and suture materials, are prepared by CROP using stannous octanoate and other catalysts as cationic initiators [8]. 

A further technically important polymer produced via cationic polymerization is poly (ethylene imine) (PEI). PEI is polymerized from aziridine via protons (Fig. 3.15). It is a highly branched product due to chain growth also through the secondary amines, and it is used, for example, for paper treatment and coatings. More recently, it has also drawn interest for biomedical applications despite its relatively high cytotoxicity [9]. 

Another very interesting class of materials that are polymerized via cationic polymerization is polyoxazolines [10]. Their properties like solubility, hydrophilicity, or special functionality can be controlled by the monomer substituent in 2-position. Methyl- and ethyloxazolines are hydrophilic and nontoxic and lead to water-soluble polymers with high potential in biomedical use [11], whereas oxazohnes with propyl, butyl, octyl, dodecyl, and phenyl in 

Coordination or transition metal catalysts a-Olefins including ethylene, dienes, and alkyl vinyl ethers 

Cationic only Free radical only Anionic only 

FIGURE 4.7 Type of chain polymerization suitable for common monomers. (Data from Billmeyer, E. W., Jr. Textbook of Polymer Science. 1962. New York, 292. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.) 

Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 220,1953. 

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Derivatives of polyisobutylene (6. in Figure 9.1) offer the advantage of control over the molecular weight of the polyisobutylene obtained by cationic polymerization of isobutylene. Condensation on maleic anhydride can be done directly either by thermal activation ( ene-synthesis reaction) (2.1), or by chlorinated polyisobutylene intermediates (2.2). The condensation of the PIBSA on polyethylene polyamines leads to succinimides. Note that one can obtain mono- or disuccinimides. The mono-succinimides are used as... 

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. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

In their polymerization, many individual alkene molecules combine to give a high molecular weight product Among the methods for alkene polymerization cationic polymerization coordination polymerization and free radical polymerization are the most important An example of cationic polymerization is... 

On the basis of the mechanism of cationic polymerization predict the alkenes of molecu lar formula C12H24 that can most reasonably be formed when 2 methylpropene [(CH3)2C=CH2] IS treated with sulfunc acid... 

Ionic polymerizations, whether anionic or cationic, should not be judged to be unimportant merely because our treatment of them is limited to two sections in this text. Although there are certain parallels between polymerizations which occur via free-radical and ionic intermediates, there are also numerous differences. An important difference lies in the more specific chemistry of the ionic mechanism. While the free-radical mechanism is readily discussed in general terms, this is much more difficult in the ionic case. This is one of the reasons why only relatively short sections have been allotted to anionic and cationic polymerizations. The body of available information regarding these topics is extensive enough to warrant a far more elaborate treatment, but space limitations and the more specific character of the material are the reasons for the curtailed treatment. 

Just as anionic polymerizations show certain parallels with the free-radical mechanism, so too can cationic polymerization be discussed in terms of the same broad outline. There are some differences from the anionic systems, however, so the fact that both proceed through ionic intermediates should not be overextended. 

A single catalyst is often not sufficient in cationic polymerizations frequently a cocatalyst is required. 

We shall consider these points below. The mechanism for cationic polymerization continues to include initiation, propagation, transfer, and termination steps, and the rate of polymerization and the kinetic chain length are the principal quantities of interest. 

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

The catalysts for cationic polymerization are either protonic acids or Lewis acids, such as H2SO4 and HCIO4 or BF3, AICI3, and TiCl4 ... 

One of the side reactions that can complicate cationic polymerization is the possibility of the ionic repeat unit undergoing the well-known carbonium ion rearrangement during the polymerization. The following example illustrates this situation. 

On the basis of these observations, criticize or defend the following proposition Regardless of the monomer used, zero-order Markov (Bernoulli) statistics apply to all free radical, anionic, and cationic polymerizations, but not to Ziegler-Natta catalyzed systems. 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Coumarone—indene or coal-tar resins, as the name denotes, are by-products of the coal carbonization process (coking). Although named after two particular components of these resins, coumarone (1) and indene (2), these resins are actually produced by the cationic polymerization of predominantly aromatic feedstreams. These feedstreams are typically composed of compounds such as indene, styrene, and their alkylated analogues. In actuaUty, there is very tittle coumarone in this type of feedstock. The fractions used for resin synthesis typically boil in the range of 150—250°C and are characterized by gas chromatography. 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

The conversion of aromatic monomers relative to C-5—C-6 linear diolefins and olefins in cationic polymerizations may not be proportional to the feedblend composition, resulting in higher resin aromaticity as determined by nmr and ir measurements (43). This can be attributed to the differing reactivity ratios of aromatic and aHphatic monomers under specific Lewis acid catalysis. Intentional blocking of hydrocarbon resins into aromatic and aHphatic regions may be accomplished by sequential cationic polymerization employing multiple reactors and standard polymerization conditions (45). 

Particular drawbacks of using alkylsiHcon and alkyltin haHdes with AlCl for the cationic polymerization of terpenes are low yields and the fact that they require rigorously dried feeds (<50 ppm H2O) to be effective. Increased water content results in lower yields and lower softening points (85). Catalyst systems comprised of AlCl with antimony haHdes in the presence or absence of a lower alkyl, alkenyl, or aralkyl haHde are particularly effective in systems containing up to 300 ppm H2O (89,90). Use of 2—12 wt % of a system composed of 2—3 parts AlCl, 0.7—0.9 parts SbCl, and 0—0.2 parts of an organic... 

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

Since the discovery of living cationic systems, cationic polymerization has progressed to a new stage where the synthesis of designed materials is now possible. The rapid advances in this field will lead to useful new polymeric materials and processes that will greatiy increase the economic impact of cationic initiation. 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

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