Cationic polymerization cocatalysts

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

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. 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

C ) using AICI3 cocatalyzed with a small amount of water. The cocatalyst furnishes the protons needed for the cationic polymerization ... 

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

Another reaction that has been applied to the generation of highly functionalized polymers is cationic polymerization [12-15]. Catalysts for cationic polymerizations are aprotic acids, protic acids, or stable carbocation salts. In these processes, the catalyst generally reacts with a cocatalyst to form an active initiated species. Initiation takes place by protonation of the monomer (Fig. 2A). Monomers that possess cation stabilizing groups, such as electron rich olefins, are preferred as they more readily undergo the desired polymerization process... 

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. 

Finally, it should be noted that cationic polymerizations are very sensitive to impurities. These can act as cocatalysts, accelerating the polymerization, or as inhibitors (e.g., tertiary amines) they can also give rise to chain transfer or chain termination and so cause a lowering of the degree of polymerization. Since these effects can be caused by very small amounts of impurities (10 mol% or less), careful purification and drying of all materials and equipment is imperative. 

Cationic polymerization has a history dating back to the early 1800s and has been extensively investigated by Plesch [1,2], Dainton and Sutherland [3], Evans et al. [4,5], Pepper [6,7], Evans and Meadows [8], Heiligmann [9], and others [10-13]. Whitmore [14] is credited with first recognizing that carbonium ions are intermediates in the acid-catalyzed polymerizations of olefins. The recognition of the importance of proton-donor cocatalysts for Friedel-Crafts catalysts was first reported by Evans and co-workers [4,5]... 

These cocatalyst effects observed in the stereospecific polymerization of aliphatic monoaldehyde by the organoaluminum catalyst are similar to those reported by Letort for free cationic polymerization. We prefer the coordinated cationic mechanism to the coordinated anionic one proposed by several workers. 

II. 2.1.4. Cyclic Olefin Polymers Benzofuran (16) gives an optically active polymer by cationic polymerization with AlEtCl2 or A1C13 in the presence of an optically active cocatalyst such as P-phenylalanine and 10-camphorsulfonic acid [12,48-50], The optically active polymer is considered to have an erythro- or threodiisotactic structure with no plane of symmetry. Initiator systems of AlCl3/(-)-menthoxytriethyltin, -germanium, and -silicon also give an optically active polymer [51,52],... 

The cationic polymerization of isobutylene (12) and styrene (13) is initiated readily by Et2AlCl in the presence of an alkyl halide, RC1. The interaction of the catalyst and cocatalyst is presumed to produce the carbonium ion R+, which initiates polymerization, and the corresponding gegenion Et2AlCl2". Alkyl halides with low R-Cl bond dissociation energies—e.g. tertiary, substituted allylic, and benzylic halides—are among the most effective cocatalysts. 

Recently Russian workers claimed that the presence of cocatalysts was not a necessary requirement for cationic polymerization of styrene and isoprene with stannic chloride catalyst provided the temperature and/or the dielectric constant of the medium was high enough (53—55). Similar ideas have also been expressed by Japanese investigators (56). 

However, all these studies were carried out under objectionable experimental conditions (absence of high vacuum) and cannot be accepted as evidence for cationic polymerization in the absence of cocatalyst. Longworth et al. (57) presented direct evidence which showed that the Russian workers were in error. 

Since there is no evidence for complex formation between alkyl halides (H=R) and unsaturated compounds, reaction c is rather unlikely when alkyl halides are used as cocatalysts. Alkyl chloride cocatalysts in cationic polymerizations were postulated by Pepper (86,87) and corroborated experimentally (88). 

There are substantial differences between the mechanisms of polymerization with single site catalysts and the closely related Ziegler-Natta catalysts (37-42). Most notably, the active centers of single site catalysts are believed to be cationic. Currently, cocatalysts are used in all commercial processes using single site catalysts, but this may change in the not-too-distant future (see p. 76). 

A reaction with a cocatalyst is necessary in cationic polymerization, as well as in coordinate polymerizations with many complex metallic catalysts. In cationic polymerization, the reaction involves traces of water and generates hydrogen ion, the actual initiator. This is shown in Reaction 3 for boron trifluoride and isobutylene. 

Lewis Acids. The consensus of opinion now seems to be emerging that strong Lewis acids such as AICI3, TiCU, SnCU, ZrCU, and BF3 are capable of initiating cationic polymerization of vinyl monomers without the assistance of a cocatalyst capable of ionizing to form a proton or carbocation. However, the presence of such a co-initiator, as either an adventitious impurity or a purposefully added component, enhances the initiation process. The mechanism of initiation may well vary with each Lewis acid employed. 

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

Several kinetic mechanisms have been proposed which explain some of the experimental results obtained with cationic polymerization systems, but since the dependence of rate on monomer, initiator and cocatalyst seems to vary considerably with the type of monomer and initiator used, no one scheme fits all the data. A mechanism which takes into account the importance of the cocatalyst and gives a correct dependence on monomer concentration is the following ... 

Monomers with electron-donating groups like isobutylene form stable positive charges and are readily converted to polymers by cationic catalysts. Any strong Lewis acid like boron trifluoride (BF3) or Friedel-Crafts catalysts such as AICI3 can readily initiate cationic polymerization in the presence of a cocatalyst like water, which serves as a Lewis base or source of protons. During initiation, a proton adds to the monomer to form a carbonium ion, which forms an association with the counterion. This is illustrated for isobutylene and boron trifluoride in Equation 2.19 ... 

For the purpose of establishing the kinetics of generalized cationic polymerization, let A represent the catalyst and RH the cocatalyst, M the monomer, and the catalyst-cocatalyst complex H+ AR . Then the individual reaction steps can be represented as follows ... 

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