Monomers and Catalysts

The data presented in Tables 9.1 and 9.2 focus on homopolymerisations copolymerisations of the mentioned heterocyclic monomers of both types proceed similarly, involving catalysts that promote their homopolymerisations. 

Heterounsaturated monomers such as aldehydes, ketones, ketones, isocyanates and isocyanides, which have been reported to undergo a polymerisation in the presence of coordination catalysts, are listed in Table 9.3 [1,3]. 

However, the most important goal that might be reached by the application of coordination catalysts for the polymerisation of heterounsaturated monomers is the possibility of the enchainment of heterounsaturated monomers, not susceptible to homopropagation, via their copolymerisation with heterocyclic monomers. This concerns primarily the coordination copolymerisation of carbon dioxide and oxacyclic monomers such as epoxides, leading to aliphatic polycarbonates [8 12]. Representative examples of the copolymerisations of heterocyclic monomers and hardly homopolymerisable heterocumulenes, in the presence of coordination catalysts, are listed in Table 9.4 [1]. 

The coordination catalysts usually applied for the polymerisation and copolymerisation of heterocyclic and heterounsaturated monomers involve a wide range of metal derivatives that are characterised by moderate nucleophi-licity and relatively high Lewis acidity. Metal derivatives possessing free p, d or f orbitals of favourable energy are used as catalysts for epoxide polymerisation. In particular, compounds of group 2 and 3 metals, such as zinc, cadmium and aluminum, and transition metals, such as iron, as well as lanthanum and yttrium, are representative coordination catalysts. The appropriate Lewis acidity of the metal and the appropriate nucleophilicity of the metal substituent in these catalysts make the monomer coordination favourable prior to the nucleophilic attack. The nucleophilic attack of the covalently bound metal substituent on the monomer molecule coordinated at the metal atom at the catalyst active 

Coordination of the monomer increases the capacity of the monomer for attack by the nucleophilic metal substituent and makes this nucleophilic attack simpler on account of repulsion of the negative charge from the monomer via the metal to its substituent [schemes (6) to (9) in Chapter 2]. 

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Among the preformed polymers cured by minor additions of aHyl ester monomers and catalysts followed by heat or irradiation are PVC cured by diallyl fumarate (82), PVC cured by diallyl sebacate (83), fluoropolymers cured by triaHyl trimeUitate (84), and ABS copolymers cured by triaUyl trimeUitate (85). 

The following details for the commercial manufacture of poly(vinyl methyl ether) have been made available. Agitated vinyl methyl ether at 5°C is treated over a period of 30 minutes with 0.2% of catalyst solution consisting of 3% BF3 2H20 in dioxane. When the reaction rises to 12°C the reaction is moderated by brine cooling. Over the next 3-4 hours further monomer and catalyst is added. The autoclave is then closed and the temperature allowed to rise slowly to 100°C. 

Many applications of novolacs are found in the electronics industry. Examples include microchip module packaging, circuit board adhesives, and photoresists for microchip etching. These applications are very sensitive to trace metal contamination. Therefore the applicable novolacs have stringent metal-content specifications, often in the low ppb range. Low level restrictions may also be applied to free phenol, acid, moisture, and other monomers. There is often a strong interaction between the monomers and catalysts chosen and attainment of low metals levels. These requirements, in combination with the high temperature requirements mentioned above, often dictate special materials be used for reactor vessel construction. Whereas many resoles can be processed in mild steel reactors, novolacs require special alloys (e.g. Inconel ), titanium, or glass for contact surfaces. These materials are very expensive and most have associated maintenance problems as well. 

In Benning el al. (146) some data on the kinetics of ethylene polymerization in the presence of TiCl2 activated by ball-milling are given. Polymerization was studied at 140-260°C (the solution process in cyclohexane). The first orders of the polymerization rate on the monomer and catalyst concentrations have been established. The polymerization decreased with temperature a sharp drop in rates at about 180-200°C was observed. 

Another difference between diese catalysts is found in dieir functional group tolerance. Catalysts such as 12 are more robust to most functionalities (except sulfur and phosphorus), moisture, oxygen, and impurities, enabling them to easily polymerize dienes containing functional groups such as esters, alcohols, and ketones.9 On die other hand, catalyst 14 is more tolerant of sulfur-based functionalities.7 The researcher must choose die appropriate catalyst by considering the chemical interactions between monomer and catalyst as well as the reaction conditions needed. 

The temperature of die oil badi in die initial stages and throughout die course of die polymerization is monomer and catalyst dependent. As a general rule, low-boiling, volatile monomers are started at room temperature (20-25°C), whereas higher boiling substrates may be started at 30-40°C. Polymerizations using catalyst 14 should be started at lower temperatures (20-30°C) compared to reactions... 

In general, for polymerization reactions, the heat generation rate is not a single-valued function of temperature, g(t), but also a function of monomer and catalyst concentrations, f(c). This is particularly important in high conversion reactions where a certain amount of peaking can be tolerated. 

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

The initial rate of polymerization of methyl methacrylate initiated by chromium allyls (12) in toluene showed identical dependences on monomer and catalyst concentrations, as Zr(benzyl)4 initiated polymerization of styrene. Some data for the monomer dependence are shown in Fig. 14. 

The discovery of pseudo-cationic polymerizations has made it necessary to re-assess a very large part of the results in this field. The situation is in many ways similar to that created some 20 years ago by the discovery of co-catalysis, but whereas in the late 1940s there was only a handful of papers, there is now a vast body of information dealing with many monomers and catalysts which needs to be scrutinised. 

This type of initiation, by reaction of monomer and catalyst only, is sometimes called direct initiation, to distinguish it from an initiation which requires the intervention of a co-catalyst. The discovery of co-catalysis proved this view to be inapplicable for many systems, and theoretical reasons against it (at least for hydrocarbons) were also put forward [42]. 

It might be thought that the question whether a particular alkyl halide is a co-catalyst for a particular monomer-catalyst combination could be settled easily, by adding some of the compound in question to a non-reacting mixture of monomer and catalyst. This approach has been used [36, 44], but it must be carried out in a polar solvent which is itself not a co-catalyst, or only a very weak one. The ideal solvent for this kind of work remains to be found it may be that S02 or even CS2 (which behaves like a polar solvent) will provide the answer. If one wants to use alkyl chloride solvents without being troubled by the possibility of solvent co-catalysis, boron fluoride should be used as catalyst, since the ion BF3C1 is not formed under the conditions generally used for polymerisations. 

The only studies on olefin polymerisations in methylene dichloride in which kp was deduced directly from the rate of reaction were carried out by Ledwith and his collaborators [9, 13] with extremely low concentrations of monomer and catalyst. They polymerised isobutyl vinyl ether and N-vinyl carbazole in a Biddulph-Plesch calorimeter with trityl or tropylium salts and obtained the first-order rate constants k1 from the conversion curves. Since different catalysts gave the same ratio of kx c they concluded that for each of them Xxr = c0 and hence identified with kp which must in fact be k p, as explained above. It seems unlikely that if several initiators give the same value of kp, they do so because they are all equally inefficient, and the inference that they do so because they are all 100% efficient, i.e., that for all of them x = c0, seems plausible - but it would be useful to have a direct check of this. 

An injection unit doses and mixes the monomer and catalyst. The mix is discharged under pressure, through an injection cone, into the closed mould. The injection pressure is not negligible and the moulds must be rather rigid and resistant. The precision of the cavity and the quality of its surfaces govern the precision and finish of the parts. 

Polymers may be made by four different experimental techniques bulk, solution, suspension, and emulsion processes. They are somewhat self-explanatory. In bulk polymerization only the monomers and a small amount of catalyst is present. No separation processes are necessary and the only impurity in the final product is monomer. But heat transfer is a problem as the polymer becomes viscous. In solution polymerization the solvent dissipates the heat better, but it must be removed later and care must be used in choosing the proper solvent so it does not act as a chain transfer agent. In suspension polymerization the monomer and catalyst are suspended as droplets in a continuous phase such as water by continuous agitation. Finally, emulsion polymerization uses an emulsifying agent such as soap, which forms micelles where the polymerization takes place. 

It may be that another process (whose rate is independent of the monomer and catalyst concentrations and can be detected at the considered... 

The observed positive value for the second term (see slope, Fig. 1) indicated that the activated complex (created between monomer and catalyst, or propagating anion) has a larger separation of charges than the reactants in the initial stage. 

The existing data show that the rate of cationic homopolymerization is increased in most cases by the application of an electric field. Fig. 1 shows typical results, in which RPE and Rpo are the initial rates of polymerization Rp) with and without electric field, respectively (5). Figs. 2 and 3 represent the dependences of Rp on monomer and catalyst concentrations, respectively (9,10). It is evident that Rt is increased under an electric field, whereas the concentration dependences (the reaction orders) are not influenced. It is likely therefore that the rate enhancement is not due to a new reaction mechnism but rather to an increase... 

In devolatilization with viscous polymeric melts, it is difficult, of course, to carry out similar experiments and prove indirectly that free-streaming nuclei may play a similar role, but microscopic particles originating from the monomers and catalyst systems are likely to be found in the polymeric product. Moreover, it is well known that the addition of fine powders and solid particles induces foaming. Therefore, the Biesenberger-Lee proposition seems plausible. 

Finally, note that coordinative interactions between monomers and catalysts usually result in a specific spatial orientation of the coordinating monomer. In many cases, owing to the determined mode of configuration of the monomer with respect to the growing polymer chain at the active site, the propagation leads to a stereoregular polymer. 

As regards the kinetics of conjugated diene polymerisation in terms of the dependence of the rate of polymerisation on the monomer and catalyst concentrations, most studies show that the polymerisation rate is first order with respect to both the monomer and the catalyst concentration. For instance, the rate of polymerisation of isoprene with the TiCU AlEt3 (1 1) catalyst, leading to a cA-1,4-polymer, is represented by [177]... 

Kinetic investigations of butadiene polymerisation to 1,2-polymers with various Ziegler-Natta catalysts such as Ti(OBu)4 AlEt3 [168] showed that the polymerisation rate is first order with respect to the monomer and catalyst concentration. 

Indicate the main features differentiating acyclic diene metathesis polycondensation and single C-C bond-forming polycondensation with respect to the kind of monomers and catalyst action. 

Indicate monomers and catalysts that will produce poly(l,4-phenylene vinylene) and poly(l,4-phenylene-l,6-hexenylene) via coordination polycondensation of various types. 

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