Case studies Polymerisation

Walsh et al. (1995) considered an industrial batch reactive distillation problem originally presented by Leversund et al. (1993) as a case study. A condensation polymerisation reaction between a dibasic aromatic acid (R1) and two glycols (R2, R3) was considered. The reaction products were a polymer product (P) and water... 

The case study presented in the last subsection can also be taken to summarize the characteristics of industrial design processes. This subsection abstracts from the concrete example. The reader, however, may also relate the following general statements to the polymerisation case study. 

Some typical NMR spectra of ethylene propylene copolymers are shown in Figures 6.9 and 6.10. The technique has been used in various structural studies on copolymers. Thus Hashidzume and co-workers [61], in an investigation of the spontaneous copolymerisation of 2,6-diisopropyl-N-methylene aniline with phthalic anhydride or with itaconic anhydride, showed that in the case of polymerisation of 2,6-diisopropyl-N-methylene aniline and phthalic anhydride, the copolymer was formed by a coupling reaction of a zwitterion one to one adduct. Also in the case of copolymerisation of 2,6-diisopropyl-N-methylene aniline and itaconic anhydride, the copolymer is formed by the addition polymerisation through the C=C bond in the itaconic anhydride moiety. 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

Simple monodentate NHCs are somewhat susceptible to dissociation when coordinated to early transition metals [6], so in most cases multidentate chelating hgands are employed in which the carbene is tethered to a strongly coordinating anchoring group. This is not universally the case however, and simple monodentate NHC complexes of Zr 1 (Fig. 4.1) have been studied [7]. The complexes were activated with MAO and tested for ethylene polymerisation, leading to moderate activities between 7 and 75 kg mol bar h for linear polyethylene. 

The fact that the results of some earlier studies can be explained better in terms of equilibrium (9) than equilibrium (11) is undoubtedly related to the relatively high level of impurities prevalent in the earlier experiments, but it has not yet been explained in detail. This matter is, however, not as irrelevant as it may seem, because (as mentioned above) in the actual solutions of aluminium halides which have been, or are likely to be, used in most polymerisation experiments, an important fraction of the ions present probably arises from reactions of the initiator with impurities, solvent or both, and the case discussed here in detail - ions arising only from reaction of the initiator molecules with their own kind - is surely an idealisation. 

There are many papers which purport to record the effect of counter-ion on such factors as transfer constants, co-polymerisation ratios, etc. It is significant that in most of these studies relatively high initiator concentrations have been used, so that counter-ion effects are more likely but before accepting that the observed effects are indeed due to change of counter-ion (derived from different catalysts or co-catalysts) it must be ascertained that these polymerisations are in fact cationic and not pseudo-cationic - in which case the effects would stem from the different reactivities of different esters (see Section 5). 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

Brindley and Sempels (1), Vaughan et al. (2) and Shabtai (3) have shown that the experimental conditions of Al intercalation influences the physicochemical properties of the clay. The nature, amount and spacial distribution of the pillars change the thermal stability, texture and acidity of the pillared clays. For example, Rausch and Bale (4) have reported that the OH/Al ratio modifies the structure of the Al complex and that monomeric [Al(0H)x(H20)6-x] " or polymeric [A1i304(0H)24(H20)i2] species can be obtained. Clearfield (5) demonstrated that the polymerisation state of Zr species depends on the temperature, concentration and pH of the solutions. In any case, the height of pillars is largely controlled by the polymerisation state of the intercalated complexes. However, in order to maintain the accessibility of the inner surface, the density or spacial distribution of the pillars has to be controlled. This parameter has been studied by Flee et al (5), and Shabtai et al (7) for Al pillared clays and Farfan-Torres et al (8) for zirconium. 

Electron transfer (ET) reactions play a key role in both natural (photosynthesis, metabolism) and industrial processes (photography, polymerisation, solar cells). The study of intermolecular photoinduced ET reactions in solution is complicated by diffusion. In fact, as soon as the latter is slower than the ET process, it is not anymore possible to measure km, the intrinsic ET rate constant, directly [1], One way to circumvent this problem, it is to work in a reacting solvent [2]. However, in this case, the relationship between the observed quenching rate constant and k T is not clear. Indeed, it has been suggested that several solvent molecules could act as efficient donors [3]. In this situation, the measured rate constant is the sum of the individual ksr-... 

Many of the polymerisable cyclic monomers exhibit ceiling temperature phenomena (126) around or even below ambient temperature, and in some cases high molecular weight products can only be obtained by polymerisation at low temperatures. Precipitation of polymeric products can also complicate kinetic studies, polymers from aldehydes and trioxane being particularly prone to this behaviour (127,128). In addition insolubility can preclude the normal methods of... 

Variables Affecting Conversion to Polystyrene. The conversion of styrene in dioxan (30% w/w) reaches a maximum at approximately 0.1M H2SO4 (Table 1). Beyond this acidity, the yield of hompolymer is not significantly increased under the radiation conditions used. At acid concentrations in excess of 0.7M there is a fall off in yield of polystyrene which may be attributed to the lower solubility of polystyrene at higher acid levels so that continued polymerisation is hampered by increasing immiscibility. The data in the same table show that the effect of water on the homopolymerisation is different from that of acid. The polymer conversion is also favored at the lower monomer concentrations (Table II) and is enhanced by the presence of acid at each monomer concentration studied. The conversion is also directly proportional to the total radiation dose (Table III) and inversely proportional to the dose rate (Table IV). The addition of acid in each case enhanced the resulting yields of homopolymer. ... 

The model with different end-groups is not realistic in all cases isotactic polypropylene and syndiotactic polypropylene are chiral, or more precisely, their structure is cryptochiral. This model is to be chosen when examining oligomers, and especially when studying the polymerisation mechanism where the structure of the reactive chain end is of extreme importance [16]. 

In the case of olefin polymerisation in the presence of homogeneous metallocene-based catalysts, the individual polymerisation stages have not been very thoroughly investigated. However, kinetic studies have helped, among others, to define the nature of the active centres and to establish the occurrence of some polymerisation elementary steps in a quantitative way. 

PVC has a complex particle morphology and a low degree of crystallinity. The crystallinity of PVC increases with increase in syndiotacticity. The level of syndiotacticity rises with fall in the temperature of polymerisation, and in the case of commercial types of PVC-S reaches 54-56%, whereas their crystallinity is only about 10%. The presence of crystalline forms in PVC has been established by studies using e.g. small-angle x-ray scattering, wide-angle X-ray diffraction (WAXS), IR spectroscopy and differential scanning calorimetry (DSC). In PVC, crystallites are formed under conversion conditions of less than 2%. These... 

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