Polymerisation initiation stage

Thus, ellipsometry gives direct evidence for a model of the initial stages of polythiophene growth, disproving the conclusions based purely on coulo-metry. In the same paper, Hamnett and Hillman were able to obtain valuable and complementary information not just on the initial stages of the polymerisation but also on the mechanism of the subsequent nucleation and growth. The unique piece of information that the ellipsometer was able to extract, the changes in film thickness (in real time), when combined with coulometric data allowed a wealth of information to be deduced, e.g. with respect to the film composition, and ably showed the power of the technique. 

During the cationic polymerisation, e.g. with sulfuric acid, the process is the following at the initial stage of initiation, when organocyclosiloxanes interact with sulfuric acid, the acid proton attacks the oxygen atom of the siloxane cycle. As a result of the redistribution of the electron density, the =Si-0 bond breaks, opening the cycle and forming an active centre at the end of the chain ... 

From the above, it follows that the kinetics of the decomposition of ketene cannot be studied by pressure measurement. A pressure decrease was observed in the initial stage of the decomposition around 500 °C and below, becoming more and more pronounced with increasing surface/volume ratio. In Young s opinion the pressure decrease may be due to the presence of polymerisable impurities. 

It is well known that several monomers,such as styrene, < ( methylstyrene,isoprene,vinyl acetate (jj) have shown formation of oharge-transfer complexes in the presence of oxygen. Polystyrene peroxide is formed by photoirradiation of charge-transfer complex in the initial stage of polymerisation and the further photoinduced decomposition of the polystyrene peroxide initiates the polymerisation of styrene. On the other way,the reaction between excited state of styrene and oxygen may induce the formation of an alternating copolymer with peroxide groups -0-0- in-backbone. 

Chlorpromazine behaves differently towards ultraviolet irradiation under anaerobic conditions. A polymerisation process has been proposed which involves the liberation of HCl in its initial stages. The polymer (V) was isolated, and upon intracutaneous injection it produced a bluish-purple discoloration typical of that observed in some patients receiving prolonged chlorpromazine medication. It was suggested that the skin irritation that accompanies the discoloration may be a... 

The second approach or microheterogeneous model [1, 19-22] is based upon the principle, that the kinetics of the reaction in its initial stage are not that of a homophase polymerisation in a liquid monomer-polymeric solution, but a heterophase one. The reaction proceeding at the boundary liquid monomer - solid polymer microgranules surface under gel conditions. 

Comparison of cnrves of the dependence of the reaction rates for the three FMA mentioned above on the degree of transformation is shown in Figure 8.6. The rates obtained nnder the same conditions, indicates the general tendency of the change of reaction rates on initial stages of the transformation on the length of the perfluorinated FMA radical is similar to that observed polymerisation for alkyl methacrylates [62, 63]. This analogy in the kinetic behaviour of two sequences of monomers (FMA and AMA) enabled authors... 

Table 8.4 Kinetic parameters of radical polymerisation of perfluoroalkyl methacrylates, at 60 °C during initial stages of transformation ...

The similarity in the behaviour of the FMA sequence of monomers with that of the AMA sequence was observed that not only on the initial stages of the polymerisation, but also of the ones that followed. The onset of the gel-effect in the FMA sequence is shifted to the higher side of the degree of transformation (Table 8.5) with growth of fluoroalkyl radical length of the monomer, all other factors being the same. 

As mentioned previously, special attention has been paid to optical polymeric materials obtained by radical copolymerisation of fluorine-containing methacrylates in mixture with various vinyl monomers [3]. Study of kinetics and mechanism of this reaction over a wide range of degrees of reaction remains one of the main problems of chemistry of polymerisational polymers as a whole, and synthesis of copolymers for optical purposes based on alkyl methacrylates, in particular. On the one hand, there is an increased interest in the studies of the mechanism during the initial stages of the radical copolymerisation [65, 66], and on the other hand, the processes occurring in specific comonomeric pairs, for the point of view of those which display the gel-effect and lead to systems with excellent optical characteristics [1,3]. 

The initiation rate of isobutylene polymerisation is sufficiently high k, kp) which is why, in process kinetics, the initiation stage is considered to be instant. The concentration of active centres I A corresponds to the catalyst concentration in this case. 

A new method is described in [91] for the stndy of the nonnniformity of AC. It is based on the mass-spectrometry control of temperature-programmed desorption of prodncts, from the catalyst surface, at the initial stage of the gas-phase polymerisation of olefins. Polymerisation conditions have been selected in a way to favonr the formation of low MW products (up to 14 monomer links in a chain). The anthors report two definite maximums in the areas of 180-210 and 280-320 °C in the process of desorption from the Si02/TiCl4-Al(C2H5)2Cl surface. Therefore, the catalyst contains at least two types of AC with different activation energies of thermal destruction of Ti-C bonds. This publication also contains calculations of the activation energy distribution of thermal Ti-C bond destruction for various types of AC. 

Figures 3.33 and 3.34 demonstrate the kinetic activity distribution functions obtained from monomodal MWD curves for PrCl3-3(tributylphosphate)-Al(i-C4H9)3 and GdHal3-3(tributylphosphate)-Al(i-C4H9)3 catalytic systems in the isoprene polymerisation process. The functions demonstrated in the fignres are also seen to be polymodal. The initial stage of isoprene polymerisation demonstrates that the activity of AC, formed by both neodyminm and praseodymium catalytic systems, is predominant in the maximum of the low molecular weight area of the /(/w P)-ln M distribution curves (Figure 3.32).

As discussed in earlier sections, UPE resins are a mixture of an unsaturated polyester, styrene and an inhibitor. When the resin is mixed with a peroxide initiator and activator (cobalt octoate/napthenate), free radicals are formed. At the initial stage, all or most of the free radicals generated are consumed by the inhibitor. The driving force for the preferable reaction of free radicals with the inhibitor is the higher stability of inhibitor radicals. Once the inhibitor molecules are depleted, free radicals, produced from the initiator, initiate polymerisation of the polyesters. Styrene serves as an agent to link the adjacent polyester molecules. The curing of UPE resin (polyester + styrene) involves different types of reactions ... 

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