Polymerisation termination stage

Initiation reactions are usually started by an active free radical such as peroxide (-0-0-), e.g. benzoyl peroxide is a good inititator for the free radical addition polymerisation of styrene to produce polystyrene AICI3 is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber or azobisiso-butyronitrile compounds (-N=N-) (abbreviated to AIBN). Propagation reactions are the continuing process and, eventually, lead to the termination stage that occurs by combination or disproportionation. This usually occurs when the free radicals combine with themselves and signals the end of the polymerisation process. All polymers formed by this process are thermoplastics. Table 4.1 is a list of common polymers prepared by the addition process. 

A development of particular importance for the controlled production of block copolymers is the perfection of various so-called living polymerisation techniques. In the classical addition polymerisations there was always a termination stage, leading to the production of chains with non-reactive groups at both ends of the polymer chain. Polymerisation could therefore stop before all monomer had been exhausted, although ideally the termination step was of much lower probability than the propagation step. In living polymerisations there is no termination step and the reaction proceeds in the ideal case until all monomer has been exhausted. The chains still have reactive ends and a second type of monomer can then be added to the reaction to produce a block of a different type of polymer. 

Polystyrene was prepared by the anionic polymerisation of styrene in toluene plus THF mixtures (4 1 volume ratio) using n-butyl lithium as initiator. After removing a sample for analysis at this stage, the remainder of the living polystyrene was reacted with a five molar excess of trichloromethylsilane for 15 min and then excess methanol introduced. The methoxy-terminated polystyrene was freeze-dried from dioxan. The method described here essentially follows the route proposed by Laible and Hamann (6). 

The state of aggregation of the polymerising system represents another important factor which may affect the kinetics of polymerisation. It is well known (96,97) that many radical polymerisations are enhanced by increase in the viscosity of the po-lymerisingsystem, and this phenomenon was explained by a decrease in the rate of termination step which may become diffusion-controlled. In fact, the effect of viscosity should be observed at any stage of radical polymerisation, and this problem has been discussed recently by Benson and North (98, 99). Of course, this type of acceleration cannot be observed when the growth involves living polymers and therefore such an explanation does not apply to polymerisation of NCA, particularly since no termination resulting from active end-active end interaction takes place in these processes. 

Irreversible termination of growing macromolecules during the final stages of ATRP are particularly disadvantageous if the synthesis of block co-polymers by sequential polymerisation is attempted. Due to different solubilities of catalyst, monomer and polymer in the ionic liquid phase, a larger amount of active molecules may be observed in the presence of an ionic liquid. 

The addition polymerisation reactions follow a three-stage standard reaction -initiation, propagation and termination. 

In our particular type of step-addition polymerisation, monomers, dimers, trimers, oligomers and polymers are the reactive species which participate in the chain growth. Initially, the monomers react with monomers and give dimers, dimers react with monomers and dimers and give trimers and tetramers, respectively. The high MW polymer is formed only in the last stages of the poly addition reaction, at high conversion rates. Chain transfer and termination reactions are absent. 

The value of degree of transformation corresponded to the inflection point on lg(V/[M]) dependencies on q after the initial stationary part was used for quantitative characterisation of the onset of copolymerisation autoacceleration [61,73]. Such method of determination of (qa) is quite often used [70, 74]. Nevertheless, the physical sense of this value demands some explanation. Since the very onset of polymerisation, the elementary stage of bimolecular termination remains unchanged up to some conversion, q. As this... 

Addition polymerisation has three stages initiation, propagation and termination. The reaction is usually initiated by the thermal decomposition of an unstable initiator molecule, such as a peroxide, to produce two free radicals. The free radical on the initiator fragment, shown as I., attacks the covalent tt bond in a monomer, leaving a free radical on the monomer. 

The polymerisation process consists of a series of sequential stages with the main ones being initiation, propagation, and material chain termination. In order to produce a quality product it is necessary to provide optimum conditions for corresponding stages. These conditions include operation temperatures, pressure, concentrations of ingredients, and a sufficient degree of field uniformity for all characteristics. At the... 

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