Termination by the Monomer

The relationship between the overall rate and the concentrations of initiator and monomer changes when termination occurs by interaction with the monomer. This kind of termination occurs, for example, in the polymerization of allyl compounds (see Section 20.2.2). The same assumptions are made for the calculation as in Section 20.2.3.1. With the kinetic expression for termination by the monomer 

The concentration of polymer free radicals, which can be calculated from equations (20-62) and (20-64), is inserted into equation (20-55) for the propagation reaction and yields 

Under the stated conditions, then, the rate of polymerization does not depend on the monomer concentration and is first order with respect to the initiator concentration when termination occurs through the monomer. The kinetics of this polymerization is more complicated, however, since the degree of polymerization is usually very small. 

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The strange form of the distribution is the result of the overlapping of two distributions of macromolecules those which are terminated by the monomer, and those which remain living until the end of the process. The two corresponding distributions are given by Equations 16 (17) ... 

Figure 12. Schematic representation of the distribution of a polymer produced in an anionic polymerization with simultaneous termination by the monomer (see...

If termination by the monomer or the polymer occurs, this is known as suicide of the polymer. In the polymerization of propylene by typical cationic initiators (that is, in the absence of a polyinsertion mechanism), a hydride shift, for example, can occur ... 

The formation of polymer can be considered as a quasi-living polymerization. After the polymerization is complete, it can be reinitiated with the addition of more monomer to the unquenched polymer. However, the degree of polymerization cannot be predicted by the monomer/initiator molar ratio, the polydispersity is 1.5-2.0, and water, or even carboxylic acids, act as inhibitors and do not terminate the polymerization [10]. 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

In early work, it was assumed that the rate constant for termination was determined by the monomer unit at the reacting chain ends. The kinetics of copolymerization were then dictated by the rate of initiation, the rates of the four propagation reactions (Scheme 7.1) and rales of three termination reactions... 

The electrophilicity and therefore the stability of the cationic chain ends are relatively limited, because, on the one hand, the electrophilicity must be large enough to aid a nucleophilic attack by the monomer, but on the other hand, not so large that a chain termination occurs due to recombination with the counterion. For this reason the stability of the cationic chain ends is a function of ... 

Figure 48-3. Schematic representation of fibronectin. Seven functional domains of fibronectin are represented two different types of domain for heparin, cell-binding, and fibrin are shown. The domains are composed of various combinations of three structural motifs (I, II, and III), not depicted in the figure. Also not shown is the fact that fibronectin is a dimer joined by disulfide bridges near the carboxyl terminals of the monomers. The approximate location of the RGD sequence of fibronectin, which interacts with a variety of fibronectin integrin receptors on cell surfaces, is indicated by the arrow. (Redrawn after Yamada KM Adhesive recognition sequences.

The hydrolytic kinetic resolution of terminal epoxides catalyzed by the monomer la and dimer lb... 

Anionic polymerization represents a powerful technique for synthesizing polymers with low PDI values, thus providing good control over the chain length. This method leads to less side reactions than radical polymerizations. For instance, unlike in radical polymerization, there is no termination by the combination of two active chains. However, the mechanism is more sensitive to impurities and functional groups, and therefore applicable for only a limited class of monomers. 

Living polymers are terminated by the deliberate addition of chain-transfer agent such as water or alcohol to the reaction system after all of the monomer has reacted. 

The termination caused by the monomer is well known in a radical polymerization, and it results usually from a hydrogen transfer from the growing polymer to the monomer or vice-versa. If a reactive radical is formed in the process, one refers to this phenomenon as a chain transfer to the monomer. If the resulting radical is unreactive one deals with selfinhibition, e.g. this seems to be the case in the polymerization of some allyl derivatives. 

If the addition of a monomer to the growing chain takes place in more than one way, then the "abnormal addition, i.e. that which does not take place in the usual propagation step, might lead to termination. For example, Sela and Berger (48) assume that the anionic polymerization of Leuch s anhydride is terminated by the abnormal additions which yields carboxylate ions instead of imide ions ... 

Monomer XIV was introduced into the polycondensation reaction. The reaction was performed in CI C solution in the presence of 10 mol. Z of tritylium perchlorate at room temperature, the removal of traces of moisture being essential. The latter was most conveniently achieved by using a high-vacuum technique although, for larger runs, it is also possible to use standard equipment. The reaction practically came to an end after 50-60 h it was then finally terminated by the addition of CF CO H and, after neutralization and solvent removal, the polymer obtained, without purification, was subjected to de-acetylation. The free polysaccharide XV was isolated by precipitation. 

Give an example of coordination polymerisation involving nucleophilic attack on the growing polymer chain terminated by the coordinating monomer. Write down the scheme. 

Anionic polymerizations, when carried out in aprotic solvents, are characterized by the long lifetime of the carbanionic (or oxanionic) sites l2). When neither spontaneous transfer nor termination reactions are involved, the polymers obtained exhibit sharp molecular weight distributions, and their number average degree of polymerization is determined by the [Monomer]/[Initiator] molar ratio, provided initiation is fast as compared to propagation. However, the major advantage of these methods, as far as synthesis is concerned, is the socalled living character of the polymers 12) After completion of the polymerization the active sites retain their reactivity and can be used for functionalizations at the chain end. 

Eventually, polymerization will be terminated by the usual methods and the final co-polymer will have a random mixture of dichloroalkene (mostly) and monochloroalkene, roughly in proportion to their availabilities in the polymerization mixture. The precise properties of the resulting polymer will depend on the ratio of the two monomers. 

One may also mention one simple and convenient procedure [277-279], which allows knowing the fractions of the triads, = P(MjMjMk)/P(Mj), centered by the monomer unit Mj to establish the adequacy of the terminal model and simultaneously to estimate its parameter rj. This procedure involves the following... 

The sequence of monomer additions is terminated by the mutual annihilation of two radicals. Such termination reactions can occur if the radicals combine to form a paired electron bond as in... 

A number of nonsteady polymerization rate techniques can be used to measure ftp [11]. The most widely used method involves pulsed-laser-induced polymerization in the low monomer conversion regime. Briefly, a mixture of monomer and photoinitiator (Section 6.5.3) is illuminated by short laser pulses of about 10 ns (10 sec) duration. The radicals that are created by this burst of ligh propagate for about 1 sec before a second laser pulse produces another crop of radicals. Many of the initially formed radicals will be terminated by the short, mobile radicals created in the second illumination. Analysis of the number molecular weight distribution of the polymer produced permits the estimation of ftp from the relation... 

All parameters are provided in LJ units, such that distances are normalized by the LJ parameter corresponding to monomer size, ob (e.g. CH2 or Si02), and energies are normalized by the U parameter b- Molecular masses are normalized by the monomer s atomic mass. The MD simulation proceeds towards minimizing the free energy and the simulation terminates when a specified minimum distance between monomers is achieved. 

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