Poly polymerization termination step

Extensive studies of radical polymerization carried out in the period 1935-1950 firmly established the basic mechanism of the poly-addition18). Termination steps have been essential in accounting for the numerous observations, and their existence was unques-tionable19). 

Coombes and Katchalski [29] have considered a slightly more complex version of this mechanism in which a second propagation coefficient operates above a critical degree of polymerization. Katchalski et al. [30] calculated the molecular weight distribution obtained in a system following scheme (12) but also including a bimolecular termination step. Various authors have analysed more complex systems in which the initiator is a polymeric species. Thus Gold [31] has shown that initiation by a poly a-amino acid with a Poisson distribution leads to a polymeric product with an over-all Poisson distribution, and Katchalski et al. [32] demonstrated that in multichain polymers synthesized from polyfunctional initiators Poisson distributions also arise. 

Anionic polymerization carried out under suitable conditions results in the formation of living poly-mers—i.e. species which may grow further, if a suitable monomer is present in the system. This characteristic feature of living polymers, which arises from the elimination of all the termination steps, permits the following preparation of block polymers, polymers possessing two terminal functional groups, monodispersed polymers, etc. studies of the thermodynamics of the propagation step—i.e. determination of A / < cl aS of the... 

As described in this chapter, the living CROP of THF and 2-oxazolines are versatile methods for the preparation of well-defined polymers, whereby both the initiation and termination steps provide the possibility of introducing a variety of functional groups. Poly(tetrahydrofuran) has a low Tg, and is often used as soft block in for example, thermoplastic elastomers. As such, it is expected that the living CROP of poly(tetrahydrofuran) will remain an important method in the preparation of soft polymeric materials, whereby the easy introduction of functional groups can be used to prepare novel functional materials. 

The polymerization of vinyl chloride monomer, in common with other vinyl monomers, proceeds by a free-radical mechanism involving the usual steps of initiation, propagation, and termination. Poly(vinyl chloride) is formed in a regular head-to-tail manner Eq. (1) [3-6]. 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

Whether the formation of poly(p-xylylene) should be included in this chapter is not clear. Decisive data are not available to indicate the classification of this polymerization as a step or chain reaction. The formation of high polymer occurs instantaneously when p-xyly-lene contacts the cool surface, precluding the evaluation of polymer molecular weight versus conversion. Also, the mode of termination for this reaction is unknown. 

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