Telechelic termination process

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

C.D. Stokes, R.F. Storey, and J.J. Harrison, Process for preparing terminally functionalized living and quasiliving cationic telechelic polymers, US Patent 7 576161, assigned to Chevron Oronite Company LLC (San Ramon, CA) and The University of Southern Mississippi (Hattiesburg, MS), August 18,2009. 

A variety of CEs with tailorable physico-chemical and thermo-mechanical properties have been synthesized by appropriate selection of the precursor phenol [39,40]. The physical characteristics like melting point and processing window, dielectric characteristics, environmental stability, and thermo-mechanical characteristics largely depend on the backbone structure. Several cyanate ester systems bearing elements such as P, S, F, Br, etc. have been reported [39-41,45-47]. Mainly three approaches can be seen. While dicyanate esters are based on simple diphenols, cyanate telechelics are derived from phenol telechelic polymers whose basic properties are dictated by the backbone structure. The terminal cyanate groups serve as crosslinking sites. The polycyanate esters are obtained by cyanation of polyhydric polymers which, in turn, are synthesized by suitable synthesis protocols. Thus, in addition to the bisphenol-based CEs, other types like cyanate esters of novolacs [37,48], polystyrene [49], resorcinol [36], tert-butyl, and cyano substituted phenols [50], poly cyanate esters with hydrophobic cycloaliphatic backbone [51], and allyl-functionalized cyanate esters [52] have been reported. 

DEP afforded the synthesis of telechelic oligomers [29]. With the telomerization process, DEP appeared to be the first free-radical polymerization leading to telechelic oligomers. Tobolsky [30] first stated the conditions of DEP, i.e., the half life of the growing species has to be equivalent to that of the initiator. These specific conditions result in an unusual high rate of termination, which allows for the synthesis of oligomers. The telechelic structure will be obtained by combining a termination mode exclusively by recombination with the use of a difunctional initiator. 

Different reactions may affect the chain-end bromine atom of PS during ATRP transfer process, bimolecular terminations, or elimination reactions induced by the Cu(II) complex. The authors showed that the loss in functionality was predominantly due to /1-hydrogen elimination reactions. This result is very important for the synthesis of telechelic polymers by ATRP, because all processes (described later) are based on the halogen transformation. 

Concerning the bimolecular process, we give some examples of their synthesis. The bimolecular process, based on the use of a functional initiator, was not employed much to get telechelic oligomers. The main reason is that, despite the use of a counter radical, it is difficult to avoid any termination re-... 

Some Japanese teams developed a novel technique, based on unimolecular termination, which allows separating both initiation and termination (or transfer) processes. After growing chains are obtained, the macroradical formed is able to react with another molecule (mainly unsaturated) to lead to a stable radical. This one may transfer to give another radical able to reinitiate a polymerization. This process was developed, aiming at synthesizing either telechelic oligomers [85] (Scheme 78) or macromonomers [86] (Scheme 79). 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoindiiced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complicalion. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0,5, Sections 6.2,2.2 and 9.3.2). How ever, these features are used to advantage when the disulfides are used as initiators in the synthesis of telechelics" or in living radical polymerizations. The most common initiators in this context are the dithiuram disulfides (88) which are both thennal and photochemical initiators. The corresponding monosulfides [e.g. 89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

Peroxide Initiators. Hydroxyl radicals formed by a redox reaction with H2O2 are highly reactive, and have not been reported for the small-scale preparation of telechelics. However, the industrial preparation of hydroxy-terminated butadiene oligomers probably involves such process (39). 

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