Chain transfer in polymerization

The proton eliminated from the Cf2 ion can continue the reaction chain by protonating the Cg olefin and thus pve rise to a different set of products [Eqs. (13)- f6)]- This process corresponds to chain transfer in polymerization. To distinguish these products resulting from chain transfer from those arising by initiation, the formers are denoted second... 

It can also be noted that reversible chain transfer, in RAFT and similar polymerizations, and reversible activation-deactivation, in NMP and ATRP,... 

Park and Smith 170 attempted to allow for chain transfer in their examination of the termination mechanism during VC polymerization at 30 and 40 °C in chlorobenzene. They determined the initiator-derived ends in PVC prepared with radiolabeled AIBN and concluded that kjk = 3.0. However, questions have been raised regarding the reliability of these measurements.171 172 Atkinson et al.x72 applied the gelation technique (Section 5.2.2.2) to VC polymerization and proposed that termination involves predominantly combination. 

Since the dithiocarbatnyl end groups 8 are thermally stable but pholochemically labile at usual polymerization temperatures, only photo-initiated polymerizations have the potential to show living characteristics. However, various disulfides, for example, 9 and 10, have been used to prepare end-functional polymers37 and block copolymers38 by irreversible chain transfer in non-living thermally-initiated polymerization (Section 7.5.1). 

Branching through chain transfer in vinyl polymerizations has been... 

Ethylene pressure studies have revealed a first-order dependence on ethylene for both the rate of chain propagation and the rate of chain transfer. This polymerization behavior together with X-ray analyses and DFT calculations has provided strong support for (1-11 transfer to an incoming monomer, which is responsible for the production of vinyl-terminated PEs. The calculations thus suggest that the catalysts disfavor (I-11 transfer to the Zr metal because of the extreme instability of the Zr hydride species that is produced in such a chain transfer process. 

The existence of chain transfer in ionic polymerizations was first found in the system isobutene-BFj at room temperature when it was discovered that very small traces of water, tert-butanol, or acetic acid would, as co-catalysts, cause the transformation of large quantities of monomer to very low unsaturated polymers [2, 5]. It was assumed that the process involved proton transfer, and there is no cause to change this view ... 

The existence of chain transfer in the polymerization of the n-butenes and of propene has also been shown [19], and for the last named a hydride ion transfer mechanism has been proposed, which yields branched chains [9]. 

The polymerization of other fluoroolefins such as TFE with hexafluoropro-pylene (HFP), TFE with ethylene, and vinylidine difluoride - " further demonstrates the broad applicability of liquid and supercritical CO in the production and processing of fluorinated polymers. Many of the aforementioned advantages associated with CO2, including tunable solvent properties, integrated synthesis, separation and purification processes, negligible chain transfer in the presence of highly electrophilic species, and relative ease of recycling, make it an ideal solvent for fluoroolefm polymerization. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

TABLE 3-14 Activation Parameters for Chain Transfer in Styrene Polymerization (60°C) I... 

Possibilities for chain transfer in cationic polymerization are abundant.119,138 Proton transfer to the monomer is a rather general transfer reaction leading to two isomeric unsaturated end structures ... 

On the positive side it is important to note that primary isotope effects are only observed when the bond concerned is undergoing rupture in the rate determining step of a reactionx. Thus, the effect is of paramount importance in mechanistic studies for the identification of rate determining steps. Two examples illustrate the point a) Chain transfer in styrene polymerization (75)... 

Fig. 57. Chain transfer in free radical polymerization with a side group of the-dead polymer. Note After termination of the radical one side chain and a free linear chain are obtained.

Such a mechanism could also explain the apparent induction period found at the beginning of the polymerizations. With very small particles and large chain transfer, the polymerization could be acting as if it were in a homogeneous medium. This produces very low rates compared with a standard emulsion polymerization. As particle size increases, the rate rises because chain-transferred monomers would not diffuse into the aqueous phase. 

Chain transfer in the polymerization of styrene. VII. Compounds containing halogens, oxygen and nitrogen. J. Amer. chem. Soc. 75, 3530 (1953). 

Hayes, R. A. Polymeric chain transfer reactions. Polymerization of some vinyl monomers in the presence of vinyl polymers. J. Polymer Sci. 11, 531 (1953). 

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