Pseudo-living

The peroxide 179 dissociates in the presence of a monomer giving rise to alkoxyl (CO-) and borinate (BO-) radicals, but the latter are believed to be too stable to initiate polymerization. It should be mentioned that the molecular weight continuously increases throughout the process implying the pseudo-living mechanism for chain growth. After the completion of the process borane residue is completely oxidized into diol <2004MM6260>. Thus, the 8-boraindane molecule not only initiates the polymerization, but also is a precursor to two functionalities in the polymer chain. 

An essential feature of a strictly living polymerization is the absence of transfer reactions [652]. This requirement was found to be valid for the polymerization of IP catalyzed by NdCl3 TBP/TIBA in hexane. The lack of chain termination reactions, the second requirement for a living polymerization [652], was confirmed by the application of a mathematical model to the experimental data [279]. Bruzzone et al. realized that transfer reactions occur in the polymerization of BD. In spite of this observation the pseudo-living character of the polymerization was assigned to the superposition of chain growth and chain transfer both of which exhibit a different dependence on monomer conversion [87]. 

Modern polymerization techniques, such as sequential iodine transfer polymerization of fluoroalkenes [11,12], lead to novel thermoplastic elastomers (TPEs). These triblock copolymers can be produced in a process, which can be emulsion, suspension, microemulsion, or solution polymerization [13], Using pseudo-living technology or branching and pseudo-living technology, A-B-A phase separated copolymers with soft (amorphous) and hard (crystalline) domains can be produced. The hard domains can be composed from the following ... 

These new systems open up applications such as the difunctional compounds (used in polycondensation) which exhibit new reactive groups (amine or isocyanate) or the pseudo living polymers which are able to initiate new polymaizations for obtaining triWockcopolymers. 

In Section 4.1.1, the general mechanistic aspects of anionic polymerisation of alkylene oxides (especially PO) were discussed. The anionic polymerisation of PO initiated by hydroxyl groups is considered as a pseudo living polymerisation. This type of polymerisation has some important aspects of living polymerisations the active centre (alcoholate type) is stable and active, and during the polymerisation reaction the number of active alcoholate centres remains constant. This characteristic of living polymerisations is very important for the synthesis of block copolymers. For example if after the addition of PO to the living polymer EO (or BO) are added, then block copolymers are obtained. 

Block copolymers have also been produced by the addition of vinyl monomers to occluded or long lived macroradicals. These "pseudo" living macroradicals are produced when vinyl monomers are polymerized in poor solvents (16-18) or in viscous medium (19). 

Sanda, F., Fueki, T., Endo, T., 1999. Cationic ring-opening polymerization of an exomethylene group carrying cyclic carbonate. Pseudo-living polymerization of 5-methylene-l,3-dioxan-2-one by the assistance of the exomethylene group. Macromolecules 32, 4220—4224. 

On the other hand, a living (33) or pseudo living (25) nature of Ln-catalyzed polymerization was proposed to account for the proportional increase of molecular weight with conversion and proved by the formation of block copolymers when the reacting butadiene was substituted with isoprene. Moreover, a Nd-polybutadiene quenched with CO was found to contain functional end groups, even if their amount was not measured (31). The presence of Ln-CH -CH -Ln species, in analogy with Li chemistry, was also indicated in the reaction of 4f metals (Y, Sm) with ethylene (3). 

C-H Bond activation, with lanthanides Ethylene polymerization, with lanthanides Zeigler-Natta catalyst, lanthanide Diene polymerization, with lanthanides Olefin polymerization, with lanthanides Butadiene polymerization, with lanthanides Isoprene polymerization, with lanthanides Anionic propagation, at lanthanides Living polymers, at lanthanides Pseudo-living polymers, at lanthanides Reaction orders, diene polymerization Active sites, diene polymerization... 

Fischer, A., Brembilla, A., and Lochon, P. (1999). Nitroxide-mediated radical polymerization of 4-vinylpyridine study of the pseudo-living character of the reaction and influence of temperature and nitroxide concentration. Macromolecules, 52(19) 6069 072. 

This concept was further extended to totally eliminate the use of iodonium salts as the component of the photoinitiating system [KAH 09]. The cationic polymerization of vinyl ethers was initiated upon irradiation at A = 350 nm with vinyl halides in the presence of zinc iodide. A mechanism involving the formation of an adduct between the monomer and the products yielded from the photoinduced homolysis of the vinyl halide followed by electron transfer is proposed. In the subsequent step, the terminal carbon-halide bond in this adduct is activated by the coordinating effect of zinc iodide. This polymerization exhibited some characteristics of pseudo-living cationic polymerization. 

Two examples of molecular calcium initiators supported by dianionic multi-dentate ONNO ligands were reported. In 2006, Bochmann and coworkers prepared the dimeric [ ONNO Ca]2 (73) (Fig. 23) [95], They used it as a singlecomponent initiator to promote the sluggish but pseudo-living ROP of CL, reaching half-conversion of 200 equiv of monomer after 12 h at 60 °C in toluene (Mw/M 1.1-1.2) the nature of the mechanism was not discussed. The related Ca homoleptic complex ON 2Ca(THF)2 (74) containing two imino-phenolate ligands NO was inactive towards CL under the same conditions. 

An inverse manner of copolymerization was proposed by Watanabe et First, they polymerized TMC using 4-(chlor-omethyl)benzyl alcohol (CBA) as an initiator and DBU as an organocatalyst. The benzyl chloride group was involved in the incorporation of dithiocarbamate for pseudo-living radical polymerization of vinyl monomers. The authors applied N-isopropylacrylamide, acrylamide glycolic acid, and 2-hydroxyethyl methacrylate as vinyl monomers for the second step of copolymerization (Scheme 93). The resulting block... 

Scheme 93 Preparation of polycarbonates with dithiocarbamate groups for pseudo-living radical polymerization of vinyl monomers.

With VLO and CLO, the polymerization has been shown to proceed via a pseudo-living character (proved by the linear dependence of the degree of polymerization on the degree of conversion) and has been explained by the tmusual balance between growth center formation, chain propagation, and side reactions. It has been calculated that less than 15% of the originally introduced lactone was responsible for the growth center formation. 

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