Quasi-living

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]. 

Some of these bi-metallic oxyalkoxides are able to polymerize oxiranes to high-molecular weight poly-ethers8 b) (M.W. of the order of 106 ). The resulting polymers reveal a quasi-living character, their DPn increases with conversion and is further... 

Quasi-living Carbocationic Polymerization of Alkyl Vinyl Ethers and Block Copolymer Synthesis... 

Alternatively, Soga and co-workers described how a series of (CsH4R)TiCl3 compounds in association with MAO could mediate 1,4-at-stereospecific living BD polymerization (e.g., R = Bu, —25°C, 1,4-m-selectivity = 93.5%, = 126000, PDI = 1.04). It was also revealed that an increase in the steric bulk of the substituent R results in an increase in both catalytic activity and 1,4-m-selectivity. Additionally, Shiono and co-workers found that stereospecific quasi-living BD polymerization could be catalyzed by a rather simple catalyst system composed of C0CI2 and MAO (0°C, l,4- j-selectivity = 98.2%, M = 361000, PDI = 1.3). ... 

The quasi living polymerization of ethene and norbornene has been reviewed, among other topics in living polymerization of alkenes (19). Specifically, arylimido-aryloxo-vanadium(V) complexes with methylaluminoxane or Et2AlCl as co-catalyst have been used as catalyst systems. The polymers exhibit a low polydispersity and a high molecular weight (20). 

Functionalization of quasi living PIB has been typically attempted through the use of functional initiators and through in situ functionalization by quenching. Most past efforts to produce functionality by quenching of quasi living chains have failed and led to tert-chloride terminal units (8). 

This is explained as quasi living PIB is composed primarily of dormant, i.e., reversibly terminated chains. Thus, most added reagents, particularly strong nucleophiles, quench the Lewis acid coinitiator... 

The synthesis of A2B miktoarm star polymers has been discussed and exemplified using PIB as a component. The synthesis involves a quasi living cationic polymerization of isobutylene from a monofunctional cationic initiator. This initiator also contains a blocked hydroxyl group. Eventually, the blocked hydroxyl group of the initiator is deblocked, and functionalized with a branching agent. This activated reagent is then used for an atom transfer radical polymerization process of /erf-butyl acrylate (18). 

In the course of quasi living carbocationic isobutylene polymerization it is possible to functionalize the polymer at their end groups by quenching with reactive compounds. The various possibilities are shown in Table 6.2. 

D.L. Morgan and R.F. Storey, End-quenching of quasi-living isobutylene polymerizations with alkoxybenzene compounds, Macromolecules, 42(18) 6844-6847, September 2009. 

The synthesis of PP-fr-EPR can be accomplished by a stopped-flow polymerization method, whose polymerization time is very short and which is a quasi-living system, in the presence of a MgCl2-supported titanium catalyst [132-135]. The results of GPC, 13C NMR, CFC, DSC, and optical microscopic observation indicated the formation of a block copolymer having a chemical linkage between PP and EPR segments. 

Another indicator in favor of a living polymerization is a linear increase of number average molar masses (Mn) on monomer conversion. Hsieh et al. reported on linear plots for the dependence of inherent viscosities on monomer conversion. These plots were highly Unear and Hsieh et al. assigned the term quasi-living to these polymerizations [134,139]. Kwag et al. attributed the living character of Nd-catalyzed diene polymerizations to the ionic character of the Nd allyl bond and the stable oxidation state of Nd. In addition, theoretical frontier orbital analysis confirmed these results [653]. 

A serious contradiction with the requirements of a strictly Uving polymerization are broad or even bimodal MMDs which are in the focus of many studies, e.g. [87,178,620]. This observation of broad and at least bimodal MMDs is the result of the presence of at least two active catalyst species which show different activities. This feature is in contradiction with a strictly living polymerization. Wilson attributed the polymer fraction with a high molar mass to insoluble catalyst species which are invisible to the naked eye whereas the low molar mass fraction of the polymer is supposedly produced by soluble sites which operate in a quasi-living manner [89]. In his study Wilson used catalyst systems of the type Nd(carboxylate)3/DIBAH/tBuCl. 

Linear increases of intrinsic viscosity, inherent viscosity, dilute solution viscosity (DSV), Mw, Mn or Mv on monomer conversion are reported in many studies. For example, Wilson emphasized the unique character of Nd-based catalysts in comparison to conventional transition-metal systems in the following terms the typically linear relationship between molecular weight and conversion of the Nd-based polymerization of BD indicates a quasi-living... 

In conclusion, styrene can be considered as an ideal monomer in photo-chemically induced polymerization and can be used successfully in quasi living polymerizations. 

Under these conditions, the radical polymerization appeared to be quasi living (i.e., Mn increases with monomer conversion) in contrast to reactions in the absence of thiuram disulfide. However, even if the authors did not mention it, it would be considered a drawback that the Mn value decreased because of the excess of terminating radical 58. The proposed mechanism, where R represents... 

The MAO-activated /3-enaminoketonato complexes of Figure 62 are moderately to highly active ethylene polymerization catalysts.1186 However, molecular masses are not very high (Mn < 100000). PEs produced with the first two systems, that is, R =GF3 and R2 = Ph or 2-theonyl, show rather narrow molecular mass distribution (Mw/ Mn < 1.35), indicative of quasi-living behavior. The post-polymerization experiments indicated that the catalyst lifetime is longer than 60 min even in the absence of ethylene. On the other hand, the third system, that is, R1 = CF3 and R2 = 2-furyl, produces PEs with rather broad polydispersities, although the only difference with the 2-theonyl-based catalyst is the replacement of an S atom with an O atom. 

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