Stille polymerization

Many of the synthetic elastomers now made are still polymerized by a free radical mechanism. Polychloroprene, polybutadiene, polyisoprene, and styrene-butadiene copolymer are made this way. Initiation by peroxides is common. Many propagation steps create high molecular weight products. Review the mechanism of free radical polymerization of dienes given in Chapter 14, Section 2.2. 

The method was used in studies of a fungal heterogalactan.150 The polysaccharide was subjected to successive tritylation, methylation, detritylation, p-toluenesulfonylation, reaction with sodium iodide, and, finally, reaction with sodium p-toluenesulfinate. The product was then treated with sodium methylsulfinyl carbanion in dimethyl sulfoxide, the product remethylated, and the polysaccharide material recovered by gel chromatography. The polymer was hydrolyzed, and the sugars in the hydrolyzate were analyzed, as the alditol acetates, by g.l.c.-m.s.1 The analysis revealed that —60% of the hexose residues that were unsubstituted at C-6 had been eliminated. As the product was still polymeric, it was concluded that these residues had constituted a part of side chains linked to a main chain of (1 — 6)-linked D-galactose residues. 

U. Why does polymerization occur only at relatively low temperatures often below 200 °C What occurs at higher temperatures Formaldehyde polymerizes only below about 100°C but ethylene still polymerizes up to about 500 °C. Why the difference ... 

The enthalpy of polymerization of 3- and 4-membered rings is so much higher than the entropy factor that substitution does not significantly reduce their polymerizability. Disubstituted oxiranes (e.g., isobutylene oxide) or oxetanes (3,3-dimethyloxetane) still polymerize practically irreversibly. Substitution may prohibit polymerization of 5-membered monomers, however. 

In order to co-polymerize the IC unit, Suzuki and Stille polymerizations have been used. First, Blouin et al. [94] were able to obtain polyindolo[3,2-fr]-carbazole derivatives with bithiophene or biEDOT as co-monomers. Unfortunately, these studies demonstrated a relatively low oxidation potential for these polymers (especially for P35 and P37), limiting their applications in OFETs and PCs. However, for doped state applications, these polymers may exhibit interesting properties [35]. For instance, when copolymerized with bithiophene, the resulting copolymer shows a good electrical conductivity (as high as 0.7 Scm 1) but a low Seebeck coefficient of 4.3 iV K 1 [35]. Finally, the UV-Vis absorption maxima are similar for poly(2,8-indolocarbazole-a/f-bithiophene) and poly(2,8-indolocarbazole-a/f-bis(3,4-ethylenedioxythiophene)). A broad absorption band is centered at 430 nm whereas, for the 3- and 9-substituted copolymers, the broad band is centered around 490-500 nm [94],... 

Scheme 9.32 The Stille polymerization to yield the organometallic polymer with Pt(ll) centers 92 for chemical sensing applications by [256].

Mentioned below are broadly eleven ways of synthesizing these polymers Method A Electrochemical Polymerization Method B Solution Polymerization Method C In situ Chemical Oxidative Polymerization Method D Dispersion Polymerization Method E Vapor-Phase Polymerization Method F SoUd-State Polymerization Method G Solvent-Free Chemical Polymerization Method H Grignard Metathesis (GRIM) Polymerization Method I Direct C-H Arylation Polymerization Method J Stille Polymerization Method K Yamamoto Polymerization... 

Factors to Consider from the Perspective of Stille Polymerization... 

The Stille reaction-based polymerization, by its polycondensation nature, falls into the major category of step-growth polymerization. Thus, characteristics concerning step-growth polymerization still exist and related fundamental principles apply, such as reaction kinetics, molecular-weight distribution and control, and end-group modifications. We will briefly discuss the latter two in the context of Stille polymerizations. 

Over the past twenty years numerous D A copolymers with a great variety of donor/acceptor moieties have been designed and synthesized. It is practically impossible and unnecessary to include all related chemistries in the limited space of this chapter. In the next section, we will select a few commonly employed monomers (including both donors and acceptors) with broad applications and excellent properties (especially for OPV devices), and describe their syntheses in detail. Then we will discuss the specifics of synthesizing D-A copolymers via Stille polymerization. To conclude this chapter, three representative syntheses of D-A copolymers will be provided as examples. 

Prior to the Stille polymerization, all these thiophene-based donor molecules need to be converted into stannylated monomers, typically via BuLi in anhydrous THF followed by treating the lithiated anion with trialkyltin chloride. Usually MesSnCl is preferred albeit its stronger toxicity than the butyl analog i.e., BusSnCl) because the latter can render it much more difficult to purify monomers through recrystallization. 

Except TPD and isoindigo, which were brominated during its synthesis, all other acceptor units introduced above can be converted into monomers for Stille polymerization, usually by subjecting them to simple brominations with NBS. 

The purity of both monomers for the Stille polymerization is crncial. This will not only ensure an accurate stoichiometric control (see Section 15.2.4 for details), but also minimize possible negative impact to the device properties by these impurities. Typically, both distaimylated donor monomer and dibromo acceptor monomer need to be recrystallized or purified by prep HPLC prior to the polymerization, in order to ensure a high level of purity. An excellent case study demonstrating monomer purity s effect on the molecular weight of the D-A copolymer was provided by Osaka et al. In this study, the authors used three different procedures to purify both monomers and observed essentially no difference in the NMR spectra of these monomers after the different purification processes. However, the purity of the monomers significantly affected the observed molecular weight of their polymers (from 13 kg/mol to 73 kg/mol), which noticeably influenced the photovoltaic properties of devices based on these polymers. 

The purity of the catalyst is equally important. This simple fact, however, is often ignored because usually these catalysts are purchased from reputable vendors and regarded as pure . However, Zalesskiy and Ananikov recently cautioned that commercially available Pd2(dba)3 could contain up to 40% Pd nanoparticles, which would not participate in the catalytic cycle. For D-A copolymerization, such a high level of impurity in the catalyst could lead to unexpectedly low molecular weight and even worse, more metal residue in the finished polymers. Therefore, it is strongly recommended that Pd2(dba)3 be carefully purified prior to the Stille polymerization. Following is a specific procedure to purify Pd2(dba)3, adapted from ref. 81. 

Although these systems polymerize much slower than the acrylate systems, they are low cost and can still polymerize at ambient temperatures. 

For 33 and 34, this situation is reversed these polymers can be better described as the Bu2Mg moiety formally acting as a Lewis base to solvate the dimeric [NaHMDS]2 unit [ie, the (NaHMDS)2 dimer acts as a Lewis acidic entity], hence the new term inverse magnesiate . Both complexes are still polymeric despite the presence of TMEDA and (J ,Ji)-TMCDA donors. 

P2 was prepared by Wei et al. with Stille polymerization of a dibromo compound and ditin compound using [Pd2dba3/P(o-tolyl)j] as the catalyst precursor. The of 9.7 kg mol with a PDI of 1.4 was determined by gel permeation chromatography (GPC) using chloroform as the eluent. P2 is readily soluble in hot chlorinated solvents sueh as chloroform, chlorobenzene, and dichlorobenzene. With the blends of P2 PC7i BM (1 1.5, w/w) as the active layer, a PCE of 7.3% was achieved using 1,6-diiodohexane (DIH) as the processing additive (Scheme 1.3). ... 

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