Cationic techniques

Telechelic and semitelechelic polymers have been synthesized by a variety of free radical, anionic and, most recently, cationic techniques (1.2.9-12). The advantages of anionic procedures include functional purity, versatility of substitution, and monodisper-sity and control of polymer molecular weights. This... 

The occurrence of definitive bimodal molecular weight distributions in polymers prepared by normal cationic techniques was first discovered by Hi-... 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

As a first approach to post-ionization of desorbed neutral pol5uners we discuss laser-generated cationization. This approach differs from other desorption/ cationization techniques because the desorption step and the cationization step are completely separate. First polymers are gently laser desorbed at low fluence wifh one laser. Independenfly atomic metal ions are generated by a second pulsed laser that is tightly focused on a metal surface fo creafe a plume of mefal ions. Gas phase collisions above the sample surface subsequenfly produce fhe cafionized complexes. Separation of fhe desorption and cationization processes allows independent optimization of fhe fwo laser/maferial interactions. This approach is especially useful in sifuations where ofher ionization methods fail, such as fhe example of PFPEs discussed here. ... 

Styrene is conspicuous among the monomers given in Table 29.2 because it can be polymerized using either anionic or cationic techniques as well as radical techniques. This characteristic particular to styrene is attributable to the fact that the phenyl group can stabilize cationic, anionic, and radical benzylic intermediates. 

Soft ionization can be achieved in the gas phase, from the liquid phase, or from the solid phase. Gas-phase cationization techniques have been discussed in previous chapters, especially in Chaps. 5 and 6. Typical examples of liquid-phase ionization techniques are thermospray ionization (TSI) and electrospray ionization (ESI), which are especially important in combining liquid chromatography and mass spectrometry (LC-MS). A wide variety of solid-phase ionization or desorp-tion/ionization techniques have been developed over the years, including field... 

CARs have been obtained on a laboratory or an industrial scale [92-94]. In this respect, thermal oUgomerization is employed [95], the cationic technique [96] or other procedures that combine thermally initiated reactions or Diels—Alder additions with cationic oligomerization [97,98],... 

Likewise, p-pinene with its exocyclic vinyl group is readily polymerized by cationic techniques however, the polymers obtained have rather low MWs of<3.4 kg/mol [80, 84-86]. High molecular weight poly(p-pinene) with MW up to 40 kg/mol (PDI 2.2) can be obtained with the H20 /EtAlCl2 system ( H2O indicates adventitious moisture impurities). The polymerizations are carried out in mixtures of methyl chloride/ methylcyclohexane (preferred composition 50 50) at —80°C. Quantitative monomer conversions are reached within 20 min or less. The repeat unit of the poly(p-pinene) is found to consist of a cyclohexene unit in the main chain (Scheme 6), which reflects isomerization polymerization [87]. With AICI3 etherates, e.g., AlCl30Ph2, the polymerization can be performed even at room temperature and low catalyst concentration (2.S-5.5 mM, [p-pinene]o = 0.55 M) to yield polymers with = 9-14 kg/mol... 

Feetures Suitable for cold emulsif cation technique... 

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