Precursor cationic polymerization

In a similar fashion, the cationic polymerization of 2-oxazolines has been extensively studied and was found to provide the first verified entry to linear-poly(alkyleneimine) architectures. These acylated polymers were first recognized as precursors to linear poly(ethyleneimines) in the early 1960s [25]. Hydrolysis experiments demonstrated that deacylation of these products to linear PEI was possible. The original polymerization mechanism proposed by Tomalia et al. 

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

Diffiuex investigated a synthesis of cyclic poly(vinyl ether) using cationic polymerization [26,28]. The reaction process is depicted in Fig. 9. They studied on the living cationic polymerization of 2-chloroethyl vinyl ether (CEVE) initiated with the HI adduct of 4-(vinylbenzyloxy)butyl vinyl ether prepared by reacting chloromethyl styrene with sodium salt of 4-hydroxy-butyl vinyl ether in THF at 80 °C. By the cationic polymerization of CEVE, o /o-hetcrofunclional linear polymer precursor of cyclic poly(CEVE) was produced. The MWDs of the polymers were unimodal and very narrow (< 1.2),... 

This technique is based on the use of a linear polymer with pendant functional groups that can be activated to initiate the polymerization of a second monomer. Based on this definition, the linear precursor polymer can be considered as a multifunctional macromolecular initiator. The importance of the grafting from technique by cationic polymerization of the second monomer increased considerably with the advent of living cationic polymerization. The advantage is the virtual absence of homopolymer formation via chain transfer to monomer. 

Photopolymerization is traditionally initiated by direct photolysis of a precursor to provide free radicals via bond homolysis. Examples of such initiators include benzoin, and benzoin ethers, disulfides, and azoalkanes or dialkylperoxides. Hydrogen abstraction chemistry, typified by benzophenone photochemistry, is also recognized as extremely useful. However, a number of viable commercial photopolymer imaging systems are based upon ionic (especially cationic) polymerization. These systems will be discussed next. 

Another important role of the pendant-functionalized vinyl ethers is that they can be precursors of initiators for living cationic polymerization of other vinyl ether and styrene derivatives, from which polymers with terminal functional groups can be prepared (see Section IV). 

Cationic polymerization is one of the most fundamental methods for synthesizing polymers.A general scheme for the cationic polymerization of vinyl monomers having a cation stabilizing group (R) is shown in Scheme 9.4. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions are used as... 

One of the significant developments in living cationic polymerization has been the synthesis of telechelics and production of diblock copolymers. PIB based macromonomers are another class of functional precursors. Copolymerization of these... 

Interestingly, arene iron salts 62 have also proven to be effective catalysts for the free radical polymerization of epoxides, acrylates, etc. [3,36]. Similar to cationic polymerization, radical processes are also photo chemically initiated reactions but are initiated in the presence of radical precursor additives (halogenat-ed solvents, bis(p-iV,i -dimethylaminobenzylide)cyclopentanone, etc.). 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

Figure 7. Degree of polymerization ( , ) and polydispersity (O, n) of the precursor polymer resulting from the cationic polymerization of 6-chlorohexylvinyl ether ( , O) initialed by HI/Ij in toluene at -40 °C, and of the final poly 6-[(4 -n-ethoxy-4"-azobenzene)hexyl]vinyl ether)s prepared by polymer analogous reactions ( , )...

Cationic polymerization is one of the most fundamental methods for synthesizing polymers [19, 20]. Although there are several types of cationic polymerization, the most important one is cationic polymerization of vinyl monomers having a cation stabilizing group (Y) (Scheme 14.1). The initiation usually involves the addition of a cationic species (A ) to a vinyl monomer to produce a carbocationic intermediate associated with a counter anion (X ), which is derived from the initiator. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions [21-23] are used as initiators. 

Microsystem-controlled cationic polymerization technology requires extremely reactive initiators and cation pools serve as effective initiators for this technology. Usually carbocations are generated by a reversible process from their precursor. Yoshida et al. developed the cation pool method [35], in which carbocations are generated irreversibly by low-temperature electrolysis and are accumulated in relatively high concentration in the absence of nucleophiles. N-Acyliminium ions, alkoxycarbenium ions [36-40] and diarylcarbenium ions [41] have been generated by this method. Such cation pools are expected to serve as extremely reactive initiators for cationic polymerization. 

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