Unprotected functional

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

The synthesis of block copolymers by macromonotner RAFT polymeriza tion has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thioearbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block formation is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared.476 638 The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

Radical polymerization is the most useful method for a large-scale preparation of various kinds of vinyl polymers. More than 70 % of vinyl polymers (i. e. more than 50 % of all plastics) are produced by the radical polymerization process industrially, because this method has a large number of advantages arising from the characteristics of intermediate free-radicals for vinyl polymer synthesis beyond ionic and coordination polymerizations, e.g., high polymerization and copolymerization reactivities of many varieties of vinyl monomers, especially of the monomers with polar and unprotected functional groups, a simple procedure for polymerizations, excellent reproducibility of the polymerization reaction due to tolerance to impurities, facile prediction of the polymerization reactions from the accumulated data of the elementary reaction mechanisms and of the monomer structure-reactivity relationships, utilization of water as a reaction medium, and so on. 

Free radical reactions are proving to be synthetically useful altern-atives for producing carbon-carbon bonds. Recently, Stork has shown that vinyl radicals are valuable in ring forming reactions since they place a double bond in a predictable position. Their compatibility with many unprotected functional groups and their ability to form quaternary centers are additional features which make vinyl radical cyclization an attractive synthetic method. 

Fig. 2.1 Divergent synthesis. C=coupling points F=active, unprotected functional group P= protected, inactive (protective group) functionality. The core of the dendri-...

It must be resistant to reagents that would attack the unprotected functional group. 

The benzyl ethers of serine and threonine are stable under the acidic conditions used in the Boc strategy. Cleavage of the benzyl ethers is performed with HF under normal conditions or with nnixtures of super adds. Especially in the case of large peptides with many benzyl ether groups, side reactions result from the intermediate benzyl halides that can react with unprotected functionalized side chains scavengers are added to avoid such side reactions. A modified TMSBr method has also been developed. ... 

Functionalization of several pyranose sugar derivatives, e.g. 28, 30, 33, was achieved with a variant of this method in which 3-oxabicyclo[4.1.0]heptanyl radicals were generated, either photochemically from benzoate esters, or from bromides. These photochemical transformations left unprotected functional groups in the substrate intact, including bromomethyl, ester, and ether groups, and rather effectively demonstrated the tolerance of free-radical intermediates for such functionality. 

Kagan and co-workers pioneered the work on the reductive behavior of the low oxidation states of the lanthanide elements in organic synthesis [2b]. Ln metals and Ln(II) derivatives were subsequently found to promote a number of important individual reactions [301]. The combination of one- and two-electron chemistry sets Sml2 apart from virtually every other reductive coupling agent currently available and exhibits exceptional properties for sequential conversions tolerating unprotected functional groups [lb]. 

This method involves the use of functional initiators with a protected or unprotected functional group. When the functional group is unreactive under the polymerization conditions, protection is not necessary. Functional vinyl ethers have extensively been used in the Bving cationic polymerizations of vinyl ethers and these functional poly(vinyl ethers) can be derivatized to the desired functionahty by simple organic reactions. Vinyl ethers carrying a variety of functional pendant groups, in a general form (Scheme 26.1) ... 

Resistant to the reagents used to transform the unprotected functional group or groups. 

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