Telechelic living” chain polymerization

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

On the other hand, a functionality of 1 on each chain end will be related to telechelic compounds (Scheme 1). This includes diols, diamines, and diacides. Of course it also comprises diolefin compounds that usually lead to gels or networks. We can also note that when the G and G functional groups are different at each chain end, the appropriate term becomes heterotelechelic (Table 1). It is also necessary to specify the particular case of macromolecules bearing a well-identified G functionality at one chain end and a thermally reactivated group at the G chain end. These groups can be nitroxides, an iodine atom, xanthate, etc.and are commonly used in living radical polymerizations (LRP). These compounds may be classified as monofunctional oligomers (Table 1). 

In recent years, there have been significant developments in the field of living carbocationic polymerization (LCCP) of vinyl monomers, such as isobutylene (IB), styrene and its derivatives, and vinyl ethers, leading to a wide variety of functional polymers (for recent reviews see Refs. 1-4). Due to the attractive properties of polyisobutylene (PIB) available only by carbocationic polymerization, coupling this hydrophobic, thermally, oxidatively, and hydrolytically stable polymer with a low Tg to a variety of other chain segments is expected to result in new useful products. For instance, methacrylate-telechelic PIB (MA-PIB-MA) obtained by LCCP and subsequent chain end derivatization has been successfully used to synthesize novel amphiphilic networks by radical copolymerization of MA-PIB-MA with a variety of monomers, such as N,N-dimethylacrylamide and 2-trimethylsilyloxyethyl methacrylate, a protected 2-hydroxyethyl methacrylate... 

Cyclic Sulfides. The three-membered cyclic thiiranes can be polymerized cationically, anionically, or by a coordination mechanism. The four-membered cyclic thietanes can be polymerized by cationic and anionic mechanisms, but five-membered rings cannot be polymerized. Polymerization of propylenesulfide initiated with sodium naphthalene yields telechelics with naphthalene groups on both ends, if the living chain is terminated 1-chloromethylnaphthalene (321). 

To obtain telechelic polymers—that are, those carrying a same reactive molecular group at each of their ends—the most direct method is to use bifunctional initiators. Examples of such initiators can be found for all living chain addition polymerizations the example shown below illustrates the bifunctional initiation of living cationic polymerization of vinyl ethers ... 

Friedel-Crafts alkylation of various aromatic rings such as benzene, toluene or phenol lead to a variety of diaromatic products (equation 46). Nitration of this telechelic followed by reduction leads to a polymer with terminal amine groups. The phenol-terminated telechelics can be further derivatized as shown in Scheme 47. 57.458 Recently, tertiary esters and ethers were used in conjunction with BCI3 to provide the first example of a living carbocationic polymerization of isobutylene. Scheme 48 shows the polymerization mechanism suggested for initiation with tertiary esters. The resulting polymers are telechelics containing one or two tertiary chlorine chain ends. 

Telechelic polymers are defined as macromolecules with reactive sites on the polymer chain, usually as endgroups on linear polymers [106]. This macro-molecular architecture has successfully produced a wide variety of block copolymers using macroinititated polymerizations. Living anionic polymeriza-... 

The versatility associated with nitroxide-mediated polymerizations, in terms of both monomer choice and initiator structure, also permits a wide variety of other complex macromolecular structures to be prepared. Sherrington201 and Fukuda202 have examined the preparation of branched and cross-linked structures by nitroxide-mediated processes, significantly the living nature of the polymerization permits subtlety different structures to be obtained when compared to traditional free radical processes. In addition, a versatile approach to cyclic polymers has been developed by Hemery203 that relies on the synthesis of nonsymmetrical telechelic macromolecules followed by cyclization of the mutually reactive chain ends. In a similar approach, Chaumont has prepared well-defined polymer networks by the cross-linking of telechelic macromolecules prepared by nitroxide-mediated processes with bifunctional small molecules.204... 

Synthesis and Characterization. The synthesis of carboxy-telechelic polyisoprenes was carried out as described previously[7], in tetrahydrofuran at -78°C using sodium naphthalide as initiator. Approximately 2-3 units of a-methylstyrene per living end were added after completion of the Isoprene polymerization to reduce the reactivity of the chain ends, which were then terminated by the addition of gaseous... 

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