Chain end functionalization

The repertoire of reactions possible with organolithium compounds is well documented in the literature The application of these functionalization reactions to polymers is also described in the anionic polymer review literature Unfortunately, many of the reported applications of these functionalization reactions to anionic polymers have not been well characterized. Accordingly, one is faced with the situation in which a variety of useful chain end functionalization reactions is potentially possible, but whose application to polymers is not well defined in terms of specifics such as side reactions, yields, solvent effects, etc. The following discussion of representative functionalization reactions is not meant to be exhaustive, but can be regarded as typical of the state-of-the-art in this area. 

The carbonation of pol3meric anions using carbon dio.xide is one of the most useful and widely used functionalization reactions. However, there are special problems associated with the carbonation of polymeric organolithium compounds . For example, Wyman, Allen and Altares reported that the carbonation of poly-(styryl)lithium in benzene with gaseous carbon dioxide produced only a 60% yield of carboxylic acid the acid was contaminated vrith significant amounts of the corresponding ketone(dimer) and tertiary alcohol(trimer) as shown in Eq. (70). 

Contrary to the conclusion of Mansson it would be expected that association of the organolithium chain ends would promote coupling to form the ketone (dimer) product, Eq. (73) conversely, dissociation 

As discussed elsewhere in this review, Lewis bases such as tetrahydrofuran are known to promote disaggregation of polymeric organolithium speciesThus, in the presence of excess tetrahydrofuran, both poly(styryl)lithium and poly(isopre-nyl)lithium would be expected to be unassociated (or at least much less associated). Therefore, in the presence of sufficient tetrahydrofuran, the carbonation reaction would take place with unassociated organolithium chain ends and ketone formation (Eq. (73)) would only be an intermolecular reaction (rather than an essentially intramolecular reaction as in the case with the aggregated species) competing with carbonation. In complete accord with these predictions, it was found that the carbonation of poly(styryl)lithium, poly(isoprenyl)lithium, and poly(styrene-h-isoprenyl)lithium in a 75/25 mixture (by volume) of benzene and tetrahydrofuran occurs quantitatively to produce the corresponding carboxylic add chain ends. The observation by Mansson that THF has no apparent influence was complicated by the use of methyl-cyclohexane, which is a Theta solvent for poly(styrene) (60-70 °C) furthermore. 

The carbonation of dilithium reagents is complicated by the occurrence of gelation phenomena which produce severe mixing problems In general, lithium 

PSLi+ C02— PSC02Li PSLi PS —C—PS- -PSCPS- U ps)3COLi H2° - (PS)3C0H 

The carbonation of dilithium reagents is complicated by the occurrence of gelation phenomena which produce severe mixing problems 145 146-323). In general, lithium derivatives of heteroatoms are highly associated in solution therefore, heteroatom functionalization of polymers with two active anionic chain ends will form an insoluble, three-dimensional network. The beneficial effect of decreasing the effects of association and gelation by solvents with solubility parameters 7.2 has been reported in the literature 140,324). 

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Ruths M and Granick S 1998 Rate-dependent adhesion between opposed perfluoropoly (alkyl ether) layers dependence on chain-end functionality and chain length J. Rhys. Chem. B 102 6056-63... 

When a polymer is prepared by radical polymerization, the initiator derived chain-end functionality will depend on the relative significance and specificity of the various chain end forming reactions. Tlius, for the formation of telechelic polymers ... 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Hyperbranched polymers generally have very low melt and indinsic viscosities. The large number of chain-end functional groups present in hyperbranched macromolecules have also been shown to dramatically affect physical properties... 

Izumisawa, S. and Jhon, M. S., "Stability Analysis and Molecular Simulation of Nanoscale Lubricant Films with Chain-End Functional Groups, /. Appl. Phys., 2002, Vol. 91,2002, pp. 7583-7585. 

Scheme 7 Fomation of chain-end functionalized nickelacycle initiators...

The bifunctional initiator 4-hydroxy-bulyl-2-bromoisobulyralc, HBBIB, promoted the ATRP of styrene as well as the cationic ring opening polymerization of THF [134], In the presence of Cu/CuBr2/PMDETA styrene was polymerized through the bromoisobutyrate function of HBBIB, to give PS chains end-functionalized with hydroxyl groups, PS-OH. The in situ... 

A second, and potentially more useful feature is the stability of these unimolecu-lar initiators to a wide variety of reaction and polymerization conditions which is in sharp contrast to traditional initiators for anionic procedures, such as n-butyl lithium. This allows the initiators to be fully characterized, purified and handled by normal techniques, thus simplifying the polymerization process. It also permits a variety of chemical transformations to be performed on the initiator prior to polymerization, which greatly facilitates the preparation of chain end functionalized macromolecules. For example, the chloromethyl functionalized al-koxyamine, 18, can be readily converted in high yield to the corresponding aminomethyl derivative, 19, followed by polymerization to give well-defined linear polymers, 20, with a single primary amine at the chain end (Scheme 12). 

The use of AB monomers for the preparation of both hyperbranched and dendritic macromolecules leads to the presence of an extremely large number of chain end functional groups for both systems. In analogy with dendrimers, the... 

Functional homopolymers can be synthesized by essentially two different methods. The first and more preferred way is to use a functional initiator which will ensure a high rate of chain end functionality. For instance, the polymerization of St initiated by a unimolecular terpyridine-functionalized nitroxide initiator yields well-defined PS homopolymers. The second technique is based on post-polymerization modifications. In this case, the reaction between mPEG and chloroterpyridine yields terpyridine-functionalized PEG building blocks, as illustrated in Scheme 13. 

A cationic mono-Cp-based cobalt complex FlO-6 can initiate the living polymerization of ethylene, which has been successfully applied to the preparation of chain-end-functionalized polymers (see Section 11.20.4.5.4). " ... 

In recent years several attempts have been made to prepare polymers possessing chain end functions capable of giving rise to free radical or to cationic sltes i This research has been mostly aimed at extending the possibilities of synthesis of block copolymers, in which only one of the blocks is obtained anionlcally. The synthesis of -hydroperoxy polymers has already been mentioned. Peroxy-or peranhydride functions have also been introduced into polymer chainsSubsequent radical polymerization of a second monomer results in block copolymers. 

Chain end functionalization of reactive carbocationic monomers, like isobutyl vinyl ether, can occur using ionic nucleophilic quenching reagents, i.e., methanol, alkyl lithium, etc. (6). 

However, chain end functionalization does not occur when these reagents are added to living polymerization of less reactive monomers such as isobutylene (7). 

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