Telechelic associative polymers

Fig. 10.15 Comparison of the phase diagrams of telechelic associating polymers (a) random hydration (cr = 1.0) for telechelic PEO, (b) cooperative hydration (a =0.3) for telechelic PNIPAM. Spinodal lines (solid lines) and sol-gel transition hnes (broken lines) are shown. The various curves correspond to polymers of different molecular weights. Other parameters are fixed at the values obtained from the single-chain study. (Reprinted with permission from Ref. [38].)...

Telechelic associative polymers have received considerable attention because of their wide range of applications as rheological modifiers of aqueous systems, either simply in water or together with other amphipiles. Up to now almost all investigations have been restricted to doubly end-capped hydrophilic polymers, i.e., bifunctional briding polymers. 

Recent theoretical efforts to address the phase behavior of associating polymer blends have involved self-consistent field theory (SCFT) and lattice cluster theory (LCT). The SCFT framework was applied to enumerate all possible linear reaction products for blends of self-complementary and heterocomplementary telechelics. Mesophase regions were identified. Dudowicz and Freed reformulated the LCT to model solvent-telechelic polymer blends and identified several trends, including an enhancement of miscibility as the molar mass of the associating polymer was increased [92, 93]. This trend was explained by unbalanced entropy and enthalpy changes that occur with increasing chain length. 

Dudowicz J, Freed KF (2012) Lattice cluster theory of associating polymers. I. Solutions of linear telechelic polymer chains. J Chem Phys 136(6) 064902... 

Figure 4.2 Structures of telechelic PCOE polymers with UPy or DAN end groups and their self-association/complementary hydrogen bonding.

Various dynamic processes occurring in solutions of associative polymers explain to a large extent their rheological behavior. Most studies have been performed using rheological methods (see Chapter 2, Section VI, and Chapter 9) and NMR (see Chapter 2, Section IV). Results for telechelic and for comblike associative pol5nners are examined successively. 

Jones, B. A. Torkelson, J. M. Large melting point depression of 2-3-nm length-scale nanocrystals formed by the self-assembly of an associative polymer Telechelic, pyrene-labeled poly(dimethylsiloxane). J. Polym. ScL, Part B Polym. Phys. 2004,42, 3470-3475. 

For comparison, a telechelic sulfonated polystyrene with a functionality f = 1.95 was prepared. In cyclohexane the material forms a gel independent of the concentration. At high concentrations the sample swells. When lower concentrations were prepared, separation to a gel and sol phase was observed. Thus, dilution in cyclohexane does not result in dissolution of the gel even at elevated temperatures. Given the high equilibrium constant determined for the association of the mono functional sample, the amount of polymer in the sol phase can be neglected. Hence, the volume fraction of polymer in the gel phase can be calculated from the volume ratio of the sol and gel phases and the total polymer concentration. The plot in Figure 9 shows that the polymer volume fraction in the gel is constant over a wide range of concentrations. 

Reed 332) has reported that reaction of ethylene oxide with the a,(a-dilithiumpoly-butadiene in predominantly hydrocarbon media (some residual ether from the dilithium initiator preparation was present) produced telechelic polybutadienes with hydroxyl functionalities (determined by infrared spectroscopy) of 2.0 + 0.1 in most cases. A recent report by Morton, et al.146) confirms the efficiency of the ethylene oxide termination reaction for a,ta-dilithiumpolyisoprene functionalities of 1.99, 1.92 and 2.0j were reported (determined by titration using Method B of ASTM method E222-66). It should be noted, however, that term of a, co-dilithium-polymers with ethylene oxide resulted in gel formation which required 1-4 days for completion. In general, epoxides are not polymerized by lithium bases 333,334), presumably because of the unreactivity of the strongly associated lithium alkoxides641 which are formed. With counter ions such as sodium or potassium, reaction of the polymeric anions with ethylene oxide will effect polymerization to form block copolymers (Eq. (80) 334 336>). 

Binder et al. [92,93] have reported on the formation of poly(etherketone) poly(isobutylene) networks formed by the respective endgroup-modified telechelics. The relevant interactions investigated relied on the 2,6-diamino-1,3,5-triazine/thymine and the much weaker cytosine/2,6-diamino-1,3,5-triazine-modified polymers (Fig. 18). In addition to the pure hydrogenbonding interaction, phase-separation energies resulting from the strongly microphase separating PEK- and PIB polymers were expected. The association behavior was followed in solution via NMR-association experiments. 

Figure 5.16 Model for associations of telechelic polymers as a function of increasing concentration. For strong associations, isolated flower micelles form just above the critical micelle concentration (CMC), which is often around 2 to 10 ppm (Winnick and Yekta 1997). At higher concentrations, the flowers are expected to be connected by bridges. (From Winnik and Yekta 1997, with permission from Current Chemistry Ltd.) 1997 Current Opinion in Colloid -i- Interface Science.

Figure 5.17 (a) Illustration of types of chain association in telechelic polymers. (b) Chain architectures that can form in solution micelles that have a network functionality greater than two are shown in black. (From Annable et al. 1993, with permission from the Journal of Rheology.)... 

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... 

In theory, it is possible to form block, graft, or crosslinked copolymers by ionic associations. However, in practice, telechelic polymers with ionic functionality at the chain ends are uncommon. Therefore, the majority of reported examples involve crosslinked copolymer formation between two immiscible polymers bearing pendent ionic groups. 

In this review, the term macromer is used to describe oligomer or polymer precursors that undergo reversible association to form supramolecular polymers or networks. Macromer synthesis, although a crucial aspect of supramolecular science, is also out of the scope of this review. Several comprehensive reviews of the synthesis of H-bonding polymers are available [10, 11,42] and primarily describe the application of controlled radical polymerization techniques, including atom-transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP). For synthesis of telechelic polymers, avoiding monofunctional impurities that can act as chain stoppers is crucially important [43],... 

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