Copolymers from Telechelic Monomers

Telechelic compounds are oligomers or low-molecular-weight polymers carrying monofunctional terminal groups or reactive terminal groups, respectively, on both chain ends. Block sulfone copolymers have been synthesized from hydroxy-telechelic sulfonated PESs and fluorotelechelic PESs. As a monomer for the sulfonated hydroxy-telechelic compound, 3,3 -sulf-onyl bis-(6-hydroxybenzene sulfonic acid) disodium salt is used. This compound is synthesized from bis-(4-hydrox5 henyl)-sulfone by sulfona-tion with concentrated sulfuric acid and subsequent neutralization. 

The momomers can be chain extended with bis-(4-fluorophenyl)sulf-one or bis-(4-hydroxyphenyl)-sulfone, respectively in a next condensation step. Eventually, in a final condensation step, block copolymers containing blocks of unsulfonated aromatic polyether sulfones and blocks of aromatic polyether sulfones sulfonated on the aromatic rings are obtained. The block copolymers provide compounds with both an adjustable degree of sulfonation and a defined length of sulfonated and unsulfonated blocks. The materials are suitable for the preparation of synthetic membranes. 

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A series of at least 14 papers [200-208] have been published dealing with the synthesis of telechelic polymers or block copolymers from the radical polymerization of various vinyl monomers with substituted 1,1,2,2-tetraphenyl ethanes. These aromatic compounds, known for over a century [209], are efficient in radical polymerization [201,210], They behave as both initiators and terminating agents [200] that can be involved in living radical polymerization as illustrated in the following reaction ... 

Aqueous miniemulsion polymerization of styrene was performed in the presence of CeFis-I as CTA, yielding particles with a good control of the molecular weights, in contrast to emulsion polymerization where the transfer agent efficiency was low due to a slow diffusion of the hydrophobic perfluor-ohexyl iodide from the monomer droplets to the active particles during polymerization.The chains were capped with iodine as evidenced by the successful chain extension upon addition of butyl acrylate. The miniemulsion process was also successfully applied to the preparation of triblock copolymers PS-b-PDMS-b-PS starting from a telechelic diiodo-poly(dimethylsiloxane) macrotransfer agent. A somewhat similar procedure was used to prepare PVAc-b-PDMS- -PVAc triblock copolymers, but the polymerization was performed under UV irradiation (instead of thermal initiation) and in the absence of radical initiator. In this case, the aqueous dispersion medium was a key parameter to achieve a controlled polymerization (Scheme 19). ... 

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

The transformation of the chain end active center from one type to another is usually achieved through the successful and efficient end-functionalization reaction of the polymer chain. This end-functionalized polymer can be considered as a macroinitiator capable of initiating the polymerization of another monomer by a different synthetic method. Using a semitelechelic macroinitiator an AB block copolymer is obtained, while with a telechelic macroinitiator an ABA triblock copolymer is provided. The key step of this methodology relies on the success of the transformation reaction. The functionalization process must be 100% efficient, since the presence of unfunctionalized chains leads to a mixture of the desired block copolymer and the unfunctionalized homopolymer. In such a case, control over the molecular characteristics cannot be obtained and an additional purification step is needed. 

Two separate topics must be considered the block copolymers produced from bistelomerization of two different monomers with a telechelic or a difun-ctionalizable telogen and the coupling of monofunctional telomers. 

In Table 10 we have gathered different 1,2-disubstituted tetraphenylethanes reported in the literature to get telechelic polymers. We can remark that few studies were undertaken in the area of telechelic polymers hence, despite a one-step reaction to get a telechelic structure, the main interest attributed to initer systems concerns the ability to restart a block copolymerization. The number of publications concerning the synthesis of diblock copolymers may prove this assumption. Under certain polymerization conditions, the chain ends, comprising the last monomer unit and the primary radical formed from the intiator, may split up into new radicals able to reinitiate further polymerization of a second monomer, leading to block copolymers. This is certainly the reason why 1,2-disubstituted tetraphenylethane does not present such interesting condensable functions (X in Scheme 10) for polycondensation reactions (Table 10). 

Appropriate telechelic polymers produced from a variety of monomers may be incorporated as reactive components in a variety of applications such as sealants, elastomers, foams, and fibers. Such telechelic polymers may impart almost any desired characteristic such as hydrophilic properties, elastomeric properties, dyeability, and solvent resistance. A dihydroxy telechelic polymer may be reacted with a telechelic polymer containing two carboxylic acid groups to produce a condensed polyester. Accordingly, from these telechelic entities one may produce block copolymers that will have alternating addition polymer residues of like or unlike repeat units. In addition, block copolymers can be produced by modifications of this procedure in which addition polymers are alternated with condensation polymer units. 

Terminal ring functionalization has been explored as a route to block copolymer synthesis. The Stille coupling approach affords Br and RsSn termini on an isolable product. Iraqi and Barker isolated a low-A/ fraction (DP 14) then homo-cross-coupled to increase Af (DP ra 28), thus demonstrating telechelic utility [43]. The nature of the nickel-initiated cross-coupling polymerization (GRIM method) allows chain extension from the quasi-living end until the reaction is terminated. Controlled blocks of HT-PHT with HT-PDDT were prepared by changing the monomer [63]. 

Cyclic Amines. The three-membered cyclic amines and the four-membered cyclic amines can be polymerized only by cationic mechanism (324). Monofunctional initiators, such as methyl triflate (325), produce monofiinctional telechelics from f-butylaziridine (TEA). Addition of the monomer to a bifunctional living PTHF solution (326) gives bifimctional poly(TBA). This is a method of making ABA block copolymers. The aziridinium end groups react with acrylic acids to form the corresponding esters (327). 

Clearly, supramolecular materials based on telechelics combine many of the mechanical properties of conventional macromolecules with the low melt viscosity of low molecular weight organic compounds. The reversibility of supramolecular polymers adds new aspects to many of the principles that are known from condensation polymerizations. For example, a mixture of different supramolecular monomers will yield copolymers, but it is extremely simple to adjust the copolymer composition instantaneously by adding an additional monomer. Moreover, the use of unimers with a functionality of three or more, will give rise to network formation. However, in contrast to condensation networks, the self-healing supramolecular network can reassemble to form the thermodynamically most favorable state, thus forming denser networks (Figure 6). 

F-NMR spectra were obtained to investigate random and block structures (Figure 10.3) [17], The 3F CF3- peaks in both block and random copolymers shift to low field relative to 3F homopolymer due to a solvent effect. Homo- and block-telechelics show a series of well-resolved peaks with 71H-19F = 8 Hz. In contrast, the random copolymer peak is broad with little resolvable structure supporting the notion that the random-copolyoxetane is comprised of random sequences with many sequence distributions. In contrast, the block copolyoxetane contains (3F) sequences paralleling those in the homotelechelic. Hence, P[3F] and P[3F- -ME3] have similar F-NMR spectra. Copolymers prepared from 3F and ME2 monomers were also block or random depending on order of addition [16],... 

The most intensively studied disubstituted monomer, a,a-dimethyl-) -propiolactone (pivalo-lactone) (PVL), has been converted into several telechelics and used to prepare block copolymers.An interesting example of an assymetric telechelic from PVL was reported by Wilson and Beaman, in which cyclic tertiary amine initiators yielded telechelics having a-cycloammonium and o>-carboxylate end groups. Lenz et al prepared telechelics of a-methyl-a-butyl-jS-propiolactone (MBPL) with a diammonium dicarboxylate initiator (tetrabutylammonium sebacate), which were then used to polymerize PVL forming ABA triblock copolymers. ... 

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