Difunctional oligomers

AB and difunctional oligomers leading to ABA types of structures. The molecular weight control of the growing segments is achieved by the ratio of the cyclic monomer to the oligomeric initiator in the original reaction mixture. 

Telechelic (or a, co-difunctional) oligomers exhibit functional groups at both ends of the oligomeric backbone. These compounds are quite useful precursors for well defined architectured polymers (e.g. polycondensates, polyadducts and other fluoropolymers previously reviewed [400, 401]). As mentioned above, a... 

ADMET depolymerizations with substituted alkenes have been done as well, thereby generating perfectly difunctional telechelic molecules. As an example, 1,4-polybutadiene has been depolymerized in an inert atmosphere with a 10-fold molar excess (based on the repeat unit) of either allyltrimethylsilane or allylchlorodimethylsilane (equation 23). In these examples, the chemistry can be driven to complete depolymerization to yield structures with either one, two, three, or four repeat units of 1,4-butadiene. The synthesis of perfectly difunctional oligomers by this chemistry offers significant opportunity, particularly for functional groups such as alcohols, esters, carboxylic acids, and amines. 

Scheme 9 Synthesis of a, >-difunctional oligomers through addition-fragmentation processes. CTA chain transfer agent...

The synthesis of a,co-difunctional telomers by cross-metathesis between cyclic olefins and acyclic functionalized olefins is possible in the presence of functional-group-tolerant catalysts. Thus, cross-metathesis of dimethyl hex-3-enedioate with cycloocta-1,5-diene (Pinazzi 1980 Reyx 1982b, 1990), cyclopentene (Reyx 1986, 1987), or norbomene (Cramail 1991a) in the presence of the catalyst system WCV Mc4Sn allows the synthesis of a,m-difunctional oligomers, e.g. reaction (2). 

The polycondensation of difunctional oligomers leads to the preparation of well-defined polymer structures. Monomers in this type of reactions must be soluble in the reaction mixture and stable when the reaction is carried out in the melt, which is the case for some aromatic polymers prepared by polycondensation [22]. As previously described, polycondensation can occur with monomers bearing the same or a different functional group at both ends of the molecule. When one of the reactive functional groups is a hydroxyl moiety, several types of materials can be prepared, such as polyethers, polyesters, and polyurethanes, independently if they are used to form homopolymers, copolymers, or hyperbranched polymers. 

Block copolymers can be prepared by polycondensation of difunctional oligomers (which are previously prepared... 

Chem. Descrip. Aliphatic urethane acrylate resin/SR 454 (ethoxylated trimethylolpropane triacrylate) blend, < 400 ppm MEHQ inhibitor Uses Urethane-acrylic for adhesives, coatings (metal, paper, PVC floor, wood), electronics, inks, photopolymers Features High performance difunctional oligomer hard, nonyellowing Properties APHA 100 clear liq. sp.gr. 1.104 vise. 2300 cps tens. str. 9000 psi tens, elong. 31% 99% reactive esters 75% oligomer, 25% EOTMPTA... 

Selected examples of each type of functional oligomers are shown below. In the case of monofunctional oligomers, the functional group can be derived from the chain transfer agent, e.g. by using allylic sulfides with one functional residue. Difunctional oligomers can be obtained by the addition-elimination mechanism, e.g. from allylic sulfides with 2 functional residues or from styryl ethers.23... 

Another method of forming polydialkyldisiloxanes is by reacting difunctional oligomers with cyclic oiganisilicon compounds. ... 

Separation of mono(-OH) and difunctional oligomers and cyclic products. 

Copolymers referred to as segmented are copolymers with short and numerous blocks. They are generally obtained by a chain extension process from difunctional oligomers. 

Symbols such as HO[POE]OH indicate an a,o>-difunctional oligomer (see Section 11.1.1 for comment on the nomenclature and designation of such substances). 

The synthesis of block copolymers follows two essential pathways (i) a difunctional oligomer initiates the formation of two or more other blocks (Scheme 1) and (ii) two or several different difunctional oligomers react together or with a coupling agent (see Chapter 2) sometimes, the second block can be prepared in the presence of the first one. 

Block copolymers can be prepared either by polycondensing a difunctional oligomer, which is often the soft block, with the precursors of the hard block, or by polycondensing or coupling two, or more, difunctional oligomers this aspect is discussed in Chapter 2. 

Some difunctional oligomers are prepared by polycondensation and then used in chain polymerization. However, most of them are applied in block polycondensation or polyaddition Pan et al. [62] prepared difunctional oligosiloxanes by polycondensation of dimethyldichlorosilane with different oligomeric diols, and the polycondensates were then reacted with diisocyanates. Yokozawa et al. [63] prepared a well defined poly(p-benzamide), with a low dispersity index, using a new polycondensation process, which is discussed in Section 6. 

The contribution of the difunctional oligomers to the preparation of TPEs should enjoy an important development their use is not limited to classical chain polymerization and polycondensation since they can be also applied to less common processes, such as the metathesis. A functional diene, for instance an a,ua-divinyl aliphatic or aromatic ester, is polyadditioned with an a,u)-divinyl-(soft-oligomer) in the presence of a metathesis catalyst, e.g., a ruthenium derivative [64,65]. 

The determination of accurate molecular mass values of the copolymers and their distribution, as well as of the functionality of difunctional oligomers remains a difficult problem. It is necessary to compare the results of, at least, two different techniques, for instance SEC and titration of the end-groups but, unfortunately, the latter is inapplicable when the chain length is high. NMR is probably one of the most efficient tools in the determination of the end-group... 

Madec P J and Mar5chal E (1994) Block copolymers by polycondensation of ot,ijo-difunctional-oligomers, Prace Naujowe Polytechniki Szczecinskiej, 514 11-19 (CA 123 56784). 

Block copolymers can be prepared by polycondensation of several a,o -di-functional oligomers or by polycondensation of o ,(j-difunctional oligomers with the precursors of the other blocks. They can be also prepared by the coupling of different difunctional oligomers. Most often, the number of the different oligomers does not exceed 2. 

The direct polycondensation of a,ca-difunctional oligomers leads to well defined block copolymers. The oligomers must be soluble in the reaction mixture or molten and stable when the reaction is carried out in the melt, which is rarely the case of aromatic polymers. Moreover, the direct polycondensation requires that the functionality of the oligomers is exactly 2. 

S, Poly condensation of an a,oj-difunctional oligomer with the precursors of another block... 

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