Polystyrene reactions with difunctional

A structure resembling that of the dumbbell polymers was made by Frechet et al. In this case the connector is linked with polyether dendritic groups [272, 273]. The synthetic approach involved the preparation of a difunctional polystyrene chain in THF using potassium naphthalenide as initiator. The living polymer was end-capped with 1,1-diphenylethylene (DPE) to reduce its nucle-ophilicity and avoid side reactions with benzylic halomethyl groups. Addition of the fourth generation dendrimer [G-4] -Br led to the final product (Scheme 100). 

In this context the experiments of Hocker et al. [96], Rempp et al. [97], Roovers et al. [98]. and other research groups [99] should be recalled. Those authors obtained high yields of cyclic polymers from preformed difunctional polymers (e.g., bisanionic polystyrene) by polycondensation with difunctional reaction partners at an IMC of 10 mol/L. The final concentration of chains in experiments (A) and (B) are by a factor 10 lower, and chain growth without any cyclization is absolutely unlikely In summary, the existence of the critical IMC is unproven, it is in contradiction to the results of Kricheldorf et al. [46, 47], it is in conflict with the calculations of Gordon et al. or Stepto et al. (see Chap. 7), and it is in conflict with the law of self-dilution combined with the RZDP ... 

A bimolecular process was reported by two independent groups, i.e., Hocker [19] and Rempp [20] in 1980. Macrocyclic polymers have been successfully prepared by the coupling reaction of a two-ended living polystyryl anion with a difunctional electrophile such as a, a -dibromo-p-xylcnc under high dilution to yield cyclic and linear mixtures. The cyclic polymer was isolated by a fractional precipitation. This bimolecular end-to-end reaction process has been used for synthesizing cyclic polystyrene by many researchers [21-25]. 

Polymeric supports can also be used with advantage to form monofunctional moieties from difunctional (Hies. Leznoff has used this principal in the synthesis of sex attractants on polymer supports (67). Starting from a sheap symmetrical diol he blocked one hydroxyl group by the polymer. Functionalization of cross-linked polymers is mostly performed by chloromethylation (65). A very promising method to introduce functional groups into crosslinked styrene-divinylbenzene copolymers is the direct lithiation with butyllithium in presence of N,N,N, N -tetramethyl-ethylenediamine (TMEDA) (69, 70). Metalation of linear polystyrene with butyl-lithium/TMEDA showed no exchange of benzylic hydrogen and a ratio of attack at m/p-position of 2 1 (71). In the model reaction of cumene with amylsodium, a kinetic control of the reaction path is established. After 3h of treatment with amyl-sodiuni, cumene is metalated 42% in a-, 39% m-, and 19% p-position. After 20h the mixture equilibrates to affort 100% of the thermodynamically more stable a-prod-uct (72). 

The simplest case of a polymer molecule which can undergo either cyclization or chain extension is that in which the groups are situated at each end of the polymer chain (which can react together). An example of this type of polymer is a,hydroxy-terminated polystyrene which reacts with a difunctional isocyanate, and our first studies were carried out with this reaction system. Obviously, the chain extension reaction leads to an increased specific viscosity, and cyclization leads to the reverse. 

Molecules of a difiinctional monomer such as X-R-Y (where X and Y are mutually reactive functional groups) are polymerized via the elimination of small molecule by-products of XY, a linear condensation polymer with a general formula of X-(-R-)w-Y is formed, whereas when molecules of an olefinic monomer such as H2C=CHR are polymerized, a vinyl polymer with a structure of -[-H2C-CH(R)-]w- is produced. The difunctional and olefinic monomers have been the major monomer sources, and their step and chain polymerization reactions have been the main synthetic routes to the conventional polymers such as polyester and polystyrene, respectively [27]. 

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