Branched-monomer approach

Frechet et al. 8fi reported a branched-monomer approach to the convergent synthesis of dendritic macromolecules this approach permits an accelerated growth by the replacement of the simplest repeat unit with a larger repeat unit of the next generation. In essence, the traditional AB2 monomer is replaced with an AB4 unit. Scheme 5.28 depicts the transformation of the AB4 unit (115) to the trimethylsilyl protected tetraester 116... 

Scheme 5.28. The branched-monomer approach leads to dendrimers possessing differentiated generation connectivity.

Frdchet et al. reported the synthesis of a unimolecular micelle (Fig. 6) generated by a convergent approach starting with 3,5-dihydroxybenzyl alcohol, as the key branched monomer unit [65]. Fourth generation dendrimer 15 with 32 carboxylic acid moieties on the periphery was synthesized and its correspond-... 

The branched molecules approach to create interactions between chro-mophores and enhance crppA values has been widely used. Different systems have been designed we will report some of them and the results for TPA response optimization. The factor F n), defined in Eq. 9 as the ratio between the TPA cross-section cr.. p of the branched molecule with n branches of monomers and TPA cross-section of the monomer, will be used to draw conclusions about interactions within the branched molecule F( ) = n will be ascribed to independent branches, while P n) n will correspond to deleterious (F( ) < n) or constructive (F( ) > n) interactions between branches. 

The bifiinctional monomer approach, with continuous addition of the monomer and the branching agent, is analogous to hyperbranched polymer syntheses using the inimer technology, but the MWD obtained are significantly narrower theD values reported vary from 1.2 to 2.0, albeit the molecular weight is also limited to circa 10 g/mol [117, 118]. 

An alternative approach is the so-called hypergrafting that relies on the use of a linear block copolymer with a poly-ftmctional (usually relatively short) second block that acts as an initiator in the grafting polymerization of the branching monomer even for a step-growth mechanism. In this case the polyfimctionality of the initiator permits control over molecular weights and polydispersity and suppresses homopolymetization. 

In 1989, Fr chet and co-workers first reported the convergent growth approach [8,9]. In contrast to the divergent growth approach, dendrimer construction is initiated at what will eventually become the outer surface shell of the ideally branched macromolecule and proceeds inward, by a stepwise addition of branching monomers, followed by the final attachment of each branched dendritic sub-unit (or dendron ) to a poly-functional core. This synthesis generated a poly(aromatic ether) dendrimer and a repetitive sequence of Williamson ether coupling and bromination reactions were employed as shown in Scheme 8.1. 

Another approach to polyesters having only few branching points consist of the copolycondensation of a2 -I- b2 monomers with small amounts of a3 or b3 monomers. In addition to trimesic acid or its trimethylester, the multifunctional alcohols outlined in Pormula 10.2 served as branching monomers. This approach was intensively studied by several research groups to modify the physical and mechanical properties of commercial polyesters such as PET, PBT, or poly (butylene succinate). A detailed discussion of syntheses, properties, and applications presented in the excellent review of Long et. al [64]. 

Dendrimers produced by divergent or convergent methods are nearly perfectly branched with great structural precision. However, the multistep synthesis of dendrimers can be expensive and time consuming. The treelike structure of dendrimers can be approached through a one-step synthetic methodology.31 The step-growth polymerization of ABx-type monomers, particularly AB2, results in a randomly branched macromolecule referred to as hyperbranch polymers. 

From these approaches, DBnmr=0.42 and DBtheo=0-49 can be obtained at y=l.l (h=0.62), respectively. Note that these values represent a rough estimate, as they are calculated based on the assumption of equal rate constants for copolymerization. For low y values (y=0.5),the DB (DBnmr=0-48) even exceeds the value for poly(inimer 1) (DBnmr=0-43) obtained by a homo-SCVP. This is an accordance with theoretical prediction that a maximum of DB=0.5 is reached at y=0.6 [73]. The effect can be explained by the addition of monomer molecules to in-chain active centers (i.e., in linear segments), leading to very short branches. For 2.5>y>0.5, DBnmr decreases with y, as predicted by calculations. 

The OEt-substituted Zr(IV)-boratabenzene complex has been employed in an interesting dual-catalyst approach to the synthesis of branched polyethylene.47 Capitalizing on the ability of this boratabenzene complex to generate 1-alkenes (Scheme 25) and the ability of the titanium complex illustrated in Scheme 27 to copolymerize ethylene and 1-alkenes, with a two-catalyst system one can produce branched polyethlene using ethylene as the only monomer (Scheme 27). The structure and properties of the branched polyethylene can be altered by adjusting the reaction conditions. 

In Fig. 48, the regions of the formation of linear or branched polymers, microgels and macrogels are shown as a function of the concentration of 1,4-DVB and of n-BuLi. Reactive microgels can be obtained at a monomer concentration below 50 g/1 and between 3 and 16 mol % of n-BuLi. The polymer structure approaches that of a macrogel when the concentration of 1,4-DVB or n-BuLi is increased. 

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