Dendronized monomers

Table 1. Dendronized monomers, polymerization conditions, as well as molecular weights of the polymers obtained. Molecular weights are not comparable (see text). (AIBN = azoisobuty-ronitrile, DBPO = dibenzoyl peroxide, BPB = ferf-butyl perbenzoate)... 

Polymerization hybrids are obtained by the polymerization of a linear chain initiated from the focal point of a convergent dendrimer or the chain ends of a dendrimer, or by polymerization or copolymerization of a dendronized monomer. 

Another type of architecture featuring a linear main chain surrounded by dendritic side-chains has emerged over the last decade [4], The highly descriptive term dendronized , coined by Schliiter [4] aptly describes this novel type of macromolecular architecture. Though three separate routes can be used to prepare such dendronized hybrids (Figure 7.8), the most successful approach to date has generally involved the polymerization of dendronized monomers. 

Controlling the size, shape and ordering of synthetic organic materials at the macromolecular and supramolecular levels is an important objective in chemistry. Such control may be used to improve specific advanced material properties. Initial efforts to control dendrimer shapes involved the use of appropriately shaped core templates upon which to amplify dendritic shells to produce either dendrimer spheroids or cylinders (rods). The first examples of covalent dendrimer rods were reported by Tomalia et al. [43] and Schluter et al. [44], These examples involved the reiterative growth of dendritic shells around a preformed linear polymeric backbone or the polymerization of a dendronized monomer to produce cylinders possessing substantial aspect ratios (i.e. 15-100) as observed by TEM and AFM. These architectural copolymers consisting of linear random... 

Figure 15.3 Various strategies leading to dendronized or hyperbranched glycopolymers. (A) grafting of dendronsto pre-formed polymers (B) polymerization of dendron monomers (C) grafting dendrimers on pre-formed polymers...

Dendrimers usually exhibit spherical (isotropic) shape. However, wedge-like dendrimer fragments ( dendrons ) that have been attached to linear polymers as side groups can be used to create anisotropic nanocylinders , leading to uncoiling and extension of the polymer chains. Synthetic macromolecules of this type can be visualized directly on surfaces and their contour length determined from the images. Unexpected acceleration effects in the self-encapsulated polymerization of dendron monomers used to prepare such polymers as well as the structural consequences of dendritic pieces of cake on linear polymer chains are discussed. 

Figure 1. Examples of dendron monomers described by Percec et al., Schliiter et al. and XI et al.

Figure 2. Dependence of self-as.sembly of dendronized polymer chains on the degree of polymerization monomers and short polymer chains assemble in spheres longer chains assemble in cylinders. This effect leads to rapid polymerization of such dendron monomers due to self-encapsulation [21],...

While the PDIs of the REMP macrocycles are higher than those typically reported for the ring-closure approach, the molecular weights obtained are by far the highest reported to date for a cyclic polymer synthesis. The catalyst design optimization has yielded improved polymerization catalysts, and promises to broaden the versatility of this route. For example, by using a dendronized monomer, Grubbs and coworkers have demonstrated the synthesis of dendronized cycHc polymers and confirmed their cyclic structure with AFM [59], and have also synthesized cycHc polymers with threaded rotaxanes [60]. 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

The authors refer to compound 9 as a monomer with tapered side chains and have only recently begun to use dendrimer terminology. Since the side chain meets the criteria for a G-l dendron, compound 9 is considered here. 

Cores Monomers Branch Cells Dendrons Dendrimers... 

It is apparent that both the core multiplicity (Nc) and branch cell multiplicity (Nh) determine the precise number of terminal groups (Z) and mass amplification as a function of generation (G). One may view those generation sequences as quantized polymerization events. The assembly of reactive monomers [48, 78], branch cells [48, 83, 89] or dendrons [85, 90] around atomic or molecular cores... 

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