Dendronized macromonomers

Recent seminal work reported by Percec et al. [45-47] has shown that either spheroidal or cylindrically shaped dendronized polymers may be obtained by the polymerization of a dendronized macromonomer . The degree of polymerization of these monomers determines the ultimate shape which appears to demonstrate the quasi-equivalence of dendritic coats . 

A number of reviews on dendronized polymers and their synthesis by various methods including radical polymerization of dendron macromonomers have... 

The DB of branching can be modified by special synthetic approaches, as demonstrated by the NMR quantification of subunits. Copolymerization - for example, the addition of bifunctional monomers AB - resulted in an increase in linear units and, therefore, in a decrease of the DB [109-112]. An enhancement of DB was realized, for example, by employing a slow monomer addition technique [113], the polymerization of prefabricated dendron macromonomers [56, 114], and by a stepwise addition of the monomer mixture for the (A2 + B3) approach [92]. Whereas dendritic and terminal units are essential for a dendritic structure, in the case of hb polymers the content of the linear units can vary greatly. To date, few examples of AB2 hb polymerizations have been reported were the linear unit is a chemically labile structure that either breaks down to the initial educts, or reacts immediately with a further terminal unit to form the stabile dendritic unit. Thus, a hb polymer containing only T and D units is formed, with 100% DB [35,115-119]. 

It is important to note that, in most cases, the DP remained low and did not complement the previous studies on the SPG protocol referred to above [6]. A logical explanation for this discrepancy is as follows. Dendronized macromonomers for all the discussed polymers are relatively complex, requiring multistep synthetic procedures, and this limits both the amounts and the level of purity of the prepared macromonomers. As monomer purity and the scale of the reaction are two key factors (to ensure sufficient stoichiometry control) in SPG [5], it is reasonable to assume that those polymers with higher molar masses would be accessible if additional efforts had been undertaken to satisfy these stringent requirements. On the other hand, an effective conjugation length for such polymers is reached already at short chain lengths hence, it is understandable that further efforts towards molar mass improvement were not pursued. 

Figure 10 Synthesis of dendronized polymer brush (with fourth-generation dendrons) from a dendronized macromonomer and a comonomer via Suzuki polycondensation. ...

Boydston AJ, Holcombe TW, Unmh DA, Frechet JMJ, Gmbbs RH (2009) A direct route to cyclic organic nanostmctures via ring-expansion metathesis polymerization of a dendronized macromonomers. J Am Chem Soc 131 5388-5389... 

Andreopoulou AK, Kallitsis JK (2002) From terphenyi-dendronized macromonomers to aromatic-aliphatic poiyethers bearing two pendant dendrons per repeating unit. Macromoiecuies 35 5808... 

Scheme 5. Structural and chronological overview of the macromonomers used for synthesis of dendronized polymers chain growth (a) and step growth polymerizations (b)...

Other efforts based on the macromonomer approach to homopolymers having dendritic side chains, include the work of Draheim and Ritter on acrylate and methacrylate derived structures having dendritic chiral side chains based on L-aspartic esters [17a], and of Xi and coworkers with poly(methacrylate) structures containing very small benzyl ether dendritic side-chains [17b]. Unfortunately, both of these approaches met with limited success due to a significant drop in degree of polymerization (DP) when the size of the dendron used as pendant group in the macromonomers increased from G-l to G-2. 

The macromonomer route assures uniform distribution of dendrons along the polymer backbone. Acrylates and styrenes with pendant dendrons proved to be well suited for polymerisation [57]. However, steric hindrance between monomers bearing higher-generation dendrons and the spatial requirements of the end of the chain cause problems The monomer reacts with the chain end only in the case of slight, if any, steric hindrance. Thus the method only provides access to dendronised linear polymers with relatively low molar masses... 

These rod-like PAMAM dendrimers [83] are formally derived from the core functionalized aziridinyl macromonomer (dendron) A, just as Frechet et al. [104] have produced similar rod-like dendrimers by polymerization of the styrene functionalized monodendron (B) which was prepared via the convergent approach (see Fig. 21). 

The macromonomer approach refers to the polymerization of a monomer that is functionalized with a dendron. The first attempts to make these compounds via the convergent approach were reported by Hawker and Frechet and utilized a fourth genera-... 

The two convergent approaches toward dendronized polymers offer complementary control over structural parameters. The macromonomer route ensures well-defined connectivity between the dendron and the polymer, but affords polymers with broad dispersity and generally low degrees of polymerization. In the... 

Figure 20 Modes of grafting dendrimers or dendrons onto polymer backbones. Shown here are (A) the graft-to route, (B) the graft-front method, and (C) the macromonomer route. Source From Ref. 106.

Scheme 10.2 Synthetic scheme of dendronized homologous polymer brushes from macromonomers carrying G2 dendrons.

P23/25 (with a fourth generation of dendrons) from a low-molecular-weight monomer 23 and macromonomer 26, following a Suzuki polycondensation protocol. 

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