Side chains, dendritic

When a linear polymer is grafted with a large number of much shorter side chains, cylindrical polymer brushes are formed [33, 108-111]. They are also denoted as bottlebrushes or molecular brushes. Although most cylindrical polymer brushes contain linear side chains, dendritic or even hyperbranched space demanding grafts can also render cylindrical shapes, which leads to the so-called dendronized [112-116] andhypergrafted[117, 118] polymers, respectively. In this review, we will focus on cylindrical brushes with linear side chains. Due to their anisotropic nature in topology, they have attracted more and more research interest in their synthesis, bulk, or solution properties, as well as applications. 

A GGS model was used to treat side-chain dendritic polymers [78]. The model is given in Fig. 21 it consists of chains bearing dendritic wedges (CBDW) in their middles. A dendritic wedge (DW) differs from a conventional classical dendrimer in that its core has one main branch less /c = (/ - 1). For such DW the results of the previous section hold [74,78]. In particular, the maximal relaxation time of a DW is the same as that of a classical dendrimer, because this time corresponds to the motion of two main branches against each other. 

This theoretical result is in quahtative agreement with rheological data obtained for side-chain dendritic polymers that consist of a polymethane main chain and of polyether wedges of second, third, and fourth generations [201]. 

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. 

The concept of making brush-type polymers in which a linear polymer is funtionalized with dendritic side-chains was first suggested by Tomalia in a 1987 patent, though actual experimental work on his approach was only reported recently recently [15]. Hawker and Frechet were first to document the preparation of a vinyl copolymer containing a few pendant Frechet-type dendrons (Figure 7.9). 

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. 

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

The affinity of Cgo towards carbon nucleophiles has been used to synthesize polymer-bound Cgo [120] as well as surface-bound Cjq [121]. Polymers involving G q [54, 68, 69] are of considerable interest as (1) the fullerene properties can be combined with those of specific polymers, (2) suitable fullerene polymers should be spin-coatable, solvent-castable or melt-extrudable and (3) fullerene-containing polymers as well as surface-bound Cgo layers are expected to have remarkable electronic, magnetic, mechanical, optical or catalytic properties [54]. Some prototypes of polymers or solids containing the covalently bound Cjq moiety are possible (Figure 3.11) [68,122] fullerene pendant systems la with Cjq on the side chain of a polymer (on-chain type or charm bracelet ) [123] or on the surface of a solid Ib [121], in-chain polymers II with the fullerene as a part of the main chain ( pearl necklace ) [123], dendritic systems III, starburst or cross-link type IV or end-chain type polymers V that are terminated by a fullerene unit For III and IV, one-, two-and three-dimensional variants can be considered. In addition, combinations of all of these types are possible. 

Dendronized polymers are a class of polymers produced by the combination of linear polymers and dendritic molecules as side chain pendant moieties [67-69],... 

When dendritic fragments are attached to polymer chains, the conformation of the polymer chain is strongly affected by the size and chemical structure of the dendritic wedges attached. Dense attachment of dendritic side chain converts a linear polymer into a cylindrically shaped, rigid and nanoscopic dimension. Frechet and Flawker [70] were one of the first to recognize these hybrid architectures . 

The self-assembly of the various polymer systems described in the above sections is only a brief summary of the attempts by chemists to create multifunctional materials based on noncovalent interactions. The unique regions present within a polymer including the (1) main-chain/backbone, (2) end groups, (3) side-chains and (4) dendritic periphery, along with the ability to functionalize any of these regions with recognition units, provide chemists with a wide array of self-assembly possibilities with which to build and create multifunctional materials. 

The second impetus to consider monomers with dendritic side groups comes from the work of Percec et al. on monomers with tapered side chains, e.g., polymerization of 3,4,5-tris (4 -dodecyloxybenzyloxy) benzoic acid ethylene glycol (n=l,2,3,4) methacrylates... 

Norbornene-based and oxa-norbornene-based monomers bearing dendritic side chains, XXX and XXXI (Fig. 19), were synthesized and polymerized via ROMP with initiator 6 [83]. Based on size exclusion chromatography data, the polymerization shows hving-like character up to DP=70. H- and C-NMR-spectroscopy revealed 35% cis and 65% tram sequences. These polymers displayed enantiotropic nematic and smectic mesophases, except for DP=5. In contrast to other classes of SCLCPs, the dependence of the DP on the transition temperatirre of the polymer was very weak. Glass transition and isotropization temperatures became independent of molecular weight above a degree of polymerization of about 10. 

The bulkiness of dendritic side chains can force the polymer backbone to adopt special geometries [84]. The overall contour of the polymer can then be spherical or rod-hke so that the polymers adopt Uquid crystalUne phases. Even when the driving force of this behavior is caused by the bulkiness of the side chain, these polymers show MCLCP-Uke behavior. 

Fig. 20 Monomers XXXII-XXXVI, featuring dendritic side chains...

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