Comb polymer architecture

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

Freed et al. [42,43], among others [44,45] have performed RG perturbation calculations of conformational properties of star chains. The results are mainly valid for low functionality stars. A general conclusion of these calculations is that the EV dependence of the mean size can be expressed as the contribution of two terms. One of them contains much of the chain length dependence but does not depend on the polymer architecture. The other term changes with different architectures but varies weakly with EV. Kosmas et al. [5] have also performed similar perturbation calculations for combs with branching points of different functionalities (that they denoted as brushes). Ohno and Binder [46] also employed RG calculations to evaluate the form of the bead density and center-to-end distance distribution of stars in the bulk and adsorbed in a surface. These calculations are consistent with their scaling theory [27]. 

The recognition of the two fundamental mechanisms of reptation and arm fluctuation for linear and branched entangled polymers respectively allows theoretical treatment of the hnear rheology and dynamics of more complex polymers. The essential tool is the renormahsation of the dynamics on a hierarchy of timescales, as for the case of star polymers. It is important to stress that experimental checks on well-controlled architectures of higher complexity are still very few due to the difficulty of synthesis, but the case of comb-polymers is an example where good data exists [7]. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

Although not extensively studied, various architectures such as star and comb polymers have also been synthesized [Bielawski et al., 2000 Goethals et al., 2000]. 

The molecular architecture of a polyphosphazene has a profound influence on properties. For example, linear and tri-star trifluoroethoxy-substituted polymers with the same molecular weight (1.2 x 104 or higher) have strikingly different properties.138 The linear polymers are white, fibrous materials that readily form films and fibers, whereas the tri-arm star polymers are viscous gums. One is crystalline and the other is amorphous. Cyclolinear polymers are usually soluble and flexible. Cyclomatrix polymers are insoluble and rigid. Linear polymers can be crystalline, but graft or comb polymers are usually amorphous. 

Tin alkoxides, on the other hand, are less sensitive to hydrolysis and can be used for controlled ROP and the synthesis of macromolecules with advanced architecture (tri-block, star, or comb polymers). Cyclic tin alkoxides offer a convenient pathway for tri-block copolymerization. 

There are many polymer architectures beyond chains such as stars, combs, and brushes. An example of a star-type oligophenylene is structure 100.276 It can be described as possessing three oli-... 

The rheological properties of polymers with more complicated branch architecture have been investigated, but unfortunately such polymers are seldom of the monodisperse variety as can be found in star and linear polymers. Work on comb polymers [37-39] has shown in general that... 

The main feature of polymers is their MMD, which is well known and understood today. However, several other properties in which the breadth of distribution are important and influence polymer behavior (see Figure 1) include physical, the classical chain-length distribution chemical, two or more comonomers are incorporated in different fractions topological, polymer architecture may differ (e.g., linear, branched, grafted, cyclic, star or comb-like, and dendritic) structural, comonomer placement may be random, block, alternating, and so on and functional, distribution of chain functions (e.g., all chain ends or only some carry specific groups). Other properties the polymers may disperse (tacticity and crystallite dimensions) are not of the same general interest or cannot be characterized by solution methods. 

To synthesise polymers with unusual properties from existing basic monomers one needs to place the monomer units in ordered arrays rather than at random. Thus polymer architecture control remains an important area of research. Possible structural elements include block, graft and comb copolymers as well as star and dendritic/hyperbranched topographies. Potential for such structures in the surface coatings and adjacent industries include use as... 

Another important feature controlling the properties of polymeric systems is polymer architecture. Types of polymer architectures include linear, ring, star-branched, H-branched, comb, ladder, dendrimer, or randomly branched as sketched in Fig. 1.5. Random branching that leads to structures like Fig. 1.5(h) has particular industrial importance, for example in bottles and film for packaging. A high degree of crosslinking can lead to a macroscopic molecule, called a polymer network, sketched in Fig. 1.6. Randomly branched polymers and th formation of network polymers will be discussed in Chapter 6. The properties of networks that make them useful as soft solids (erasers, tires) will be discussed in Chapter 7. 

Although in controlled radical polymerisation, termination reactions cannot be excluded completely, they are limited in their extent and consequently the molecular weight is controlled, the polydispersity index is small and functionalities can be attached to the macromolecules. These features are indicative of the realisation of well-defined polymer architectures such as block copolymers, starshaped and comb-shaped copolymers. 

A wide range of architectures of PAIs is available, and the general polymer architecture can be varied to include linear, star, branched, comb and even cyclic structures. The amines can be spaced two, three or even four or more carbons apart, or even patterned by DIP, and the allqrl spacer can include further side-chain modifications. The nature of the amine can be altered, with secondary, tertiary and quaternized amines being accessible (and terminal primary amines), and the latter two may (or must) have allq l chains attached to the amines. However, despite the diversity offered by these various synthetic methodologies, only a few of these systems have been tested for biomedical applications, leaving a large area for future exploration. 

The subject of this review is complexes of DNA with synthetic cationic polymers and their application in gene delivery [1 ]. Linear, graft, and comb polymers (flexible, i.e., non-conjugated polymers) are its focus. This review is not meant to be exhaustive but to give representative examples of the various types (chemical structure, architecture, etc.) of synthetic cationic polymers or polyampholytes that can be used to complex DNA. Other interesting synthetic architectures such dendrimers [5-7], dendritic structures/polymers [8, 9], and hyperbranched polymers [10-12] will not be addressed because there are numerous recent valuable reports about their complexes with DNA. Natural or partially synthetic polymers such as polysaccharides (chitosan [13], dextran [14,15], etc.) and peptides [16, 17] for DNA complexation or delivery will not be mentioned. 

Polymers are long-chain molecules synthesized by linking monomers through chemical reactions. Types of polymer architecture include linear and branched chains, as well as those with comb, ladder, and star structures. 

The possibilities are not restricted to flexible polymers. One of the blocks can be rigid rodlike, in which case a rod-coil block copolymer [54-57] is formed if the architecture is of the diblock type (see Section 2.3.1 for another example [15]). Other interesting cases comprise diblock copolymers where one of the blocks is a side-chain liquid-crystalline polymer [4, 58-62]. Finally, we mention the important class of hairy rods obtained for a comb copolymer architecture consisting of a rigid backbone and flexible side chains [4, 58-61, 63-66], to be discussed in more depth later in this review. 

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