Comb-like architecture

At the same time, the macromolecules might be classified according to whether their chains have only one kind of atoms - like carbon - in the backbone (isochains) or different elements (heterochains). Concerning their chain architecture, polymers are subdivided into linear, branched, comb-like, crosslinked, dendritic, or star-like systems. 

A similar evaluation was made for the other series of CA-g-PHAs to establish a general relationship between their molecular architecture and thermal transition behavior [24]. Of particular interest is the finding that the composition dependence of the Tg of the cellulosic graft copolymers was represented well in terms of a formulation based on a comb-like polymer model [29], when CAs of acetyl DS 2 were employed as a trunk polymer. 

Fig. 1 a—f. Various branched architectures obtained by the macromonomer technique a,b comb-like c,d star-like e brush f flower-like, a c, and e are poly(macromonomers) obtained by homopolymerization, while b, d, and f are graft copolymers obtained by copolymerization... 

Depending on the composition of the monomer feed and the polymerization procedure, different types of heterogeneities may become important. For example, in the synthesis of tailor-made polymers telechelics or macromonomers are frequently used. These oligomers or polymers usually contain functional groups at the polymer chain end. Depending on the preparation procedure, they can have a different number of functional end groups, i.e. be mono-, bifunctional, etc. In addition, polymers can have different architectures, i.e. they can be branched (star- or comb-like), and they can be cyclic. 

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. 

Polymers are normally classified into four main architectural types linear (which includes rigid rod, flexible coil, cyclic, and polyrotaxane structures) branched (including random, regular comb-like, and star shaped) cross-linked (which includes the interpenetrating networks (IPNs)) and fairly recently the dendritic or hyperbranched polymers. I shall cover in some detail the first three types, but as we went to press very little DM work has been performed yet on the hyperbranched ones, which show some interesting properties. (Compared to linear polymers, solutions show a much lower viscosity and appear to be Newtonian rather than shear thinning [134].) Johansson [135] compares DM properties of some hyperbranched acrylates, alkyds. and unsaturated polyesters and notes that the properties of his cured resins so far are rather similar to conventional polyester systems. 

A recent trend is to design polymers with multiple functions including cationic units to bind nucleic acids as well as pH-sensitive carboxylic acids for endosomal dismption. Using a block copolymer architecture, comb-like polymers with cationic grafts were extended from aspartic acid units via a hydrazone linkers.The second block was composed of... 

Molecular architecture In addition, there are different ways in which the different monomers can be bonded chemically to each other, leading to the formation of block-like versus random comonomer sequences (Figure 2.1). Moreover, the polymer may be Hnear, branched, dendritic, or comb-like (see Figure 2.2). 

To obtain comb-like or side chain-like architecture, a monofunctional, low molar mass mesogen (or monomer liquid crystal, MLC) can be complexed to a polymer possessing complementary groups. The non-covalent bond may then be located within or near the polymer... 

Hydrogen bonded PLC structures 3.2.2 Comb-like or side chain architecture... 

The use of ionic bonds to couple dissimilar components has been shown to be a viable alternative to hydrogen-bonded assemblies in constructing PLCs. The ionic bond concept has been applied almost exclusively to comb-like PLCs to date. In general, the architecture of the examples published resembles that of the (potentially) hydrogen-bonded system in Figure 3.15, except that the hydrogen bond is replaced by an ion pair that is, the flexible spacer is part of the small molecule constituent. 

WUG Wu, G., Chen, S.-C., Zhan, Q., and Wang, Y.-Z., Well-defined amphiphilic biodegradable comb-like graft copolymers Their rmique architecture-determined LCST and UCST thermoresponsivity. Macromolecules, 44, 999, 2011. 

In this work, poly(ethylene glycol) was grafted onto a cationic poly-L-lysine backbone, forming the comb-like polymer, poly(L-lysine)-graft-poly(ethylMie glycol) (PLL-g-PEG). The PLL backbone acts as an anchor onto anionic surfaces such as mica (Fig. 1 a). This type of copolymer exhibits an extraordinarily strong adsorption onto many different surfaces. Different PLL-g-PEG architectures are known to exhibit different degrees of protein resistance (Kenausis et al., 2000 Pasche et al., 2003). In this article, we have focused on the most efficient architecture for protein resistance. 

As a unique method of controlled/living radical polymerization, ATRP has had a tremendous impact on the synthesis of macromolecules with well-defined compositions, architectures, and functionalities, including star- and comb-like polymers as well as branched, hyperbranched, dendritic, network, cyclic type structures and so forth. 

Gitsov, I. (2000) Hybrid dendritic capsules properties and binding capabilities of amphiphilic copolymers with dendritic architecture. In J.E. Glass (ed.), Associative Polymers in Aqueous Media, ACS Symposium Series 765. American Chemical Society, Washington, DC, pp. 72-92. Kurjata, J., Chojnowski, J., Yeoh, C.T., Rossi, N.A.A. and Holder, S.J. (2004) Synthesis of poly[dimethylsiloxane-block-oligo(ethylene glycol) methyl ether methacrylate] an amphiphilic copolymer with comb-like block. Polymer, 45,6111-6121. 

In the following sections, we will review the various different ways in which comb copolymer-like architectures (supramolecular comb copolymers) mi t be obtained using noncovalent physical interactions, e.g., ionic, coordination complexation, or hydrogen bonding see Figure 8. 

Molecular architectures can be structurally classified as being more comb-like or Cayley tree-like. Structure has impact on the radius of gyration, which is larger for linear molecules than for branched molecules of the same weight (number of monomer units), since the latter are more compact. The ratio between branched and linear radius is usually described by a contraction factor . Furthermore, Cayley tree-like structures are more compact than comb-like structures [33, 56]. We will show here how to obtain the contraction factor from the architectural information. The squared radius of gyration is expressed in monomer sizes. According to a statistical-mechanical model [55] it follows from the architecture as represented in graph theoretical terms, the KirchhofF matrix, K, which is derived from the incidence matrix, C [33] ... 

Cycloolefin macromonomers have been recently used in ring-opening metathesis polymerization reactions to manufacture block and graft copolymers of novel macromolecular architectures [84]. For this purpose, a- and co-norbornenyl-polybutadiene macromonomers, a-NBPB (R = CH2) and co-NBPB (R = COO), were reacted in the presence of molybdenum alkylidene complex, Mo(NAr)(CHtBu)(OtBu)2, to form polynor-bornene-polybutadiene diblock copolymers (117), with comb-like structure [85] [Eq. (49)]. 

When multifunctional polymers are used in reactive compatibilization (see Alternatives 2 and 3), their reactive group content (RGC) is expected to influence the extent of the interfacial reaction (at constant blend composition) and the molecular architecture of the in situ formed compatibilizer. For instance, there must be a change from a single graft to a multiple-graft comb-like structure, as the RGC of one of the two reactive polymers is increased. This change in the molecular architecture can affect the stability of the compatibilizer at the interface and its physical entanglement with the chains in the phases to be compatibilized. Finally, the RGC of multifunctional preciu ors may also have an effect on the miscibility with the nonfunctional chains in which they are dispersed. 

Figure 19.23 Schematic diagrams of linear, Y-shaped, and comb-like rod-coil block copolymer brushes. (Reproduced with permission from C.-S. Li, W.-C. Wu, Y.-J. Sheng and W.-C. Chen Effects of chain architectures on the surface structures of conjugated rod-coil block copolymer, Journal of Chemical Physics, 128, 154908, 2008. 2008 American Institute of Physics.)...

Direct visualization of isolated macromolecides by atomic force microscopy (AFM) imaging and the statistical treatment performed on a large set of these molecides is an interesting and efficient approach that was recently used to study the stracture of comb polymers (Dziezok et al., 1997 Sheiko et al, 2003 VivUle et al., 2001 ViviUe et ai, 2000 Viville etai, 2004). Besides the analysis of single macromolecular objects with linear and star-like architecture this method was used to investigate the selectivity of various controlled-radical polymerization processes (Beers et al., 1999 Beers et al., 1998 Bomer et al., 2002 Qin et al., 2003 Min et al., 2007). 

Reversible deactivated radical polymerization processes, which have been referred to as living/controlled radical polymerizations, allow for producing polymeric materials with controlled molecular masses, low dispersities, and complex maaomolecular architectures, such as block and comb-like copolymers as well as star-shaped (co)polymers. In addition to nitroxide-mediated polymerization (NMP) ° and atom-transfer radical polymerization (ATRP), ° reversible addition fragmentation chain-transfer (RAFT) polymerization is an attractive new method. " ... 

Guzman, E., Ortega, F., Prolongo, M.G., Starov, V.M., Rubio, R.G. Influence of the molecular architecture on the adsorption onto solid surfaces comb-like polymers. Phys. Chem. Chem. Phys. 13, 16416-16423 (2011)... 

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