Branched “comb-shaped” polymers

It has been hypothesized by Graessley (30) and others (3, 31) that the g1/2 rule is applicable to star-shaped and slightly branched polymers whereas the g372 rule describes the behavior of highly branched "comb-shaped polymers for which [rj] B/ [y] l < 0.5. The results of this study indicate that the g1/2 rule in combination with Equation 11 adequately represents the random trifunctionally and tetrafunctionally branched... 

Randomly branched comb-shaped polymers can be prepared by three general synthetic methods the grafting onto , the grafting from , and the grafting through or macromonomer method.88... 

Anionic grafting methods (vide infra) can be applied to the synthesis of comb-shaped polymers. As an example, a polystyrene backbone is partially chloromethylated (under mild conditions) and used as an electrophilic deactivator for a living polystyrene 89). The grafting onto process yields well defined species that have been characterized accurately. The branches are distributed randomly along the backbone 90). 

Graft copolymers are comb-shaped polymers consisting of a backbone and two or more branches which differ chemically from the backbone. Branches are usually distributed randomly along the backbone, although recent advances in synthetic methods have allowed for the preparation of better-... 

Experimental data on branched polystyrenes were used to test the theory. There are some discrepancies in particular the parameter A varies from one polymer to another, especially for comb-shaped polymers y> also varies. It is suggested that the reason for the discrepancies in A may be the failure of the assumption of a Gaussian distribution of segment density. 

Homopolymerization of macromonomer provides regular star- or comb-shaped polymers with a very high branch density as shown in Fig. 1 a,c,e. Such polymacromonomers, therefore, are considered to be one of the best models for understanding of branched architecture-property relationships. Their properties are expected to be very different from the corresponding linear polymers of the same MW both in solution and the bulk state. Indeed, during the past decade, remarkable progress has been accomplished in the field of static, dynamic, and hydrodynamic properties of the polymacromonomers in dilute and concentrated solutions, as well as by direct observation of the polymers in bulk. 

The branching sites can be introduced onto the backbone either by postpolymerization reactions or by copolymerization of the main backbone monomer ) with a suitable comonomer, with the desired functional group (unprotected or in a protected form if this functional group interferes with the polymerization reaction). Branches of comb-shaped polymers are commonly prepared by anionic polymerization, and backbones with electrophilic functionalities such as anhydrides, esters, pyridine, or benzylic halide groups are employed.88 The actual average number of branches in the final copolymer can be found by the determination of the overall molecular weight of the copolymer and the known molecular weights of the backbone and the branches. 

Recently, Yamamoto and Hyodo have employed the DPD method for studying Nafion membranes [20]. The systems considered in this study were built using two distinct molecular species, denoted comb-shaped polymer ip) and water (w). The polymer was presented as a branched sequence of beads. It consisted of a main chain (backbone) of iV = 20 effective monomer units (-CF2CF2CF2CF2 ) linked with rig = 5 short side chains of = 2 units [-0CF2C(CF3)F0 and F2CF2S03H] the total number of interaction sites in the macromolecule was Np= N/, + n xn = 30. A water-like particle was modeled as the same size as the units of the Nafion fragment (<7 = 6.1 A) and represented four water molecules. The x parameters were found using an atomistic calculation. The DPD simulation was performed for water volume... 

FIGURE 1.1. Shapes of polymeric molecules, (a) linear polymer, (b) branched polymer, (c) star>shaped polymer, (d) comb-shaped polymer, (e) ladder polymei (f) semiladder polymer, and (g) network structure. 

The type of polymers which form these monomers are the so-called comb-shaped polymers (Volkov et fl/., 1992). They are branched polymers that contain many side chains. For instance, each monomeric unit can contain such a chain. It is important that the length of these side groups is significantly greater than their cross-section. Only under this condition can the unique properties of comb-shaped polymers occur (for example, the autonomous behaviour of the side chains, their capacity to form layered structures, and to crystallise regardless of the main chain configuration) (Plate and Shibaev 1987). 

Study of the structure and physicochemical properties of comb-shaped polymers in dilute solutions, gels, and the solid phase genoalized in [8] permitted describing the structural features of this special class of branched polym systems in detail within the framework of the so-called rotational-crystalline state, a variety of the LC state. 

On the whole, it is possible to state that the tendency toward orientation of the side branches of the macromolecules forming the LC phase with the corresponding dielectric and diamagnetic anisotropy is the driving force of orientation processes in comb-shaped polymers. The interaction of the side groups with the external field sets the main chains in motion, which also determines certain specific features of polymer liquid crystals wtih respect to the effect of external fields. 

Comb-shaped polymers are derived from polymerizing or copolymerizing macromonomers. Macromonomers can be synthesized by a variety of synthetic techniques. Asami and co-workers prepared a methacrylate-terminated polystyrene by anionic polymerization. The macromonomer was then polymerized using GTP [41] to yield an oligomer with a polystyrene backbone and PMMA grafts. McGrath and co-workers prepared a poly(dimethyl siloxane) macromonomer end-capped with a methacrylate group. This macromonomer was polymerized by GTP to yield a comb-shaped polymer with PDMS branches [19]. 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

Macromers have been used to produce thermoplastic elastomers. Generally, the backbone serves as the elastomeric phase while the branches serve as the hard phases. These structures are often referred to as comb -shaped because of the similarity between the rigid part of the comb and its teeth and the structure of these graft polymers. 

In several cases the melt viscosity of a series of lightly-branched polymers has been determined as a function of MW, and compared with that of linear polymers, and it has been found or may be deduced from the published data that there is a cross-over molecular weight, below which the branched polymer is less viscous, but above which it more viscous, than the linear polymer of equal MW. This behaviour is observed with some comb-shaped polystyrenes (35) and poly(vinyl acetate)s (59, 89), star polybutadienes (57, 58, 123), and randomly-branched polyethylenes (56,61). Jackson has found (141) that if the ratio ZJZC of the number of chain atoms at the cross-over point, Zx, to the number at the kink in the log 0 — logM curve, Zc, [as given in Ref. (52)], is plotted against nb, the number of branches, a reasonable straight line is obtained, as in Fig. 5.1. 

Graessley s theory, though satisfactory for linear polymers, has not yet been shown to apply to branched polymers. Fujimoto and co-workers (65) attempted to apply it to comb-shaped polystyrenes, but obtained only poor agreement with experiment. They attributed this to the failure of the assumption that the state of entanglement is the same in branched polymers as in linear ones. It is not surprising that this theory fails, for (in common with earlier theories) it predicts that the zero shear-rate viscosity of all branched polymers will be lower than that of linear ones, contrary to experiment. 

An alternative to a linear polymer is a branched one. The branches can be long or short. Low-density polyethylene, for instance, can have both short and long branches. Linear and branched molecules are illustrated in Fig. 1.1a and b. Branched polymers can also be star or comb shaped (Fig. 1.1c and d). In addition to the above, polymer molecules can also be double stranded. Such polymers are called ladder polymers (Fig. 1. le). It is also possible for polymers to have semiladder structures (Fig. l.lf). 

Explain what is a linear, a branched, a star-shaped, a comb-shaped, and a ladder polymer. 

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