Strictly Linear Macromolecules

Strictly linear macromolecules would be, for examjde I, IX, X, and XIII to XV of Table 6.1. Of these not all have been made or are chemically stable and little is known about their structure. Polyparaphenylene, as one example, was described in Sect. 5.1.4 and linked to the better known oligomers (Sect. 5.1.1-5.1.3). The LC glass state proposed for the polymer would be difficult to dissolve into a lyotropic liquid crystal because of the seeming lack of high temperature solvents that could reduce the glass transition temperature sufficiently. By itself decomposition occurs before the glass transition temperature is reached. 

More successful was the handling of poly(p-phenylme henzohisthiazole) (PPB) 

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Strictly linear macromolecules such as polyfp-phenylene) cannot be brought into the molten state, and thus have solid state structure set by the polymerization condition. Conformational mobility is usually limited to ring rotation. Little details are known about these polymers. For dissolution, solvents with strong interactions and of high temperature stability are needed, as for example found for poly(p-phenylene-... 

Many difficulties in polymer processing are caused by chain branching The macromolecules of technical polymers consist of thousands of monomeric units. The ratio of the rates of regular addition and of the branching reaction should be > 103 for strictly linear polymers. 

Wales and his coworkers. " Direct use of the relations developed for branched macromolecules in solution depends on the availability of a strictly linear polymer of the same type. Wales and coworkers overcame this difficulty by application of the Flory-Fox treatment, to yield... 

PPEs and PAEs are ideally conjugated linear macromolecules (in a strict sense) if all bond angles... 

Polymerizations lead to linear macromolecules only if the monomers are strictly bifunctional. Monofunctional molecules were shown in Fig. 3.13 to limit the molecular length of step-growth polymers. Adding monomers with a higher functionality than, f = 2 to bifunctional monomers leads to network formation with stractures as discussed in Sect. 1.2. A sudden gel formation is illustrated in Fig 3.48. Gel formation is marked by the appearance of a macroscopic network. The network has a practically infinite molar mass. In polymerizations from the pure monomer the network is swollen by monomer, oligomers, and linear and branched polymer molecule. In polymerization from solution, the solvent makes up a large number of the small molecules. As the polymerization continues, most or all of the polymerizable material joins the network. A gel can hold a large number of small... 

The importance of macromolecular substances, or polymers, is matched by their ubiquity. Examples too numerous to mention abound in biological systems. They comprise the structural materials of both plants and animals. Macromolecules elaborated through processes of evolution perform intricate regulatory and reproductive functions in living cells. Synthetic polymers in great variety are familiar in articles of commerce. The prevailing structural motif is the linear chain of serially connected atoms, groups or structural units. Departures from strict linearity may sometimes occur... 

It is noteworthy that strict topological constraints do not exist for systems of linear chains with free ends and do not affect the statistical properties of linear polymers nevertheless, they significantly influence the dynamics of macromolecules. 

Birr 156) finds that the reactions on supports like 1 % cross-linked polystyrene in suitable solvents are not strictly solid-phase reactions, even if the particles look solid. Under these loosely cross-linked conditions, the polymer exists in a quasi-dissolved gel state, and the reactivity of the chain molecule is not considerably reduced in any way by the gel character. The free motion of the large chain segments in the swollen state of the macromolecular gel phase — which is only very lightly cross-linked just to give macroscopic insolubility — guarantees reactivity which is not much different from the corresponding linear, soluble macromolecule. 

Cellulose is one of the most important aixl most abundant natural polymers. This circumstance and certain specific features of its molecular and supermolecular structures (the linear, strictly regular structure of the macromolecules, the great rigidity dF tte polymeric chains, the possibility of formation elements of the supermolecular structure havii a high d ree of ordering, i.e. crystallites, etc.) are the factors that have stimulated extensive work on the chemistry, physicochemistry and technology of cellulose and its derivatives. 

Cross-linked macromolecules are sometimes designated as three-dimensional. Strictly speaking each macromolecule is three-dimensional, even when regarded in hypothetical, exactly linear conformation, not spatially ordered in a crystal or in a random coil. With non-branched macromolecules, the transversal dimensions are sometimes neglected with respect to length. All three dimensions of cross-linked (network) macromolecules are mutually comparable. 

Branched macromolecules fall into three main classes star-branched polymers, characterized by multiple chains linked at one central point (Roovers, 1985), comb-branched polymers, having one linear backbone and side chains randomly distributed along it (Rempp et al., 1988), and dendritic polymers, with a multilevel branched architecture (Tomalia and Frechet, 2001). The cascade-branched structure of dendritic polymers is typically derived from polyfunctional monomers under more or less strictly controlled polymerization conditions. This class of macromolecules has a unique combination of features and, as a result, a broad spectrum of applications is being developed for these materials in areas including microencapsulation, drag delivery, nanotechnology, polymer processing additives, and catalysis. 

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