Linear, Branched, and Crosslinked Polymers

Polymers can be classified as linear, branched, or crosslinked polymers depending on their structure. In the previous discussion on the different types of polymers and polymerizations we have considered only those polymers in which the monomer molecules have been linked together in one continuous length to form the polymer molecule. Such polymers are termed 

It is important to point out that the term branched polymer does not refer to linear polymers containing side groups that are part of the monomer stmcture. Only those polymers that contain side branches composed of complete monomer units are termed branched polymers. Thus polystyrene XXVIII is classified as a linear polymer, and not as a branched polymer. 

IUPAC name for this polymer is poly(oxycarbonyloxy-l,4-phenylenedimethylmethylene- 

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The discussions until this point have been concerned with the polymerization of bifunctional monomers to form linear polymers. When one or more monomers with more than two functional groups per molecule are present the resulting polymer will be branched instead of linear. With certain monomers crosslinking will also take place with the formation of network structures in which a branch or branches from one polymer molecule become attached to other molecules. The structures of linear, branched, and crosslinked polymers are compared in Fig. 1-2. 

Macromolecules having identical constitutional repeating units can nevertheless differ as a result of isomerism. For example, linear, branched, and crosslinked polymers of the same monomer are considered as structural isomers. Another type of structural isomerism occurs in the chain polymerization of vinyl or vinylidene monomers. Here, there are two possible orientations of the monomers when they add to the growing chain end. Therefore, two possible arrangements of the constitutional repeating units may occur ... 

For carbohydrates to meet these requirements, diversity is needed on both the molecular and the size-level. Only large carbohydrate molecules, polysaccharides, can provide the wide spectrum of storage, structural, and gel-forming abilities required by nature. Meeting these requirements has made it necessary for plants to produce polysaccharides that can be classified as linear, branched, and crosslinked polymers, as well as homo- and heteropolymers in accordance with terminology in common use in the polymer community (O Fig. 1) [26]. Nature has found need to adopt all different kinds of macromolecular architectures in pursuit of the three different functions of carbohydrates. 

The rearrangement of more than one inner site in a flexible or semi-flexible chain molecule can be conveniently performed if the geometric constraints that guarantee chain closure are taken into account every time a site is repositioned [50] (see Fig. 1). Performance can be enhanced by favoring low-energy trial positions for each growing site (extended continuum configurational bias, or ECCB method). Since this method can be applied to inner sites of arbitrary functionality, it has been used to study linear, branched, and crosslinked polymers [50-52]. 

Figure T.6 Linear, branched, and crosslinked polymer structures. (Ref Baker, A.M.M., and Mead, J., Thermoplastics , Modem Plastics Handbook, C.A. Harper, ed., McGraw-Hill, New York, 2000)...

In polymer science and technology, linear, branched and crosslinked structures are usually distinguished. For crosslinked polymers, insolubility and lack of fusibility are considered as characteristic properties. However, insoluble polymers are not necessarily covalently crosslinked because insolubility and infusibility may be also caused by extremely high molecular masses, strong inter-molecular interaction via secondary valency forces or by the lack of suitable solvents. For a long time, insolubility was the major obstacle for characterization of crosslinked polymers because it excluded analytical methods applicable to linear and branched macromolecules. In particular, the most important structural characteristic of crosslinked polymers, the crosslink density, could mostly be determined by indirect metho ds only [ 1 ], or was expressed relatively by the fraction of crosslinking monomers used in the synthesis. 

A broad variety of l.c. polymers is conceivable because of the wide range of well known mesogenic molecules, e.g. tabulated in the book of Dcmus27), and the different types of polymers. Further variations are possible by copolymers or systems, where each monomer unit carries more than one mesogenic moiety ( en bloc systems28)). Furthermore the synthesis of linear, branched and crosslinked systems has to be mentioned. Because of this broad variety a manifold influence on the phase behavior of the systems via the chemical constitution is feasible. In the following chapter we will discuss some basic considerations on the phase behavior of l.c.-side chain polymers. 

Linear, branched and cyclolinear polymers are usually soluble. Lightly crosslinked polymers (VII) are swelled by, but are insoluble in liquids. Highly crosslinked or cyclomatrix polymers are insoluble in all media. Oligomers dissolve in liquids to give non-viscous solutions. High polymers dissolve to give highly viscous solutions. 

No, and this is another way that chemists classify these really big molecules. The major types of polymer shapes (technical term topology) are linear, branched, and crosslinked networks. Linear polymers are chains of monomers joined together, like a noodle or a rope. If there is a point along a polymer chain where a second chain starts, like a fork in the road, this arrangement is referred to as branched. 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

Abstract Polymers are macromolecules derived by the combination of one or more chemical units (monomers) that repeat themselves along the molecule. The lUPAC Gold Book defines a polymer as A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. Several ways of classification can be adopted depending on their source (natural and synthetic), their structure (linear, branched and crosslinked), the polymerization mechanism (step-growth and chain polymers) and molecular forces (Elastomers, fibres, thermoplastic and thermosetting polymers). In this chapter, the molecular mechanisms and kinetic of polymer formation reactions were explored and particular attention was devoted to the main polymerization techniques. Finally, an overview of the most employed synthetic materials in biomedical field is performed. 

R and R in these reactions may be a part of the polymer or oligomer. Also, the functionality in the polymers or oligomers may vary, and thus the network structure of the cured polymers can be varied. For example, difunctional reactants give linear polymers, whereas branched and crosslinked polymers are obtained when the functionality of the reactants is raised to more than two. 

The topological structure of condensation polymers is predetermined by the functionality of the initial monomers. If all of them are bifunctional then linear polymers are known to form. Branched and crosslinked molecules are prepared only when at least one of the monomers involves three or more functional groups. 

A microgel is an intramolecularly crosslinked macromolecule which is dispersed in normal or colloidal solutions, in which, depending on the degree of crosslinking and on the nature of the solvent, it is more or less swollen. Besides linear and branched macromolecules and crosslinked polymers, intramolecularly crosslinked macromolecules may be considered as a fourth class of macromolecules. 

Experimental and analytical studies over the past 25-30 years revealed that microgels are intramolecularly crosslinked macromolecules, which represent a new class of polymers besides linear and branched macromolecules and crosslinked polymers of macroscopic dimensions. In some ways microgels may be considered as a transition from molecules to larger polymer particles or macroscopic polymer materials. 

Due to these different primary structures of the main chain, important modifications and a broad variety of systems is realizable. While linear polymers can be essentially characterized by the number of the monomer units, for branched and crosslinked systems e.g. the way of branching and their quantity is of significance for the polymer specific properties. In cases of crosslinked systems the molecular dimension is the macroscopic dimension of the sample. 

As a consequence, together with linear chains, branched and crosslinked structures are also formed. They strongly affect molecular masses, MMD, and solution properties. Moreover, these non-crystal 1izable units cause a decrease of both the polymer melting temperature and the crystallization rate, as well as a poorer thermo-oxidative stability (16). 

Silicone Polymers - Laser flash photolysis studies on poly(silylenes) generates radical cations along with silyl radicals and polysiloxane composities for the space shuttle have been found to be stable to far UV light exposure. Linear polysiloxanes have been found to be more unstable than branched or crosslinked polymers while the transparency of poly(methylphenylsilane) increases with light exposure. Photooxidised polysiloxanes doped with iodine are converted into semiconductors. ... 

If one introduces a component into a polycondensation with a functionality of 3 or more the situation dramatically changes. Branches, branched branches, and crosslinks between polymer chains will cause a more rapid increase in molecular weight as the polymerization proceeds, and ultimately to higher final values than possible from linear polycondensations (see Section 20.2). The extent to which branches and crosslinks will occur will be proportional to the ratio of the polyfunctional to the bifunctional components present, and to the number of functionalities on the polyfunctional component. Usually 3, but up to 8, functionalities per monomer may be used for this component of polyfunctional polycondensations. 

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