Linear polymers polymerisation

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

An alternative route to cross-linking is to start with a linear polymer and then cross-link the molecules by tying the molecule through some reactive group. For example, it is possible to cross-link unsaturated polyesters by an addition polymerisation across the double bond as shown schematically in Figure 2.12. 

In the case of linear polymers it is often difficult to obtain high molecular weight polymers. The degree of polymerisation i will be given by... 

Although polyethylene is virtually defined by its very name as a polymer of ethylene produced by addition polymerisation, linear polymers with the formula (CH2), have also been prepared by condensation reactions. For example in 1898 von Pechmann produced a white substance from an ethereal solution of... 

Polymerisation of conjugated dienes can frequently lead to the formation of linear polymers containing main chain double bonds. Examples of such diene... 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

The cyclic trimer (trioxane) and tetramer are obtained by a trace of sulphuric acid acting on hot formaldehyde vapour (i) Figure 19.1). Linear polymers with degrees of polymerisation of about 50 and a terminal hydroxyl group are obtained by evaporation of aqueous solutions of formaldehyde (ii). In the presence of strong acid the average chain length may be doubled. Evaporation of methanol solution leads to products of type (iii). 

PTEE is a linear polymer of the addition type, formed by polymerisation of the unsaturated monomer tetrafluoroethylene, CF2=Cp2. Despite the fact... 

The species IA, IB, IC represent the chain-propagating ester molecules, stabilised by styrene, and they are equivalent from the point of view of the polymerisation. At the end of the polymerisation, the now unstabilised ester II reacts with its own kind to give the indanyl ion III and a saturated linear polymer IV. It is also in equilibrium with unsaturated polymer V, and it reacts with acid and/or V to give the polymer with indanyl end groups VI. This is equivalent to the transfer reaction with monomer which gives the indanyl end groups [23]. The oligostyryl ion VII can only be present in very small concentration, as it is much less stable than the other SD ions which co-exist with ion III [7] these other SD ions have been omitted from the scheme so as not to complicate it unnecessarily. 

The process described is referred to as ion-exclusion as discussed by Asher and Simpson 9. The resins used are normal and the non-ionic molecules are assumed to be small enough to enter the pores. When large non-ionic molecules are involved, an alternative process called ion-retardation may be used, as discussed by Hatch et al. W]. This requires a special resin of an amphoteric type known as a snake cage poly electrolyte. The polyelectrolyte consists of a cross-linked polymer physically entrapping a tangle of linear polymers. For example, an anion exchange resin which is soaked in acrylic acid becomes entrapped when the acrylic acid is polymerised. The intricacy of the interweaving is such that counter-ions cannot be easily displaced by other counter-ions. On the other hand, ionic mobility within the resin maintains the electro-neutrality. The ionic molecule as a... 

Cellulose is a linear polymer (polysaccharide) made up of glucose monomers. Figure 46 shows the molecule structure of cellulose. It has a degree of polymerisation (DP) on average 10 000 glucose units, which corresponds to a length of 5 im. The chemical formula for cellulose is usually written as [(CgHjjO,) ]. [64,65]... 

Inorganic heterocycles in which other p-block elements are inserted into a phosphazene ring are accessible via a variety of synthetic approaches. The primary interest in these hybrid ring systems is as potential precursors of linear polymers via ring-opening polymerisation (Section 11.4.3). Consequently, recent work has focused on halogenated derivatives. For example, the... 

B16. Butler, G. B., and A. Crawshaw The formation of linear polymers from diene monomers by a cyclic polymerisation mechanism. II. Polyacrylic anhydride and the derived polyacrylic acid. J. Am. Chem. Soc. 80, 5464 (1958). 

The macromonomer route assures uniform distribution of dendrons along the polymer backbone. Acrylates and styrenes with pendant dendrons proved to be well suited for polymerisation [57]. However, steric hindrance between monomers bearing higher-generation dendrons and the spatial requirements of the end of the chain cause problems The monomer reacts with the chain end only in the case of slight, if any, steric hindrance. Thus the method only provides access to dendronised linear polymers with relatively low molar masses... 

The dimensions of the dendritic cylinder depend on the one hand upon the degree of polymerisation, which determines the length, and on the other hand upon the generation of the dendron, which determines the diameter (about twice the size of the attached dendron). Conventional polymers have diameters in the Angstrom range, whereas the dendronised linear polymers described here have nanometre diameters. 

If the monomers are bifunctional, as in the above example, then a linear polymer is formed. Terminating monofunctional groups will reduce the average degree of polymerisation. Polyfunctional monomers, such as glycerol and phthalic acid, are able to form branching points, which readily leads to irreversible network formation (see Chapter 9). Bakelite, a condensation product of phenol and formaldehyde, is an example of such a space-network polymer. Linear polymers are usually soluble in suitable solvents and are thermoplastic - i.e. they can be softened by heat without decomposition. In contrast, highly condensed network polymers are usually hard, are almost completely insoluble and thermoset - i.e. they cannot be softened by heat without decomposition. 

It is difficult to introduce phosphorus into a linear polymer by conventional polymerisation or co-polymerisation of phosphorus containing monomers. The polymers containing... 

Allene has been polymerised to high molecular weight linear polymers by various Ziegler-Natta catalysts based on compounds of such transition metals as Ti, V, Cr, Mn, Fe, Co and Ni as precursors and alkylaluminium compounds as activators [439-441], Crystalline and amorphous polymers have been produced, in various proportions, with each of the catalysts used. The crystalline polymers consist predominantly of 1,2-linked (head-to-tail) monomeric units formed by insertion polymerisation as in scheme (68), but some regioirregular-ities resulting from the 2,1-insertion, leading to head-to-head and tail-to-tail arrangements, may be present in the polymer chain ... 

Substituted allenes undergo polymerisation to linear polymers in the presence of both Ziegler-Natta catalysts, e.g. VOCl3 Al(i-Bu)3 [440], and monometallic complexes such as Ni complexes [442], Amorphous polymers consisting of 1,2-linked monomeric units (predominant) and 2,3-linked monomeric units have been obtained from 1,2-butadiene (H2C=C=CHCH3) ... 

The metathesis polymerisation of dicyclopentadiene (endo, exo) yields a linear polymer when carried out with single-component homogeneous metathesis catalysts of the metallacyclobutane or metal alkylidene type in diluted toluene solutions [142, 143]. 

If the heat is removed from the polymerisation system through dilution or by cooling the solution, a soluble linear polymer of dicyclopentadiene remains the final product [143]. 

Non-linear polymers maybe formed by polymerisation, or can be prepared by linking together (i.e. crosslinking) pre-existing chains. 

Condensation polymerisation occurs when multifunctional monomers, which possess more than one chemically reactive group per molecule, react together with the elimination of small molecules, typically either water or HC1. With a bifunctional monomer the product is a linear polymer, e.g. the polyamide Nylon-6 is derived from e-amino caproic acid as follows ... 

Hasegawa and Higashimura reported an interesting application of their recent studies on the selective dimerisation of styrenes. By choosing the conditions which give an optimum yield of linear dimer, they polymerised divinylbenzene with acetyl perchlorate to a linear polymer with the structure... 

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