Chain Comb-like

Prior to discussing LC elastomers, we will consider in Sect. 2 in some detail the conformations and chain anisotropy of their polymer counterparts because the polymer backbone generates the shape anisotropy and elastic response of the elastomer network. In this context, note that two different classes of thermotropic LC polymers exist main-chain and side-chain (comb-like), as depicted in Fig. 1). In side-chain LC polymers, the pendant mesogenic groups are linked to a linear polymer backbone... 

Copolymer macromolecules are composed of a single backbone having simple grafts attached to it, i.e., the macromolecules are of the comb-like type. No further grafting of grafted chains is contemplated (4). 

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. 

The synthesis of comb-like polymers with regular branching (in contrast to random branching) has been performed in the following way 91) A linear polystyrene precursor fitted with carbanionic sites at both ends is reacted first with 1,1-diphenylethylene (to decrease the nucleophilicity of the sites) and then with a calculated amount of triallyloxytriazine to get chain extension. Each triazine residue still carries one allyloxy... 

An alternative route for the preparation of styrenic macromonomers is the reaction of living chains with 4-(chlorodimethylsilyl)styrene (CDMSS) [192]. The key parameter for the successful synthesis of the macromonomers is the faster reaction of the living anionic chain with the chlorosilane group rather than with the double bond of the CDMSS. Anionic in situ copolymerization of the above macromonomers (without isolation) with conventional monomers leads, under appropriate conditions, to well-defined comb-like chains with a variety of structures. 

The feasibility of the fabrication of comb like fluorocarbonpolymer LB films has been shown. These films can be deposited onto different kinds of substrates as y-type layers by the usual LB technique. In this case the deposition procedure is much simpler than the one for polyimide LB films, but the temperature, chemical and mechanical stability, and dielectricproperties of the fluorocarbonpolymers are not inferior to those of polyimides. The fluorocarbons are more hydrophobic than ordinary hydrocarbons, hence shorter hydrophobic chains can be used and thinner monolayers can be prepared (the PFHA-AA LB monolayer thickness investigated was 16.5 x l(L8cm). 

In naming non-linear copolymer molecules having linear subchains of two or more types, the italicized connective for the skeletal structure is placed between the source-based names of the types of constituent linear subchains. In the case of branched and comb-like macromolecules, the linear chain named before the connective is that which forms the main chain, whereas that (those) named after the connective forms (form) the side-chain(s). The names of different species of side-chain are separated by semicolons. In the case of variegated star macromolecules the prefix is placed before the name of the macromolecule with the different species of arms separated by semicolons. 

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. 

Shibaev, V. P., Moiseenko, V. M., Plate, N. A. Thermotropic liquid crystalline polymers, 3, Comb-like polymers with side chains simulating the smectic type of liquid crystals. Makromol. Chem. 181, 1381 (1980)... 

Considerations by V. Shibaev and N. Plate (see this issue) led to a similar conclusion. Investigations on comb like polymers26), where each monomer unit of the macromolecule carried a non-branched alkyl chain of m methylene groups, have shown that for m > 8 side chain crystallization takes place independently to the main chain conformation. Consequently, if mesogenic molecules are linked to the side chains, they should occupy a l.c. order without influence of the backbone conformation. Following these considerations, alkyl chain lengths m > 8 are necessary for the formation of the l.c. order. As shown later, however, nearly only smectic polymers are possible under these conditions (see Chap. 2.3.3.). 

A synthetic pathway that appeared to be most convenient and promising for the synthesis of LC polymers was proposed and proved in Moscow State University in 19736,8 10) and involved the use of so called comb-like polymers containing long paraffinic fragments in each monomer unitl2,33) (Fig. 2). Macromolecules of comb-like polymers are constituted of two types of structural units — the main chains and the side chains. Their behaviour is mutually dependent as they are chemically linked and at the same time both parts are sufficiently independent because the side chains are long enough. 

Fig. 2. Scheme of a comb-like macromolecule 1 junction) 3 — n-aliphatic side chain... 

As the formation of LC phase in comb-like polymers is predetermined by the interaction of mesogenic groups, it would have seemed, that the temperature range of LC state for such systems should not depend on the length of the main chain, i.e. on the degree of polymerization (DP). However, studies 44,49) on the dependence of Tc] on DP carried out for some polyacrylic and polymethacrylic derivatives of cyanobiphenyl, as well as for polyparabiphenylacrylate 51 (Fig. 5), have shown that... 

Thus, polymers with mesogenic groups in side chains form structural mesophases of the same types as low-molecular liquid crystals. This makes it possible to apply traditional mesophase classification for the description of the structure of LC polymers. At the same time, the structure of some of comb-like polymers (see Table 5) considered as crystalline, may probably be treated as one of highly-ordered smectic mesophases (SH or Sj), whose study is only started74). 

Poly(ethylene oxide) (PEO) macromonomers constitute a new class of surface active monomers which give, by emulsifier-free emulsion polymerization or copolymerization, stable polymer dispersions and comb-like materials with very interesting properties due to the exceptional properties of ethylene oxide (EO) side chains. They are a basis for a number of various applications which take advantage of the binding properties of PEO [39], its hydrophilic and amphipathic behavior [40], as well as its bio compatibility and non-absorbing character towards proteins [41]. Various types of PEO macromonomers have been proposed and among them the most popular are the acrylates and methacrylates [42]. 

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. 

Diblock, triblock, and multiblock copolymers are typically prepared by sequential monomer addition to polymerization systems in which the chain-breaking reactions are sufficiently suppressed. Polymer properties can thereby be varied by manipulating the constituent blocks compatibilities, hydrophilicities/hydrophobicities, and hardness/softness. New and/ or novel topologies can also be prepared by controlled processes, including cyclic polymers and/or copolymers, comb-like macromolecules, and star polymers. The synthetic range of cationic vinyl polymerizations will be discussed in detail in Chapter 5. 

Oligosilanes and polysilanes initiate multidirectional growth. For example, poly(/V-vinyl carbazole) and poly(vinyl ether) side chains have been initiated from polysilanes containing several triflate groups to prepare comb-like copolymers [200-202]. 

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