Substituted ethene monomers

Tacticity is common in polymeis based on substituted ethene monomers, CH j = CHR, and the pattern of optically active sites in a polymer chain can sharply alter the properties of polymers such as polypropylene. Even in atactic polymers, the frequency of repeated, identically oriented, active sites can influence polymer properties. The fiequeney of two identically oriented sites in a row, diads, and three identically oriented sites in a row, triads, is often determined in some polymers with identical chemical composition to determine how differences in properties are produced 1 differences in spatial orientation within the polymer samples. 

Long chain molecules called polymers are formed from the small ethene molecules, the monomers. The long, thin molecules of a polymer may also be called linear macromolecules. Polythene is an unreactive solid it can be easily moulded and is used for plastic bags, bottles, washing-up bowls and plastic piping. The word plastic means easily moulded . Substituted ethene molecules can undergo the same addition reaction to give other polymers ... 

Next to isobutene, other 1,1-dialkyl substituted ethenes can also be polymerized cationically. Suitable monomers are 2-methyl-1-butene, 2-methyl-1-pentene, and 2,3-dimethyl-1-butene [613] Polymers with very high molecular weights of Mn>300000 are obtained by catalysis with aluminum alkyl halogenides. Also, cyclic hydrocarbons with a methylene group (methylenecyclopropane, methylene cyclobutane, methylene cyclohexane, a-pinene) are suitable monomers [614-619]. 1,1-Disubstituted ethenes with stronger steric hindrance as camphene or 2-methylene-bicyclo-[2.2.1] heptane, however, could not be polymerized cationically [574]. 

Alkyl- or aryl-substituted ethenes are the monomers for many of the most widely produced polymers in the world. Tables 21.2 and 21.3 give details of some of the most common ones, how they are made, and what they are used for. 

Remarkably high activities for the polymerization of ethene similar to those of active Ziegler-Natta systems have been reported and the physical properties of the polyolefins produced can be tailored by the choice of the metal center and the substitution pattern of the hgand backbone [12-14]. Some of these catalytic systems are not only compatible with polar monomers, but also copolymerization of... 

The monomers used most commonly in chain-growth polymerization are ethylene (ethene) and substituted ethylenes. In the chemical industry, monosubstituted ethylenes are known as alpha olefins. Polymers formed from ethylene or substituted ethylenes are called vinyl polymers. Some of the many vinyl polymers synthesized by chain-growth polymerization are listed in Table 28.1. 

Related to ROMP is the acyclic diene metathesis (ADMET) procedure. Here, the starting monomer is a substituted p-divinylbenzene. During the polymerization, ethene is ejected. However, only low-molecular-weight polymers can be obtained by ADMET. 

Ethene bears on its periphery only H atoms. In propene, one H atom is replaced by a methyl group, in styrene by a phenyl ring, in vinyl chloride by a Cl atom. In Fig. 2, this type of substitution is indicated by an X. Monomers with more than one substitution (e.g., polytetrafluorethylene) are used to make high-grade halogenated plastics. 

Substituted polyethylenes comprise an entire class of polymers. For example, polyvinyl chloride (PVC)—the plastic used to make certain kinds of pipes and plumbing fixtures—is composed of monomers in which a chlorine atom has been substituted for one of the hydrogen atoms in ethene. 

A drawback of E-N copolymers endowed with high Tg values is their brittleness at high norbomene content. The substitution of norbomene with a bulkier cycloolefin monomer could result in a more ductile COC by obtaining the same Tg values at a lower amount of cycloolefin content and thus copolymers with a higher amount of flexible ethene units in the chain. The properties of a COC based on bicyclic mmiomers can be varied by varying the norbomene content or the stmcture of the bicycloolefin this includes norbomadiene, dicyclopentadiene (DCPD),... 

When going from ethene as the simplest possible monomer for radical polymerization to monomers with higher reactivity, one or more hydrogen atoms are formally substituted by groups activating the double bond and stabilizing the macroradical. Of course, steric hindrance is added at the same time, so a complete separation of entropic and enthalpic effects on the propagation rate seems impossible. 

---