Sigma -bonded carbon-backbone

The second common method of synthesising polymers (Fig. 5.3) is chain (addition) polymerisation. The most common type of addition polymer is based on ethene CH2 = CHj in which the monomer contains at least one double (tt) bond which on being activated, by free radical attack say, opens up to produce two single sigma bonds and the homopolymer poly(ethene). (Note in Fig. 5.3 the resultant polymer backbone is joined together by carbon-carbon bonds, unlike the condensation polymer systems (Fig. 5.1).)... 

For reasons not explained, but perhaps due to improved grafting efficiency, an addition lithiation step was included in the copolymerization procedures (see Figure 2.3(b)) [74]. sec-BuLi in the presence of tetramethylethylendiamine (1 1 mol ratio) was used to lithiate the cw-polybutadiene carrier backbone in cyclohexane at room temperature. Formation of car-banion at the allylic positions, not limiting to 1,2-vinyl units, was expected. The carbanions are capable of an alkylation or transmetallation exchange reaction with Ti(OBu)4 to form new Ti-carbon sigma bonds. The Ti(IV) was then allowed to reduce to the active Ti(III) species by another Li-polymeryl species before acet-... 

These three views of the ethylene molecule emphasize different aspects of the disposition of shared electron pairs in the various bonding orbitals of ethene (ethylene), (a) The backbone structure consisting of sigma (a) bonds formed from the three sp2-hybridized orbitals on each carbon, (b) The % (pi) bonding system formed by overlap of the unhybridized pz orbital on each carbon. The pi orbital has two regions of electron density extending above and below the plane of the molecule, (c) A cutaway view of the combined sigma and pi system. 

---