Syndiotactic alternating copolymers

All resonances in the C NMR spectra of all-HT polymers of racemic 1-methylnorbomene are insensitive to tactieity, but C-6 in the spectrum of the hydrogenated polymer does show mtr splitting (0.05 ppm), when the precursor is made with OSCI3 as catalyst. The downfield component (39.12 ppm) is dominant when the experiment is repeated with partially resolved monomer and is therefore assigned to m dyads. The upheld component (39.05 ppm) is the only peak observed in the spectrum of the hydrogenated all-cis, all-HT polymer made from racemic monomer with ReCls as catalyst, and it may be concluded that this is a syndiotactic alternating copolymer of the two enantiomers. This stracture is forced on the polymer by the steric exclusion of cis HH stmctures, which are never observed in any polymers of this monomer (Hamilton 1983, 1984a also see Couturier 1992). 

Copolymers. The CA Index name is written by citation of the name of the senior comonomer, in inverted format, followed by a comma. The modification contains the expression pol5mier with B, C, D,... and Z , where B, C, D,... and Z are the names of the other comonomers, alphabetized, and in iminverted format, and also stereochemical and structural terms such as isotactic, syndiotactic, alternating, block, or graft whenever applicable. The senior comonomer is chosen by a set of principles that include a strict order of precedence of chemical functions and compoimd classes (33). The format is A, Pol5mier with B, C, D,... and Z in which A, B, C, D,... and Z are the comonomers. 

Alternating copolymers prepared with 16/MAO and 17/MAO are predominantly isotactic. However, the low amounts of syndiotactic alternating structures present are sufficient to prevent... 

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

It has been discovered that styrene forms a linear alternating copolymer with carbon monoxide using palladium II—phenanthroline complexes. The polymers are syndiotactic and have a crystalline melting point - 280° C (59). Shell Oil Company is commercializing carbon monoxide a-olefin plastics based on this technology (60). 

Figure 2.20 Sequences of torsion angles in chains of syndiotactic and isotactic alternating styrene-CO copolymers, according to tc and s(M/N) symmetries, respectively.

Styrene Vinylarenes such as styrene and its derivatives, show different behaviors from those of aliphatic 1-alkenes described above. Completely alternating head-tail copolymer of styrene and CO can be obtained by using a Pd catalyst with an achiral bidentate nitrogen ligand, 1,10-phenanthroline or 2,2 -bipyridyl, but this copolymer is >90% syndiotactic (Eq. 7.16)... 

Styrene and ring-substituted styrenes also form copolymers with carbon monoxide, similarly to ethylene and a-olefins. These styrene/carbon monoxide copolymers are of alternating, highly regioregular head-to-tail structure and are characterised by different stereoregularity (syndiotactic, isotactic) [115-117]. 

ReCl.s-initiation. All monomers give high-cis syndiotactic polymers with this catalyst. It seems likely that the propagating species are mainly of the type P(., with r<. very large. The polymer of is not only syndiotactic but also allHIT. It therefore has the structure lAP and is an alternating copolymer of the two enantiomers. 

Recently, this type of deuterated monomer method was applied to the copolymerization of MM A and styrene in the presence of BC13 at low temperature, which gives a heterotactic-rich alternating copolymer.245,246 Using styrene-/3-dj, Goto et al. revealed that the addition of MMA to the styrene radical is syndiotactic-specific while the addition of styrene to the MMA radical is isotactic-specific, and that the alternation of these addition steps gives rise to the formation of a heterotactic-rich alternating sequence. 

Finally, IR spectroscopy contributed much to the finding of Zam-belli et al. (96) that C2-C3 copolymers tend to have an alternating sequence distribution if prepared by syndiospecific catalysts (i.e. catalysts that polymerize propylene to syndiotactic polymer) and a random distribution when prepared by isospecific or non-stereospecific catalysts. 

Diene polymerization may involve either or both of the double bonds. Geometric and structural isomers of butadiene, for example, are indicated by using appropriate prefixes — cis or irons, 1,2 or 1,4 — before poly, as in cw-l,2-poly(l,3-butadiene). Tacticity of the polymer may be indicated by using the prefix i (isotactic), s (syndiotactic), or a (atactic) before poly, such as 5-polystyrene. Copolymers are identified by separating the monomers involved within parentheses by either alt (alternating), b (block), g (graft), or co (random), as in poly(styrene-g-butadiene). 

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