Styrene/CO copolymers

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.

Figure 2.21 Side views and projections along chain axis of conformations of chains in syndiotactic121 and isotactic123 alternating styrene-CO copolymers.

Figure 12.24. Chiral bisoxazoline complexes giving chiral 4-tBu-styrene/CO copolymers...

Propylene/CO and styrene/CO copolymers possess side chains (methyl and phenyl groups, respectively) thus, multiple possibilities for regioisomers and stereoisomers exist. Several mechanistic studies have proposed that the key step which determines the regio- and stereochemistry of olefin/CO copolymers is olefin insertion into the acylpalladium species. Polymers with high regioregularity are produced if the olefin insertion reaction is regioselective to either 1,2- or 2,1-addition. As shown in Scheme 22.2, propylene insertion is mostly 1,2- whereas styrene insertion is usually 2,1-. 

It should be noted that, unlike for polypropylene or polystyrene, there exist asymmetric centers in the main chain of the propylene/CO and styrene/CO copolymers. Thus, one enantiomer of a... 

SCHEME 22.6 Stereochemical analysis for styrene/CO copolymers, (a) Highly diastereoselective diad formation with 2,2 -bipyiidine. (h) Amplification of stereoselectivity occurs as the polymer chain grows using a diimine ligand 9. 

When a bulky P N ligand was used, the electronically favored 2,1-insertion and the sterically favored 1,2-insertion compete with each other, providing a regioirregular styrene/CO copolymer. Regiocontrolled but atactic copolymers were obtained using pyridine-imidazole or diimine ligands. 

This type of analysis requires several chromatographic columns and detectors. Hydrocarbons are measured with the aid of a flame ionization detector FID, while the other gases are analyzed using a katharometer. A large number of combinations of columns is possible considering the commutations between columns and, potentially, backflushing of the carrier gas. As an example, the hydrocarbons can be separated by a column packed with silicone or alumina while O2, N2 and CO will require a molecular sieve column. H2S is a special case because this gas is fixed irreversibly on a number of chromatographic supports. Its separation can be achieved on certain kinds of supports such as Porapak which are styrene-divinylbenzene copolymers. This type of phase is also used to analyze CO2 and water. 

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Bamford and Mullik [62] have succeeded in photografting a vinyl monomer onto a styrene-MMA copolymer using the Mn2(CO)io/C2F4 photoinitiating system in acetic acid. The following scheme was reported for this process ... 

The most frequently used organic supports are polystyrene and styrene-divinylbenzene copolymer beads with functional groups such as diphenylpho.sphine covalently bonded. The polymer-anchored catalyst complex can then be obtained, for example, by displacement of a ligand already co-ordinated to a soluble metal complex (Cornils and Herrmann, 1996) ... 

We can readily copolymerize styrene with a variety of comonomers. Commercially, the two most important random styrene copolymers are styrene co-acrylonitrile and styrene cobutadiene, the general chemical structures of which are shown in Fig, 21.3. 

We commonly copolymerize styrene to produce random and block copolymers. The most common random copolymers are styrene-co-acrylonitrile and styrene-co-butadiene, which is a synthetic rubber. Block copolymerization yields tough or rubbery products. 

The same authors proposed an alternative methods for obtaining soluble poly(/i-vinylborazine) homopolymers and poly(styrene-co-B-vinylborazine) copolymers 28 In fact, gentle polymerization conditions in solution at 80°C using Azobisisobutyronitrile (AIBN) (1.6 mol%) as an initiator provided soluble homopolymers. The polymer displays typical Mw and Mn values of —18,000 and 11,000, respectively, whereas an increase in the AIBN concentration results in a decrease in the molecular weight, contrary to what is usually observed in free-radical polymerization. 

The copolymers were obtained following this method by exploiting the similarity of 5-vinylborazine and styrene. Several poly(styrene-co-B-vinylborazine) copolymers soluble in ethers were formed in solution at 80°C using AIBN.28... 

Hot melt adhesives based on poly(3HB-co-3HV) have also been described [119]. Hot melts are commonly used in bookbinding, bag ending and case and carton sealing and are mostly based on synthetic materials such as polyethylene, polypropylene ethylene-vinyl acetate and styrene block copolymers [119]. Hot melts based on PHAs alleviate the dependence on petroleum based materials and allow the development of biodegradable alternatives based on natural raw materials. 

A combination of TEMPO living free radical (LFRP) and anionic polymerization was used for the synthesis of block-graft, block-brush, and graft-block-graft copolymers of styrene and isoprene [201]. The block-graft copolymers were synthesized by preparing a PS-fo-poly(styrene-co-p-chloromethylstyrene) by LFRP [Scheme 110 (1)], and the subsequent re-... 

In figure 1 we present the experimental and calculated mK values of the copolymer poly(styrene-co-p-bromostyrene). From this study (3) we were able to show unequivocally that the tacticity of this polystyrene sample is pr — 0.55, where pr is the probability of racemic dyad replication. 

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