Tacticity block copolymers

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

Poly(styrene)-poly(acrylate) colloidal suspensions, 7 275 Polystyrene/polyfmethyl methacrylate) commercial block copolymers, 7 648t Polystyrenes. See also Polystyrene (PS) commercial, 23 364 general-purpose, 23 364 specialty, 23 364-366 stabilized, 23 366 tactic, 23 365... 

Ethylene has a symmetrical monomer, so the concept of tacticity does not apply. Consequently, the crystallinity of polyethylene is controlled either by chain branching or by copolymerisation. Copolymers are classified into random and block copolymers (Fig. 2.7) depending on whether the monomer locations are random, or whether long blocks of each monomer exist. Polyethylene copolymers are random. The figure suggests that the local composition of a random copolymer is the same as that of the monomer mixture. However, in a batch copolymerisation, monomers tend to add to the end of a growing chain at different rates. The monomer ratio drifts as the polymerisation proceeds, so polymer formed at the end of the polymer-... 

The molar optical rotation of configurational copolymers of (S) and (R) isomers of the same monomer is generally, in the case of poly(a-olefins), a hyperbolic and not a linear function of the optical purity of the monomers. Thus, the molar optical rotation of the copolymers is always greater than that obtained by additivity rules. Whether this is caused by tactic blocks in the polymers or by mixtures of (S) and (R) unipolymers has not been established yet. 

Equation 3.116 assumes the chain structure is a random copolymer. Similar expressions have been obtained for non-random (block) copolymers and for partly tactic polymers. A second example is chain ends. These may be excluded from crystals and act as impurities in which case equation 3.116 is modified... 

Applications summarized in Table 1,2 and 3, document that CEEC maybe found for many individual polymers as well as structural segment of a polymer sample. The unique sensitivity of LCCC to different characteristics of macromolecules, has been, to date primarily utilized for analysis of block copolymers (AB or ABA) as well as analysis of functionality of homopolymers (Fig. 12). Blends of polymers and structurally different macromolecules, i.e., cyclic versus linear versus branched and even with different tacticity, have been separated. Graft copolymers has been studied [43,117] as well as random copolymers [111,153,154] (Fig. 13). 

Crystalline polyolefins as polypropylene (PP) with high tacticity and copolymers of propylene with ethylene and/or higher a-olefins containing sufficiently long isotactic PP blocks can only be dissolved under conditions (solvent/temperature) which cause complete melting of the crystalline domains. Therefore, SEC of PP and crystalline copolymers of propylene must be carried out at elevated temperatures which requires the special equipment of high-temperature SEC (HT-SEC) which is commercially available from several sources, e.g. Millipore-Waters Corp. (Milford, MA, USA) and Polymer Laboratories Ltd (Church Stretton, Shropshire, UK). 

PSSA may also be prepared by sulfonation of polyst5rrene or by hydrolysis of poly(re-propyl p-vinylbenzenesulfonate). The latter cases allow preparation of tactic structures. Copol5uners can also be prepared by free-radical copolymerization of appropriate monomers or post-reaction. Poljuners are purified by precipitation of aqueous solutions with methanol, alkaline methanol, or other alcohols. Controlled structure PSSA homopolymers and block copolymers may also be prepared directly in aqueous media via RAFT using 4-cyanopentanoic acid dithiobenzoate as the CTA and V-501 as the radical source (48). NMP has been successfully... 

---