Polymers, stereoblock tacticities differences

Figure 1, Stereoblock polymer, showing distinctly different tacticities for same monomer in homopolymer.

Another way of preparing stereoblock polymers is the use of binary catalyst systems, where the exchange of growing alkyl chains with different tacticity might be expected.The alkyl chains are presumably carried by alkylaluminums and transferred between zirconocenes, and the products are a mixture of homopolymers from each catalyst and a stereoblock polymer containing both segments. 

It is clear that a determination of any two triad fractions allows a complete definition of both the triad and dyad structures of a polymer via Eqs. 8-10 through 8-13. An atactic polymer is one in which (r) = (m) = 0.5 and mm) = (rr) = 0.25, mr) = 0.5 with a random distribution of dyads and triads. The (aU-) isotactic polymer has (w) = mm) = 1. The (all-) syndiotactic polymer is defined by (r) = (rr) — 1. For random distribution with (m) 7 (r) 7 0.5 or mm) / rr) / 0.25, one has different degrees of syndiotacticity or isotacticity. Isotacticity predominates when (m)>0.5 and mm) >0.25 and syndiotacticity predominates when (r)>0.5 and (rr)>0.25. These pol3miers are random tactic polymers, containing random placement of isotactic and syndiotactic dyads and triads. When the distribution of dyads and triads is less than completely random, the polymer is a stereoblock polymer in which there are block (which may be short or long) of isotactic and syndiotactic dyads and triads. 

Stereoblock polymers are defined analogously to constitutional block polymers each block has a different species of stereorepeating unit to the block preceding it, but has the same constitutional monomeric units. Tactic... 

PLA is a fascinating material and there are big differences in the thermomechanical properties of the polymer depending on the tacticity formed. For example, atactic and heterotactic PLA are amorphous and have no defined melting point. Polymerisation of either the pure l- or d- stereoforms produce isotactic PLA with a Tg of ca. 50 °C and a T , of 170-180 °C. However, a 50 50 mixture of PLLA and PDLA has a melting point of ca. 220-230 °C, this enhancement is due to the formation of a stereocomplex between opposite chiral chains. It is also possible, and indeed preferred, to produce the stereocomplexed polymer from the isoselective polymerisation of rac-LA forming stereoblock PLA. There are many examples of zirconium, and indeed group-4 metals as a whole, in the literature for the polymerisation of rac-LA, the next section will focus on the stereoselective examples of ROP. 

A major 1981 lUPAC document (9) addresses polymer stereochemistry in depth. Key chapters of this document discuss basic definitions (configurational unit, configurational base and repeating units, stereorepeating imit, different types of tacticity, tactic block polymers, and stereoblock polymers) sequences conformations and supplementary definitions. Most of the illustrations use the rotated Fischer projections, but some three-dimensional representations are included. 

---