Atactic block

Three types of elastomeric polypropylene are known (1) a large molecular weight atactic polymer [125], (2) polymers with alternating isotactic and atactic blocks [108,109,126-129], and (3) polymers formed by the dual-site mechanism [110]. The structure of the obtained elastomeric polypropylene was elucidated by comparison of the 13C-NMR spectra with those of an atactic oily polymer and isotactic polypropylene prepared by the zirconium complex 4. On the basis of the NMR data, for all the samples the statistical lengths of the isotactic blocks between two neighboring epimerization stereodefects were calculated [41]. It was found that for the isotactic polymer 90%), the... 

Stereoblock polypropylene is synthesized by unbridged metallocenes such as bis(neomenthyl)zirconium dichloride (7) with cyclopentadienyl or phenyl substituted indenyl ligands [55, 56]. The stereoblock length increases at lower polymerization temperatures. Products containing isotactic and atactic blocks are elastomeric if the isotactic block length is short. Rieger [57] obtained similar polypropylenes by bridged fluorenyl-substituted indenyl zirconium complexes. 

Figure 14 Structures of the rac and meso rotomers that lead to isotactic and atactic polymer blocks in the polymerization of polypropylene. An example of an isotactic-atactic block copolymer is shown. The composition of the block copolymer is determined by the equilibrium constant for the interconversion between rac and meso forms of the catalyst. (Redrawn from Ref. 88.)...

A microstructural analysis suggested the presence of 10 atactic blocks per chain, each of which contained 100 propylene units, while the isotactic blocks between have only 50 monomer units.This structure was postulated to arise from an alternation of polymer growth between isospecific and aspecific sites, where site epimerization occurs on a time scale much slower than monomer enchainment (Scheme 8 Figure 8, pathway 7). This mechanism is a topic of considerable debate (vide infra). 

Statistical modeling of pentad distributions of polypropenes prepared with these catalyst systems can be satisfactorily done using a two-site modeF - based on a mixing of a chain-end-controlled site (to model the atactic blocks) and an enantiomorphic site (for isotactic blocks). ... 

In 1995 Coates and Waymouth ° reported a catalyst that produced polypropylene-containing blocks of atactic polypropylene and isotactic polypropylene. This novel elastomeric material was referred to as elastomeric homopolypropylene (EHPP). The unit cell of the crystal structure of the EHPP catalyst precursor bis(2-phenylindenyl)zirconium dichloride was observed to contain two distinct conformers see Figure 1 for the structures. In one conformer. 8M. the indenyl ligands were syn to one another (a meso stereochemistry), while in the other conformer. 8R. the indenyl ligands were anti to one another (a rac stereochemistry). Production of isotactic blocks of polypropylene could be explained by polymerization from the rac active site and production of atactic blocks by polymerization from the meso active site. In 1996 Pietsch and Rappe published a molecular... 

Compounds containing blends include different polymer systems where the polymers can contain blocky segments, crystalline portions, isomers such as cis-, trans-, and vinyl-butadiene, or isotactic or atactic blocks. 

Atactic block n. A block of chain units in a polymer of copolymer that has a random distribution of equal numbers of the possible configurational base units. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. Mark JE (ed) (1996) Physical properties of polymers handbook. Springer-Verlag, New York. 

A special case of control over the structure of copolymers may include the first stereoblock copolymers made by CRP. By applying either RAFT or ATRP to polymerization of acrylamides in the presence of rare-earth triflates such as Y(OTf)3 and Yb(OTf)3, it was possible to enhance isotacticity of A(A/-dmiethylacrylamide (DMA) from 50% meso to 90% meso dyads (170). At the same time control of molecular weights and polydispersity was preserved. Similar results were obtained for RAFT of A(-ispropylacrylamide (171). The ATRP and RAFT of DMA was applied to the first one-pot stereoblock synthesis by radical mechanism. RAFT or ATRP of DMA were started without Lewis acid to produce the first atactic block. Subsequently, the complexing agent was added at the desired conversion to continue the chain growth with the preferential isotactic placement (170). 

Razavi, A. Syndiotactic/atactic block polyolefins, catalysts and processes for prodncing the same. PCT Int. Pat. Appl. WO 98/02469A1 (Fina Research S.A.), January 22, 1998. 

In the case of inherently more complicated stereoblock polypropylenes containing (close-to-)atactic blocks, with all possible stereosequences present in comparable amounts, the requirements for a meaningful microstructural analysis are even more stringent. Routine NMR characterization of propylene polymers gives access to the stereosequence distribution at the pentad... 

After rinzinger s initiator discussed above, a variety of metallocene- and non-met-allocene-based classes of ethylene polymerization initiators also including lanthanide metallocene complexes have been developed In the metallocene series, particularly noteworthy are ordan s MAO-free 1 -electron catalyst p r 1 h and Waymouth s bis 3 phenyl indenyl zirconium dichloride, the latter providing isotactic-atactic block polypropylene... 

In the polypropylene catalyst shown in Figure 14, an isotactic-atactic block copolymer can be formed by rotation of one ring relative to the Zr-centroid axis. (For descriptions of polymer stereochemistry, see Fig. 9.) The isospecific rac rotomer of the catalyst gives rise to the isotactic block, while the aspecific meso form gives rise to the atactic block (Fig. 14). Using UFF (4) and RFF (85,86) (as weU as ab initio methods and DFT), the workers were able to confirm experimental evidence (89,90) that the indenyl substituent, R in Figure 14, could influence the equilibrium between the rac and meso rotomers. Using RFF the workers were able to successfully predict the relative amount of isotactic and atactic blocks in the polymer and to correlate that with R. 

The polymerization of styrene anionically produces an atactic block. The typical end block molecular weight for polymers used in pressure-sensitive adhesives is below the 18,000 Da entanglement molecular weight of polystyrene (see Table 15.3). Thus the softening point of these polymers is less than that of pure polystyrene. Like the tackifiers and oils discussed below, the polystyrene end blocks are in the oligomeric region where properties still depend on molecular weight (see O Fig. 15.18). 

The ease of ligand design in metallocene catalysts has made possible novel stereoregulated polymers that have not been seen with conventional Ziegler-Natta catalysts. Hemi-isotactic polyolefins, and isotactic-atactic block polymers are examples that will be discussed in Sect. 2.5. 

---