Sequence-controlled polymers insertion

S. Motokucho, A. Sudo, F. Sanda, T. Endo, Controlled monomer insertion into polymer main chain synthesis of sequence ordered polystyrene containing thiourethane and trithiocarbonate units by the RAFT process, Chem. Commun. (2002) 1946-1947. 

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

It has been shown recently (10) that such block structures could be tailored precisely by the general method summarized hereabove. It is indeed possible to convert the hydroxyl end-group of a vinyl polymer PA (f.i. polystyrene, or polybutadiene obtained by anionic polymerization terminated with ethylene oxide),into an aluminum alcoholate structure since it is well known that CL polymerizes in a perfectly "living" manner by ring-opening insertion into the Al-0 bond (11), the following reaction sequence provides a direct access to the desired copolymers, with an accurate control of the molecular parameters of the two blocks ... 

Occasional defects in the polymer chain remain isolated. When propylene reacts from the opposite prochiral face bringing about a syndiotactic error (Scheme 13.7, a), the original chirality center preceding the error continues to be formed. The same is true for a secondary defect or type 2-1 insertion (Scheme 13.7, b) which is a head-to-head addition and creates a CH2-CH2 sequence. If the chirality were controlled by the growing chain end, an error would be perpetuated, giving rise to a polymer block with altered chirality. Instead, isotacticity is maintained in both cases. 

Catalysts based on the Hf pyridyl amine complexes 126-128 have been used for the preparation of ethylene/ propylene co-polymers as well as of ethylene/propylene/l-octene terpolymers. These co-polymers are characterized by having at least 60 wt% propylene units, Mw around 300000, and Mw/Mn in the range 2.0-2.4. The NMR analysis of these co-polymers showed that the propylene sequences are remarkably isotactic mm > 90%) and showed the presence of regioirregularly inserted propylene units(<0.5% mol). The most interesting property of catalysts based on 126-128 is their high thermal stability.1119 Using modifications of isospecific bis(phenoxy-amine)-based catalysts, such as complex 164, the controlled synthesis of iPP- /orjf-poly/E-co-P) diblock co-poly-mers has been achieved. This is a remarkable result since iPP and PE are both polymeric materials of extreme industrial relevance.1206... 

NMR m and r dyad analysis of syndiotactic polystyrenes prepared with MAO activated CpTiCb or Cp TiCl3 systems was in agreement with the Bernoullian symmetric statistical model for stereoselective propagation. The polymer microstracture is a long sequence of syndiotactic dyads with only isolated m defects and no consecutive isotactic dyads (Figure 14.17), which is consistent with a chain-end control mechanism directed by the configuration of the tertiary carbon of the last inserted styrene unit (unlike 1,3-asymmetric induction). Thus, the syndiotactic preference exerted by the chain end arises from the phenyl-phenyl repulsive interaction between the last inserted... 

These two mechanisms can be distinguished by observation of the stereoerrors found in the polymers. Catalytic-site controlled isotactic polymers have an error described in Fig. 4. One misinsertion has little effect on the face selectivity of the next insertion, because it is governed by the catalyst structure. It results in a sequence of mmmrrmmm. Thus mmmr, mmrr and mrrm pentads are found besides mmmm. On the other hand, in chain-end control, once a monomer is misinserted, the opposite chirality governs the next monomer insertion. It gives an mmmrmmm sequence, as shown in Fig. 4, and mmmr and mmrm appear in the spectra. 

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