Chain shuttling agent

Fig. 18 Several samples of ethylene-octene copolymers having similar comonomer content, crystallinity, and melt index. Sample 3 is a physical blend of high and low density copolymers. Samples 4-6 are OBCs prepared with several different levels of chain shuttling agent. Reproduced by permission from [10]...

The catalytic system used to make OBCs uses a chain-shuttling agent (CSA) to shuttle or transfer growing chains between two distinct catalysts with different comonomer (alpha-olefm) selectivity." This is shown in Figure 9. Synthesis of olefin block polymer via chain shuttling requires the chain transfer to be reversible. OBCs are produced in a continuous solution polymerization process more economically favorable than the batch processes employed to make styrenic block copolymers. 

Pathway (1) has recently found an interesting demonstration in the related area of ethylene/1-alkene multiblock copolymerization, with ZnR2 acting as the chain shuttling agent. In propylene homopolymerization, presumptive evidence of isotactic/atactic stereoblock chain formation with a similar mechanism was reported by Chien et al. for polypropylenes obtained with mixtures of... 

The demonstration of the viability of chain shuttling between different active species in mixed catalyst systems may offer alternative and possibly easier routes into well-controlled stereobloek polypropylenes and block (co)polymers in general. The fundamentals of this process still need to be better understood, but the fact that existing catalysts with a known behavior ean be employed makes the search for effective chain shuttling agents and conditions potentially of very broad scope and perspective. 

The [Zn]/[C2H4] ratio is defined as the molar ratio of chain shuttling agent to ethylene in the reactor. 

Figure 24 Differential scanning calorimeter (DSC) traces for OBCs made using two catalysts and a chain shuttling agent. The polymers have nominally the same soft segment composition but varying hard segment compositions.

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