Diblock OBCs, CCTP

The advantages provided by the continuous reactor prompted us to explore CCTP in a reactor with two CSTRs connected in series [11]. This reaction scheme depicted in Scheme 6 provides a highly flexible process for production of a wide range of diblock OBC compositions. The block composition can easily be varied by changing the production rate in either reactor. The comonomer content of either block can also be independently tailored by varying the feed compositions because the process operates in two independent reactors. This CCTP scheme also produces multiple chains per catalyst, an advantage over stoichiometric living polymerization systems, but is necessarily stoichiometric in CSA. The reaction produces... 

The two reactor feeds were controlled to give copolymers with the desired densities, and a physical blend and a diblock OBC were produced. DEZ was added to the first reactor to achieve the desired melt index (/2 = 20 dg min1, equivalent to a Mn of -15-20 kg mol1). This material was fed to the second reactor, and production was continued under different conditions. The material collected after the second reactor had a lower melt index (/2 = 3.9 dg min1), indicating a higher molecular weight consistent with the chain extension reaction from the CCTP process. 

Table 6 Product and process details for production of a diblock OBC using CCTP...

The GPC traces in Fig. 24 reveal a broad molecular weight distribution, MJMn = 4.42, for the dual reactor blend sample. On the other hand, the diblock OBC displays an overall MJMn of 1.67. The narrowing of the distribution indicates that the polymerization has CCTP characteristics. The theoretical molecular weight distribution from an ideal living polymerization in a series of two CSTR reactors is given by the following equation, where/j and/2 are the mass fractions of polymer comprising the two blocks [11] ... 

The theoretical lower limit of the molecular weight distribution for the diblock OBC is 1.58. The observed MJMn of 1.67 indicates that the sample contains a very large fraction of polymer chains with the anticipated diblock architecture. The estimated number of chains per zinc and hafnium are also indicative of a high level of CCTP. The Mn of the diblock product corresponds to just over two chains per zinc but 380 chains per hafnium. This copolymer also provides a highly unusual example of a polyolefin produced in a continuous process with a molecular weight distribution less than that expected for a polymer prepared with a single-site catalyst (in absence of chain shuttling). 

Diblock OBCs from continuous CCTP are different from either of the previous two families of copolymers. The continuous process produces blocks lengths with MJMn approaching 2.0. However, the CCTP process in a series of reactors results in a narrower overall distribution, with M JMn approaching 1.5 in the case of a symmetric diblock. The number of blocks per chain is determined by the number of reactors connected in series. Finally, the dual reactor scheme ensures precise block junctions with homogeneous copolymer compositions. 

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