Okay, let's see. The question is about polymerization using ethylene with a dual catalyst system. The scientist wants to add a second catalyst to introduce regular branches in the polymer backbone. He consults a senior scientist who gives four statements, and we need to figure out which one is correct.

First, I need to understand the context. Ethylene polymerization typically forms linear polymers. To introduce branches, which would make the polymer more branched, you need a different reaction step. So, the first catalyst is probably for the main polymerization, and the second catalyst would handle the branching.

The options are A, B, C, D. Let's go through each statement.

Statement A: "Certain noble metal catalysts can be used but are too expensive." Noble metals like platinum, palladium, etc. are often used as catalysts. But if they're too expensive, that might be a reason why they aren't used. However, the statement says "can be used but are too expensive." So it's possible, but I need to check the other statements.

Statement B: "Such combined systems are already implemented on an industrial scale in the US." If the senior scientist says they are already implemented, that would be a strong statement. But the question is about the correctness of the statements. If the answer is about which statement is correct, then if B is true, that's a possible answer. However, I need to think if such systems are actually in use. From what I remember, most industrial polymerizations like polyethylene use transition metal catalysts, but adding branching might require different catalysts. I'm not sure if dual catalyst systems for branching are common. Maybe not. So maybe B is incorrect.

Statement C: "One can use a catalyst of a group VIa transition metal in combination with specific activators." Group VIa transition metals are like chromium, molybdenum, tungsten. These are known for homogeneous catalysts. For example, metallocene catalysts. If they're used with activators, maybe for cross-coupling reactions or something else. But in polymerization, activating certain steps? Maybe. But I'm not sure if group VIa metals are typically used for branching reactions. They are more often used for the main polymerization.

Statement D: "Aluminum-based activators do not work for the essential additional reaction step." Aluminum-based activators are used in some polymerizations, like in the Ziegler-Natta catalysts. But if the additional step (branching) requires something else, then aluminum-based might not work. For example, some branching reactions use different initiators or initiators with metals. So D could be correct if aluminum activators can't handle the branching step.

Now, the question is which statement is correct. Let's think about each.

If statement B is correct, that would mean that such combined systems exist. But I'm not sure if that's true. I recall that most industrial polymerizations are done with a single catalyst. For branching, sometimes they use different initiators or different transition metals. For example, in polypropylene production, sometimes a different catalyst is used for the isoprene polymerization. But dual catalysts? Maybe not. So perhaps statement B is incorrect.

Statement A is about noble metal catalysts being expensive. Noble metals are indeed expensive, so this makes sense. But the statement is "certain noble metal catalysts can be used but are too expensive." That seems plausible. However, the question is which statement is correct. If the senior scientist is saying that, then A would be correct. But wait, the question is about the correctness of the statements. So if the statement is true, then which one is correct. So if the senior scientist is correct, then which statement is correct according to him.

Wait, the question is asking which of the four statements is correct regarding the formation of a polymer with regular branches using only ethylene and a dual catalyst system. The senior scientist's statements are given as possible answers, and we need to determine which is correct.

So the options are the statements A, B, C, D. We need to pick the correct one.

If the statement is correct, then which is true. Let's think about each.

Statement B says such systems are already implemented. If that's true, then B is correct. But I believe that most industrial polymerizations use a single catalyst. Branching is often achieved by different steps or different catalysts. For example, in polypropylene production, they use different monomers or different catalysts. So maybe dual catalysts for branching aren't common. So B might be incorrect.

Statement C: Group VIa transition metals. I know that group VIB (like Ni, Pd) are used in metallocene catalysts. Group VIa (Cr, Mo, W) are used in something else. Wait, for example, in homogeneous catalysts, group VIa metals are used. For example, in some emulsion polymerizations. But for ethylene polymerization, group VIa might not be the primary choice. So maybe C is incorrect.

Statement D: Aluminum-based activators don't work for the essential additional step. If the additional step (branching) requires a different type of activator, then D would be correct. Aluminum-based activators are used in some cases, but not sure if they can't handle branching. For example, in Ziegler-Natta catalysts, Ti-based activators are used with metals like Zn. So if the additional step needs, say, a different initiator or a different type of metal, then aluminum-based activators might not work. So D could be correct.

Alternatively, if the additional step requires a different metal, then group VIa (Cr, Mo) might be used with specific activators. So statement C could be correct. But I'm not sure.

Wait, the question is about which statement is correct. Let's think again.

If the senior scientist says that group VIa catalysts can be used with specific activators (C), but noble metal catalysts are expensive (A). Also, whether such combined systems are implemented (B), and whether aluminum-based activators don't work (D).

I think that group VIa transition metals are not typically used for branching in ethylene polymerization. They are more often used for the main polymerization. So statement C is incorrect.

Aluminum-based activators are used in some processes, but for branching, maybe they are not suitable. So D might be correct.

Alternatively, if the additional step requires a different catalyst, then D is correct. If the additional step can be done with the same catalyst, then maybe not. But the question states that a dual catalyst system is intended. So they need a different catalyst for the additional step.

If the additional step (branching) requires a different type of catalyst, then aluminum-based activators might not work. So D is correct.

But I'm not entirely sure. Let me think again.

Another angle: in ethylene polymerization, branching can be achieved by using a different initiator or a different type of catalyst. For example, in free radical polymerization, different initiators can lead to different branching. But the question mentions a dual catalyst system. So maybe they are using a different metal for the branching step.

If the main catalyst is a transition metal (like a group VIB metal) and the secondary catalyst is another transition metal (group VIa) with specific activators, then statement C could be correct. But I'm not sure if group VIa are typically used for the additional step.

Alternatively, the additional step might require a different type of metal, like a noble metal, but they are expensive (A). If the statement is correct, then A is correct. But I don't know if noble metals are used in dual systems.

Wait, the question is asking which of the four statements is correct. The answer choices are A, B, C, D. The correct answer is one of them.

Given that, I think statement D is correct. Because aluminum-based activators are not suitable for the branching step. For example, in some cases, the branching requires a different initiator or a different type of metal. So if the additional step needs a different type of catalyst, then aluminum-based activators wouldn't work. So D is correct.

Alternatively, if the additional step can be done with the same catalyst, then D is incorrect. But the question says "intends to add a second catalyst system", so they need a different catalyst. Therefore, the activators for the additional step would need to be different. Hence, aluminum-based activators might not work. So D is correct.

But wait, the question is about the correctness of the statements according to the senior scientist. The senior says that noble metal catalysts can be used but are too expensive. If noble metal catalysts are too expensive, then statement A is correct. But if the answer is D, then statement D is correct.

Alternatively, maybe the statement B is correct because the senior says that combined systems are already implemented. But I don't think that's the case. Most industrial polymerizations are done with a single catalyst. So B is likely incorrect.

Therefore, among the options, D is correct. Because aluminum-based activators don't work for the additional step. So the correct answer is D.
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