Catalyst system: the low-pressure Ziegler process for polyethylene employs which catalyst combination?

Introduction / Context:
Ziegler–Natta polymerisation revolutionised polyolefins by enabling low-pressure synthesis of linear, high-crystallinity polymers. Recognising the hallmark catalyst system helps connect process conditions with product properties (e.g., HDPE vs LDPE).

Given Data / Assumptions:

• We refer to classic heterogeneous Ziegler–Natta catalysis for polyethylene.
• Transition-metal halides and organo-aluminium compounds are typical components.
• Low-pressure operation is implied.

Concept / Approach:
The archetypal system uses a titanium halide (commonly TiCl4 supported on MgCl2 in modern variants) combined with an organo-aluminium cocatalyst such as triethylaluminium (AlEt3). The pair activates insertion polymerisation of ethylene with controlled stereochemistry and suppressed branching, yielding HDPE. Nickel, vanadium pentoxide, and iron alone are not the characteristic Ziegler polyethylene catalyst pair in the low-pressure route posed here.

Step-by-Step Solution:
Recall Ziegler–Natta pair → TiCl4 + AlEt3 (or related Al-alkyl).Associate with low-pressure PE → HDPE via insertion mechanism.Eliminate single metals/oxides that are not the classic combination.

Verification / Alternative check:
Polymerisation texts and industrial practice cite TiCl4/Al-alkyl systems (and later MgCl2 supports) as standard for Z-N catalysis of ethylene/propylene.

Why Other Options Are Wrong:
Ni, V2O5, Fe: not the canonical low-pressure Ziegler PE catalyst pair indicated in the question.

Common Pitfalls:
Confusing Phillips chromium catalysts (Cr/SiO2) with Ziegler–Natta; the question specifies the Ziegler route with organo-aluminium cocatalyst.

Final Answer:
Aluminium triethyl combined with titanium tetrachloride