JEE Challenger
More from p-Block Elements
ChemMedAdv 2015 • Paper 2

Linear Polymer Preparation and Chain Termination Compounds in Hydrolysis

Under hydrolytic conditions, the chemical species utilized for the synthesis of a linear polymer and for terminating the chain, respectively, are

Options

A

CH3SiCl3\text{CH}_3\text{SiCl}_3 and Si(CH3)4\text{Si}(\text{CH}_3)_4

B

(CH3)2SiCl2(\text{CH}_3)_2\text{SiCl}_2 and (CH3)3SiCl(\text{CH}_3)_3\text{SiCl}

Correct
C

(CH3)2SiCl2(\text{CH}_3)_2\text{SiCl}_2 and CH3SiCl3\text{CH}_3\text{SiCl}_3

D

SiCl4\text{SiCl}_4 and (CH3)3SiCl(\text{CH}_3)_3\text{SiCl}

Topics & Concepts

p-Block Elementsp-Block Elements

Step-by-Step Solution

To determine the correct compounds used for the synthesis of a linear polymer and for chain termination under hydrolytic conditions, let us analyze the hydrolysis and condensation reactions of substituted chlorosilanes:

  1. Preparation of Linear Polymer:

    • A difunctional monomer is required to form a two-dimensional, linear chain polymer (straight-chain silicone).
    • Hydrolysis of dichlorodimethylsilane, (CH3)2SiCl2(\text{CH}_3)_2\text{SiCl}_2, yields dimethylsilanediol: (CH3)2SiCl2+2H2O⟶(CH3)2Si(OH)2+2HCl(\text{CH}_3)_2\text{SiCl}_2 + 2\text{H}_2\text{O} \longrightarrow (\text{CH}_3)_2\text{Si}(\text{OH})_2 + 2\text{HCl}
    • Subsequent intermolecular condensation polymerization (elimination of water molecules) of (CH3)2Si(OH)2(\text{CH}_3)_2\text{Si}(\text{OH})_2 leads to the formation of a continuous linear silicone polymer: n (CH3)2Si(OH)2⟶–[O–Si(CH3)2–]n–+n H2On\,(\text{CH}_3)_2\text{Si}(\text{OH})_2 \longrightarrow \text{--}\left[\text{O--Si}(\text{CH}_3)_2\text{--}\right]_n\text{--} + n\,\text{H}_2\text{O}

  2. Chain Termination:

    • A monofunctional monomer is required to cap the active terminal −OH-\text{OH} groups and control the polymer chain length.
    • Hydrolysis of chlorotrimethylsilane, (CH3)3SiCl(\text{CH}_3)_3\text{SiCl}, gives trimethylsilanol: (CH3)3SiCl+H2O⟶(CH3)3SiOH+HCl(\text{CH}_3)_3\text{SiCl} + \text{H}_2\text{O} \longrightarrow (\text{CH}_3)_3\text{SiOH} + \text{HCl}
    • Since (CH3)3SiOH(\text{CH}_3)_3\text{SiOH} possesses only one reactive hydroxyl group (−OH-\text{OH}), it condenses with a growing polymer chain end, blocking further propagation and terminating the chain: ∼Si(CH3)2–OH+HO–Si(CH3)3⟶∼Si(CH3)2–O–Si(CH3)3+H2O\sim\text{Si}(\text{CH}_3)_2\text{--OH} + \text{HO--Si}(\text{CH}_3)_3 \longrightarrow \sim\text{Si}(\text{CH}_3)_2\text{--O--Si}(\text{CH}_3)_3 + \text{H}_2\text{O}

  3. Analysis of Other Options:

    • CH3SiCl3\text{CH}_3\text{SiCl}_3 (trifunctional) yields cross-linked, three-dimensional network polymers.
    • SiCl4\text{SiCl}_4 (tetrafunctional) forms highly cross-linked silica networks.
    • Si(CH3)4\text{Si}(\text{CH}_3)_4 lacks reactive Si–Cl\text{Si--Cl} bonds and does not undergo hydrolysis.

Therefore, the species used for linear polymer preparation and chain termination are (CH3)2SiCl2(\text{CH}_3)_2\text{SiCl}_2 and (CH3)3SiCl(\text{CH}_3)_3\text{SiCl}, respectively.

Correct Option: B

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