How to deduce the polymerisation reaction for any alkene
4 marks · NormalThe polymer poly(methylpropene) is formed from the monomer CH2=C(CH3)2.
Draw the displayed formula of the monomer CH2=C(CH3)2.
Show the displayed formula of the repeat unit of poly(methylpropene). Label the continuation bonds at each end.
Explain why the repeat unit retains both methyl groups on the same carbon as in the monomer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Displayed formula for CH2=C(CH3)2: central C=C, one carbon bearing two CH3 groups and one H, the other bearing two H.
(b) Displayed repeat unit showing -[CH2–C(CH3)2]- with continuation bonds; two methyl groups on one carbon.
(c) During addition polymerisation, the side groups (such as methyl groups) remain attached to the same carbon atoms as in the monomer; only the double bond opens.
Must show: Correct arrangement of CH3 groups in a and b; continuity of structure in repeat unit
Common mistakes: Placing methyl groups on separate carbons; Missing or misplacing bonds
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly drawn displayed formula for CH2=C(CH3)2: central C=C, one carbon has two methyl groups and one hydrogen; other carbon has two hydrogens.
M1 (b) · 1 mark · Double bond opened to single bond with correct repeat unit formed.
A1 (b) · 1 mark · Continuation bonds shown at both ends; side groups positioned correctly.
B2 (c) · 1 mark · Explains that the methyl groups remain on the same carbon as the monomer because only the double bond opens; side groups remain unchanged.
Worked solution
a) Displayed formula of CH2=C(CH3)2 (can be shown as:
)
b) Repeat unit:
with continuation bonds shown at each end.
c) During addition polymerisation, only the C=C double bond opens; the methyl groups, being side chains, stay on the same carbon atom as in the monomer.