Anionic be carried out at low temperature, since

Anionic polymerization is a chain reaction
which, under appropriate conditions, proceeds without chain transfer reaction
and chain termination. Anionic polymerisation follows the principle of chain
growth polymerisation whereby firstly, it will initiate, followed by propagate
and lastly terminate. During the initiation stage, a negatively charged
carbanion group will be formed. The carbanion then reacts with another molecule
in the same way as how the first monomer molecule was reacted with the
initiator. This process continues and the polymer chain grows. This step is
called propagation. However, termination does not take place if the system has
no impurities. The carbanion will remain active unless a chain-terminating
agent, such as water, acid, or alcohol is added. This process is called living
anionic polymerisation. Living polymerisation is a chain polymerisation which
proceeds in the absence of kinetic steps of termination or chain transfer.1

 

 In
anionic polymerisation, an anionic initiator starts the process. Typical
initiators used are alkali metals, sodium naphthalene and Grignard reagents. Monomers
such as acrylic and methacrylic esters, styrene, acrylonitrile derivatives, butadiene
and vinylidene esters can be polymerised by anionic initiators.1 Anionic polymerisation of alkenes is difficult to achieve because
only a limited anions can add readily to alkene double bonds. Anionic
polymerization occurs readily only with alkenes substituted with sufficiently
powerful electron-attracting groups. The anionic polymerisation can only be
carried out at low temperature, since the Grignard compound can react with the
ester group of methyl methacrylate at higher temperature.2

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Thus, in comparison between all types of
polymerisation, anionic polymerisation is the most common technique to
synthesize a polymer with remarkable uniformity of chain length and predictable
molecular weight.3  In addition, anionic polymerisation has the
ability to differ the stereo-regularity of the synthesized polymer by varying
the factors such as solvent, counter anion, additive and temperature. The
alteration of factors will change the states of the growth chain and affect the
stereo-regularity of the resulting polymer.4 Ether solvents will produce syndiotactic whereas
hydrocarbon solvents isotactic polymers.

The tacticity of polymers can be determined
by using characterising techniques such as nuclear magnetic resonance (NMR)
spectroscopy.  NMR spectroscopy is a
technique used to determine a compound’s unique structure. It identifies the
carbon-hydrogen framework of an organic compound. Even though there are many
different modes of NMR, such as C-NMR and N-NMR, hydrogen (H-NMR) is most
commonly used.

The principle of NMR is that many nuclei
have spin and are electrically charged. The nuclear spins randomly and in
various directions without the presence of an external magnetic field. When an
external magnetic field is applied, the nuclei will align themselves and energy
will transfer from the base energy to a higher energy level. The energy
transfer takes place at a wavelength that corresponds to radio frequencies and
when the spin returns to its base level, energy is emitted at the same
frequency.5 The greater the
strength of the applied magnetic field, the larger the energy difference
between the two spin states.5 The frequency produced
is processed to generate an NMR spectrum. One of the most important applications
of H-NMR in the field of polymers is the determination of chain configuration.

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