Polymer Initiators

LDPE

Polymerization of ethylene can be done according to three mechanisms:

• Free radical polymerization yielding LDPE. Polymeric chains are branched allowing the material to have specific performances (rheology and mechanical properties).
• Ziegler Natta catalysis of HDPE or LLDPE by addition of co-monomers. Macromolecules are linear with some short branched segments.
• Metallocene catalysis yields linear and tailored structures leading to formation of special resins.

There are two main processes by which LDPE is produced via free radical chemistry:

• The tubular process
• The autoclave process

The table below compares their main characteristics:

	Tubular process	Autoclave process
Reactor volume (m3)	0.6-10	0.4-2.3
Reactor design	Tube hundreds of meters Inner diameter < 5cm	Multi-zone reactor, continuous stirring
Reactor cooling	yes	no
Monomer injection	mono or multi-injection	mono or multi-injection
Oxygen as initiator	yes	no
Peroxides as initiators	yes	yes
Peroxide injection	mono or multi-injection	multi-injection
Residence time(s)	20-80	20-80
Pressure (bar)	2200-2700	1300-2200
Temperature (°C)	130-325	160-310
Monomer conversation (%)	15-30	15-20

The mechanism of polymerization is the same as the one described in the mechanism of action section, but in this case, the propagating macroradicals can transfer to PE chains to generate short or long branched structure.

Short segments affect mechanical performance while long branched segment affect the rheological behavior.

The reaction takes place at temperatures between 100 to 300°C and at high pressure (1200-3000 bars). At these pressures ethylene liquefies and acts as a solvent under supercritical conditions.

Molecular weight and density is regulated by controlling the temperature and pressure, and by the addition of a chain transfer agent (e.g. propane).

The selection of the right organic peroxide is a critical parameter to consider for LDPE polymerization. The "initiator package" affects directly the polymer properties and also the productivity (% coversion, Cost /kg of monomer converted).