What is a good radical initiator?
Ideally, a thermal free radical initiator should be relatively stable at room temperature but should decompose rapidly enough at the polymer-processing temperature to ensure a practical reaction rate.
What is initiator efficiency?
Initiator efficiency f is defined as the fraction of primary (initiator-derived) free-radical species that add to monomer molecules. The primary fragments from DTBP decomposition cannot undergo cross-disproportionation, as hydrogen atoms in β-position to the oxygen-centered free-radical site are not available.
What does a radical initiator do?
In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions. These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such as polymer synthesis.
Why is benzoyl peroxide a good initiator?
The by far most important organic peroxide initiator is benzoyl peroxide (BPO). The two fragments with unpaired electrons are called free radical initiators. Following its generation, the initiating free radicals react with a monomer unit thereby creating growing polymer chains.
What is a radical initiator organic chemistry?
A radical initiator is a species that acts as the reactant of the initiation step of a radical chain reaction but does not participate in any of the propagation steps. Since the only function of a radical initiator is the generation of the first radical in the system, it is used in trace amounts.
What is initiator efficiency in chain initiation step of free radical polymerization?
Free-Radical Polymerization A significant proportion of the primary radicals that are generated are not captured by monomers, and the initiator efficiency f in Eq. (8-10) is normally in the range 0.2–0.7 for most initiators in homogeneous reaction systems.
What is the role of initiator in free radical polymerization?
To initiator: a polymer chain reacts with an initiator, which terminates that polymer chain, but creates a new radical initiator (Figure 21). This initiator can then begin new polymer chains. Therefore, contrary to the other forms of chain transfer, chain transfer to the initiator does allow for further propagation.
What is the role of initiator in addition polymerization?
initiator, a source of any chemical species that reacts with a monomer (single molecule that can form chemical bonds) to form an intermediate compound capable of linking successively with a large number of other monomers into a polymeric compound.
What is the efficiency of an initiator in a reaction?
A significant proportion of the primary radicals that are generated are not captured by monomers, and the initiator efficiency f in Eq. (8-10) is normally in the range 0.2–0.7 for most initiators in homogeneous reaction systems. It will be lower yet in polymerizations in which the initiator may not be very well dispersed.
What is the efficiency of azo compound initiators?
The efficiency for generation of ‘useful’ radicals from azo compound initiators is low, typically 50–70%. 17–19 The remaining radicals are lost as cage recombination products.
Why does F decrease with conversion to initiator?
These include cage reaction of the initiator-derived radicals, primary radical termination, and transfer to initiator. The relative importance of these processes depends on monomer concentration, medium viscosity, and many other factors. Thus f is not a constant and typically decreases with conversion.
What is the efficiency of an initiator for styrene?
For a styrene (S) polymerization in 50% (v/v) benzene initiated by AIBN, the ‘instantaneous’ initiator efficiency varies from 76% at low conversion to 20% at 90–95% conversion.20 The common assumption that the rate of initiation (kdf) is invariant with conversion cannot be supported.