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VCM POLYMERIZATION
MECHANISM

Individual polymer chains by Yurko - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4285334
Free radical polymerization is commonly summarized with the following reactions:
A. INITIATION, which includes both initiator decomposition to form two radicals and the subsequent addition of the first monomer unit to initiator radical.
B. PROPAGATION, the fast addition of successive monomer units into a growing polymer chain.
C. TERMINATION, the reaction of two growing macroradicals that stops chain growth and form either one (by combination) or two (by disproportionation) dead polymer chains.
D. CHAIN TRANSFER, the loss of a small molecular fragment from a growing macroradical to a molecular entity that results in one dead polymer chain and a new growing chain.
However, the chemistry of vinyl chloride polymerization is much more complex, as can be seen in the following scheme that relates the main reactions that have been identified by several Research groups by analyzing the chemical structures present in commercial PVC.

This complexity arises from the fact that vinyl chloride radical is highly reactive and react with various molecular entities when monomer local availability is restricted, so reaction pathways are strongly influenced by conversion since VCM polymerization passes through 3 stages defined by PVC/VCM compatibility.:
STAGE 1. Homogeneous, below 0.01% conversion
Consisting of a single phase of liquid VCM where formed PVC chains are in solution
STAGE 2. Heterogeneous, between 0.01% and around 68% conversion
Where a liquid VCM-rich phase that decreases in quantity coexists with a PVC/VCM gel phase with constant composition that increases in quantity until liquid VCM-rich phase is exhausted
STAGE 3. Homogeneous, above 68% conversion
Consisting of a single PVC/VCM gel phase with decreasing content of VCM while being consumed

Note: Although VCM can also polymerize in small amounts in vapor and aqueous phases, these reactions are commonly overlooked and not considered significant for the fabrication of PVC resins, but they influence very important aspects that affect process performance such as droplet stability (copolymerization with Primary Suspending Agents) and fouling of reactor internals (polymerization in gas and aqueous phases).
As each individual reaction has a different reaction rate dependence, molecular weight can be controlled by selecting a suitable reaction temperature (commercial process is mostly isothermal) to increase or decrease the relative importance of the propagation reaction versus transfer and termination reactions.
Global Kinetics will not only define Conversion and Production rate (Productivity) but also the population distributions of polymer chain lengths (MWD) and chemical structures (defects) that determine both Mechanical strength and Thermal stability as well as grain's Internal Morphology.
MAIN VCM POLYMERIZATION REACTIONS
1. INITIATOR DECOMPOSITION
The decomposition of initiators by homolytic cleavage generates free radicals necessary to initiate polymerization.
The most important property of an initiator is its rate of radical formation, which can be expressed by its "Half Life", which is defined as the time necessary to halve initiator concentration in a dilute solution at constant temperature.

2. CHAIN INITIATION
Approximately 20% of polymer chains have initiator groups. It happens mainly in VCM liquid phase.

3. HEAD-TO-TAIL ADDITION
It is the main propagation step.

4. HEAD-TO-HEAD ADDITION
Around 4.7-4.9 per 1,000 monomeric additions show structures related to this addition, increasing as polymerization temperature increases.

5. CHLORINE SHIFT
It is so rapid that head-to-head structures are negligible in commercial PVC.

6. CHLORINE ABSTRACTION
0.7 chloroallyl end groups are formed per 1000 units of polymerized monomer. It is considered as a single reaction as there is no evidence of kinetically free chlorine radicals. Analysis of the structures in commercial PVC by Hjerteberg shows that at least 80% of the PVC chains are started by this reaction, since there are 0.8-0.9 terminal structures of the type (ClCH2-CHCl) - for each molecule.


7. H-ABSTRACTION FROM POLYMER
The attack of radicals on polymer (<0.7 times per 1000 polymerized monomeric units) is the origin of long branches (by propagation on secondary macroradical) and internal chloroallyl structure (by chlorine abstraction from VCM), which strongly affect Thermal Stability.

8. TERMINATION BY COMBINATION
Studies indicate that this reaction is carried out in a maximum of 5% of polymer chains.

9. TERMINATION BY DISPROPORTIONATION
It has been determined that between 19% and 40% of the molecules end up through a bimolecular mechanism, most of them (15-35%) being by disproportionation.

10. TERMINATION BY INITIATOR
The importance of this reaction grows when there is a local excess of initiator.

11. TERMINATION BY CHLORINE ABSTRACTION
Together with (16), this reaction ends between 60% and 80% of all polymer chains.

12. TERMINATION BY INHIBITION
An inhibitor reacts with any free radical much faster than VCM so that polymerization “stops” until all the inhibitor has been consumed, at which point polymerization resumes at the same rate as it would in the case of have no inhibitor.

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(compared with resins of similar K-value?
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