Michael Fisch

Degradation and stabilization of poly (vinyl chloride). V. Reaction mechanism of poly(vinyl chloride) degradation

The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate and a series of chain reactions that generates additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures the thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. It can be catalyzed by a molecule of hydrogen chloride that makes possible 1,2-elimination through a transition state of six centers with much lower activation enthalpy but also with very low activation entropy. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis- to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans- double bond is formed and no hydrogen chloride is present to catalyze trans-cis- isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn- or 1,4-cis- mechanisms promoting a so-called ‘catastrophic’, very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination due to a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.

Degradation and stabilization of poly(vinyl chloride). I. Kinetics of the thermal degradation of poly(vinyl chloride)

Abstract The kinetics of the thermal degradation of solid powdered poly(vinyl chloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution and rate of polyene sequence formation. Monoalkenes are present in commercial PVC and their concentration increases during thermal degradation. Sequences of 2 to 25–30 conjugated double bonds are formed in parallel reactions and their concentrations increase linearly versus time and extrapolate back through the origin. Their rate constants decay exponentially with the number of double bonds. These results can be accommodated by a chain mechanism involving initiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugated polyene sequence (Scheme 1). Structural irregularities such as allylic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation.

Degradation and stabilization of poly(vinyl chloride). II. Simulation of the poly(vinyl chloride) degradation processes initiated in the polymer backbone

Abstract The thermal degradation of PVC consists of a slow initiation followed by a chain of fast reactions generating a series of active intermediates with an increasing number of double bonds. Each of these intermediates partitions between a relatively stable polyene (termination reaction) and the next intermediate of the chain (propagation reaction) (Scheme 1). Experimental time dependencies of hydrochloric acid evolution, rates of polyene sequence and benzene formation in solid PVC degradation, data from the previous paper of this series, were simulated using a simplified model (Scheme 2). Mean rate constants and activation parameters for random initiation, propagation and termination reactions of the PVC degradation chain were calculated. All three types of reactions have low activation enthalpies and entropies suggesting concerted hydrogen chloride eliminations.

Degradation and stabilization of poly(vinyl chloride). III. Correlation of activation enthalpies and entropies for dehydrochlorination of chloroalkanes, chloralkenes and poly(vinyl chloride)

Abstract Activation enthalpy-entropy correlations for the experimental data available for dehydrochlorination of chloroalkanes and chloroalkenes in the gas and in the liquid phase or non-polar solvents show that this reaction has two possible mechanisms: chloroalkanes, 3-chloro-1-alkenes and trans 1-alkyl-3-chloro-1-alkenes (gas phase) eliminate HCl through a transition state of four centers requiring a synperiplanar conformation of the group (Scheme 1). Cis 1-alkyl-3-chloro-1-alkenes and trans 1-alkyl-3-chloro-1-alkenes (liquid phase) and conjugated 1-alkyl-5-chloro-1,3-alkadienes eliminate HCl through a transition state of six centers requiring a cis configuration of the double bond (Scheme 2). The activation parameters for the elementary reactions of PVC degradation fit on the same correlation lines supporting, for the initiation reaction, a mechanism involving a transition state of four centers and for propagation and termination reactions, a mechanism involving a transition state of six centers.

Degradation and stabilization of poly (vinyl chloride). IV. Molecular orbital calculations of activation enthalpies for dehydrochlorination of chloroalkanes and chloroalkenes

Abstract Molecular orbital calculations at the MNDO level with AM1 or PM3 parametrization give reliable activation enthalpies for chloroalkane and chloroalkene dehydrochlorination. Dehydrochlorination of chloroalkanes and some allyl chlorides is a molecular 1,2-elimination through a four center transition state generated in a synperiplanar conformation. The transition state for chloroalkanes requires very strong polarization of the carbon-chlorine bond. When structure allows, allyl chlorides eliminate hydrogen chloride in a 1,4 process through a six center transition state generated from a cis -configuration of the double bond. This transition state requires much lower activation enthalpy and less polarization of the molecule than does the 1,2 process. Initiation of PVC degradation by 1,2-elimination from normal chain residues, or at the very beginning of degradation from structural irregularities such as tertiary chlorine atoms, takes place through a four center transition state. Allylic chlorine atoms formed in this way or pre-existing in small amounts as structural irregularities have a reactivity not much different from the secondary chlorine atoms of the main chain of the polymer if they adopt a trans - configuration of the double bond. On the other hand, if allylic chlorine atoms have a cis - configuration, they rapidly eliminate hydrogen chloride, forming conjugated polyenes through a transition state of six centers. This elimination constitutes the chain reactions in the PVC degradation process. The chain stops when a relatively stable trans - conformation of the allylic chloride is formed.