Studies on the synthesis and physico- chemical properties of polyperoxides and copolyperoxides

Abstract

The synthesis and spectroscopic characterization of three polymeric peroxides of styrene monomers with substituents in the para position are discussed. NMR spectroscopy revealed the alternating copolymer structure with labile peroxy bonds in their main chain. The thermal reactivity of the polymers was studied by differential scanning calorimetry and thermogravimetry. The measured heat of degradation of these polymers is nearly the same as that of poly(styrene peroxide). The mechanism of the primary exothermic degradation has been substantiated by thermochemical calculations. Chain?dynamics studies of these polyperoxides in terms of ¹³C spin?lattice relaxation time (T?) have been carried out to understand their main?chain flexibility. The temperature?dependence of the correlation time has been used to determine the activation energy for overall segmental motion and internal group rotation. Their flexibility in terms of glass?transition temperature (Tg) has also been examined. The flexibility trend observed in solution parallels that in the bulk. The peroxide macroinitiator has been used for the polymerization of styrene. The peroxide macroinitiator can be used as an effective high?temperature initiator, and the polymerization obeys classical kinetics.

The thermal degradation of three polymeric peroxides of styrene monomers with para?substituted groups was studied at various temperatures (65, 75, 85, and 95?°C). A continuous?distribution model was used to evaluate rate coefficients for random?chain and chain?end scission degradation from the evolution of molecular?weight distributions (MWDs). The activation energies from temperature?dependent rate coefficients fall in the range 18–22?kcal?mol?¹, indicating that thermal degradation is controlled by dissociation of the peroxide (–O–O–) bonds in the polymer backbone. The thermal stability of poly(p?methylstyrene peroxide) (PPMSP) lies between that of poly(p?tert?butylstyrene peroxide) (PPTBSP) (highest stability) and poly(p?bromostyrene peroxide) (PPBrSP) (lowest stability).

Oxidative copolymerization of indene with styrene, ??methylstyrene, and ??phenylstyrene has been investigated. Copolyperoxides of different compositions were synthesized by free?radical?initiated oxidative copolymerization. Compositions obtained from ¹H and ¹³C NMR spectra were used to determine monomer reactivity ratios. These ratios indicate that indene forms:

ideal copolyperoxides with styrene and ??methylstyrene alternating copolyperoxides with ??phenylstyrene

Thermal degradation studies by DSC and EI?MS support the presence of alternating peroxide units in the copolyperoxide chain. Activation energies again suggest degradation via peroxide bond dissociation. Flexibility in terms of Tg has also been evaluated.

The copolymerization of methyl methacrylate (MMA) with vinyl acetate (VAc) under high oxygen pressure was investigated. Copolyperoxides of various compositions were obtained by free?radical?initiated oxidative copolymerization. NMR?based compositions were used to determine reactivity ratios, which indicate a larger proportion of MMA units randomly placed in the copolyperoxide. Semi?empirical AM1 theoretical analysis supports these ratios. NMR studies showed irregularities in the chain due to cleavage reactions of the propagating peroxide radical. Thermal analysis by DSC confirmed the presence of alternating peroxide units. Activation?energy measurements confirm degradation via peroxide bond dissociation.

This is the first report on high?pressure free?radical?initiated oxidative copolymerization of styrene with ??methylstyrene (AMS) at temperatures 45–65?°C at 100?psi oxygen, and at pressures 50–300?psi at constant 50?°C. Reactivity ratios (from ¹H NMR) indicate that styrene forms an ideal copolyperoxide with AMS, and the copolymer is richer in AMS. Effects of temperature and oxygen pressure on reactivity ratios were studied. Rates of copolymerization (Rp) were used to determine overall activation energies (Ea) and activation volumes (?V‡). Unusually high ?V‡ values may originate from:

oxygen as a pressurizing reactive fluid, side reactions, and chain?transfer reactions occurring during copolymerization.

Copolyperoxides of indene with methyl, ethyl, and butyl acrylates were synthesized by free?radical oxidative copolymerization, although their ordinary copolymers cannot be synthesized under similar conditions. NMR data show random placement of indene units. Chain irregularities due to peroxide?radical cleavage were observed. DSC analysis supports alternating peroxide units. Activation energies indicate degradation via peroxide bond dissociation. Flexibility (Tg) has also been evaluated.

Polymethacrylonitrile peroxide (PMNP) has been synthesized from methacrylonitrile by free?radical?initiated oxidative polymerization and characterized spectroscopically. NMR confirmed an alternating copolymer with labile peroxy bonds. Extreme instability of PMNP was seen in FTIR spectra. Thermal degradation studies (DSC and TGA) revealed:

highly exothermic degradation heat of degradation ? 42.5?kcal?mol?¹ (similar to vinyl polyperoxides)

EI?MS fragmentation was examined. Thermochemical calculations substantiate the mechanism of primary exothermic degradation. Chain dynamics were studied via ¹³C T? relaxation of main?chain and side?chain carbons. Temperature?dependence of T? shows that PMNP is more flexible than poly(styrene peroxide). Keywords: Polymethacrylonitrile peroxide; Exothermic thermal degradation; Chain dynamics.

Vinyl polyperoxides-alternating copolymers of vinyl monomers and oxygen-are a unique class of polymers that degrade highly exothermically, unlike most polymers which degrade endothermically. These polyperoxides are typically amorphous viscous materials due to flexible peroxide linkages in the backbone. The present investigation addresses:

synthesis, structure–flexibility correlation, chain?dynamics behaviour, and oxidative copolymerization mechanisms

of vinyl polyperoxides.

The synthesis, characterization, thermal reactivity, and comparative chain dynamics of three polyperoxides-poly(p?methylstyrene peroxide), poly(p?bromostyrene peroxide), and poly(p?tert?butylstyrene peroxide)-have been studied. Oxidative copolymerization of indene with styrene, ??methylstyrene, and ??phenylstyrene was studied. Reactivity ratios show that indene forms ideal copolyperoxides with styrene and ??methylstyrene, but alternating copolyperoxides with ??phenylstyrene. Similarly, while normal copolymerization of indene with methyl, ethyl, and butyl acrylates is not feasible, their copolyperoxides were successfully synthesized and characterized. Oxidative copolymerization of methyl methacrylate and vinyl acetate was investigated to study how peroxide linkages (–O–O–) influence transmission of reactivity information along the hydrocarbon backbone. Temperature and oxygen pressure strongly influence the oxidative copolymerization of styrene with ??methylstyrene. Reactivity ratios show styrene forms an ideal copolyperoxide with AMS, richer in AMS content. Temperature and oxygen?pressure effects on reactivity ratios were also studied.