Chain dynamics and reactivity studies on polysulfide polymers

Abstract

Two new classes of polysulfide polymers, viz., PPMEPs and PPEPs, were synthesized and structurally well?characterized using spectroscopic techniques. The presence of unsaturated end groups in PPMEPs was confirmed by NMR, while the presence of hydroxyl terminals along with vinyl end groups in PPEPs was concluded based on NMR and FTIR studies. The occurrence of minor levels of monosulfide linkages in PPMED and PPED, and the presence of mono-, di-, and trisulfide linkages to some extent in PPMET and PPET, was ascertained based on the DP?MS investigations on these polymers. Finally, the ¹³C?T? relaxation studies showed that chain flexibility could be increased by incorporating heteroatoms, weak links of group VIA elements, and flexible spacers in the side chain of polysulfide polymers, while the flexibility in polysulfide polymers could be decreased by increasing the rank of the polysulfide linkage. Among the weak links of group VIA elements, the polysulfide linkages impart more flexibility than the peroxide linkages. The present study has shown that metamorphic oxidation of a disulfide polymer can be brought about by using either a strong oxidizing agent such as KMnO? or a mild oxidizing agent such as KHSO?. However, KMnO? also causes some degradation of the polymer chain during oxidation. It was also found that the degradation could be curtailed when the oxidation was carried out under heterogeneous conditions. The degradation of PSD during oxidation was confirmed by FTIR, FAB?MS, and [?] studies. The actual oxidation process was found to be complicated, and the product has a randomly oxidized structure. FTIR analysis of the oxidized polymers shows that both sulfoxide and sulfone groups are present. Increased rigidity of the oxidized polymers is reflected in their Tg. The flexibility of the different oxidized backbones was also examined by molecular dynamics (MD) simulation studies. It was found that flexibility decreases up to the structure with three oxygen atoms per repeat unit, but thereafter it increases for the disulfone. However, for the disulfone structure, the increase in flexibility was such that it was still lower than that of the parent disulfide polymer. The reversal in flexibility is explained on the basis of electrostatic repulsion of adjacent SO? groups in the fully oxidized disulfone structure, which induces a staggered conformation and consequently increases flexibility. The present study has practical relevance because many of these completely oxidized and partially oxidized sulfide polymers have potential industrial applications as compatibilizers, polymeric oxidants, etc. Another important aspect of the present study is that polydisulfones, which cannot be synthesized directly, can be prepared by oxidation of the disulfide polymer itself. Regarding the complete oxidation of PSD, it may be pointed out that in the present study a low?molecular?weight PSD was used, and complete oxidation was not carried out since the polymer would have degraded significantly. Hence, if the studies are conducted starting with a high?molecular?weight disulfide polymer, one can make polymers having only disulfone linkages. Also, to obtain an all?polydisulfone structure, further investigations are necessary to find suitable oxidizing agents and conditions. However, a copolymer having both SO and SO? groups can easily be prepared by the present method, and such polymers may exhibit novel properties. Another significant feature of the present study is that MD simulations could be successfully employed to compare the flexibility of polymer chains. A novel macroiniferter, PTBPMED, containing main?chain disulfide and side?chain peroxide linkages was synthesized by an interfacial polycondensation route and characterized by spectroscopic techniques. The ¹H NMR studies indicate the presence of pendant hydroxyl groups in PTBPMED, formed by the base hydrolysis of the peroxide bonds, while FTIR studies confirmed their presence. DP?MS studies reveal the presence of some mono? and trisulfide linkages along with the major disulfide linkages in the backbone of PTBPMED. Bulk free radical polymerization of styrene and MMA in the presence of PTBPMED showed an increase in the conversion to polymer with time as well as an increase in molecular weight with conversion— a characteristic feature of a polyfunctional initiator in free radical polymerization. The chain?transfer reactions of the main?chain disulfide linkages lead to a decrease in molecular weight with an increase in the concentration of PTBPMED, indicating that it acts both as an initiator and a chain?transfer agent, hence functioning as a macroiniferter in free radical polymerization. Finally, the g’ values for both PS and PMMA show that these polymers synthesized using PTBPMED are randomly branched.