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Haloaldebyde Polymers. I. Chain Termination in Anionic Chloral Polymerization

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Haloaldebyde Polymers. I. Chain Termination in Anionic Chloral Polymerization Polymer Journal, Vol. 7, No. 2, pp 186-194 (1975) Haloaldebyde Polymers. I. Chain Termination in Anionic Chloral Polymerization P. KusisA* and 0. VoGL Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01002 (Received May 7, 1974) ABSTRACT: Anionic chloral polymerization can be carried out under certain circum­ stances in the presence of acylating agents, such as acetyl chloride and benzoyl chloride. Parts of these polymers exhibit a much improved thermal stability. Introduction of acyl end groups into the polymer permits an estimation of the molecular weight of the polymer. DTG was found to be an excellent technique with which to follow the extent of reaction, to assist in the examination of this kind of end group and to follow the thermal stability of chloral polymers. KEY WORDS Haloaldehyde Polymers I Chloral 1Anionic Polymeri- zation I Chain Transfer I Chain Termination I Acetyl Chloride 1 Benzoyl Chloride I Triphenyl Phosphine as Initiator I End Groups 1 Thermal Degradation I Our knowledge of the polymerization of chlo­ stable end groups into polychloral. It also ral has recently been greatly advanced with the provided a method for estimating the minimum development of a new technique to make homo­ number-average molecular weight of the stable and copolymers of chloral directly in a useful polychloral fraction. form. 1•2 Chloral polymerizes rapidly to high conver­ EXPERIMENTAL sion with anionic initiators. Cationic polymeri­ zation is now also well established, however, it Materials is extremely slow. 3 Chloral (obtained from the Diamond Shamrock Because chloral polymers of high chloral con­ Company) was allowed to reflux over phospho­ tent are insoluble and infusable, much of the rous pentoxide for 24 hr. It was then transferred recent work on the characterization of these into a carefully dried distillation apparatus which polymers was done by infrared techniques and consisted of a 100-cm long column with an particularly by the studies of thermal degrada­ electric heating jacket, filled with glass helices tion of chloral polymers. of 1/2-cm diameter and an automatic distillation Very little is known about the end groups of head. The distillation was carried out under a chloral polymers, particularly of the homo­ reflux ratio of 1 : 100 under purified nitrogen. polymer, although it has been reported that The distillation of chloral was carried out chloral polymers are like the polymers of for­ continuously. An initial fraction of 20% was maldehyde, substituted polyoxymethylene glycols collected which is not useful for the preparation which can be acetylated to the corresponding of good chloral polymers. The progress of the acetates. 4 distillations was monitored by gas chromato­ We found that anionic chloral polymerization graphy. Polymerization grade chloral, the main can be carried out readily in the presence of fraction, contains only two impurities: water, strong acylating agents and that acylation occurs less than 100 ppm, preferably less than 20 ppm in the form of a chain termination reaction. and dichloroacetaldehyde (less than 0.1%). This method provided a new way to introduce It is not possible to store pure chloral mono­ mer for extended periods. Consequently, the * On leave from Center of Molecular and Macro­ molecular Studies, Polish Academy of Sciences, necessary amount of chloral was collected from Lodz. the distillation apparatus before use. 186 Haloaldehyde Polymers. I. Triphenylphosphine (Aldrich) was purified by were then separated, the polymer film was cut crystallization from benzene and used as a one to the proper size, extracted with acetone for molar solution in benzene. 48 hr in a Soxhlet extractor and dried under re­ Lithium tertiary butoxide (Alpha Inorganic duced pressure at room temperature for several Company) was purified by vacuum sublimation hours. and used as a one molar solution in methyl­ Measurements cyclohexane. Acetyl chloride, acetyl anhydride, and benzoyl All infrared spectra of polychloral films were chloride (Eastman Kodak Chemical Company) recorded on a Perkin-Elmer 727 spectrophoto­ were purified by distillation. meter. The thickness of different samples of chloral Polymerization of Chloral film varied depending on several variables, es­ It was convenient to prepare chloral polymers pecially the thickness of the spacer material. for various measurements in the form of films. Consequently, an internal standard method was Since the polymer is insoluble and infusable the used to compare the intensity of the carbonyl film must be cast directly from the monomer. absorption of the end groups of chloral polymers. The initiated monomer solution was prepared The peak at 2625 cm-t, which appears in the as follows: A large test tube (50 ml) was flamed spectra of pure polychloral was chosen as the out and dried under nitrogen. It was fitted with standard. The "relative peak intensity" which a serum cap and a nitrogen inlet and outlet and describes the ratio of the carbonyl peak placed into an oil bath at 70°C. Freshly dis­ (1750 cm-1 for the carbonyl group of the benzoyl tilled chloral was transferred from the receiver group or 1780 cm-1 for the acetyl group) to the of the distillation apparatus with a predried 2625 cm-1 peak (measured in the absorbance hypodermic syringe into the test tube. The test scale) was used as the value for comparison. tube was immersed into the oil bath and after To determine quantitatively the. dependence 5 min the temperature of the contents of the test of the concentration of the ester end groups on tube has reached the temperature of the oil bath. "the relative carbonyl peak intensity," a series The initiator solution and the additive used as of "homogeneous" mixtures of polychloral and the chain-transfer agent were injected with small poly(methyl methacrylate) with a poly(methyl syringes and the solution was shaken until it methacrylate) content of less than 3 mol5'6 was was homogeneous. prepared by "single phase polymerization." 5 The monomer is initiated and all the han­ The molar ratio of the components was deter­ dling of this solution must now be carried out mined by elemental analysis and the "relative with equipment which is preheated to at least carbonyl peak intensity" of these polymer films 65°C. were measured. The solution from the test tube was then From these results the concentration of the transferred with a hypodermic syringe into the ester end groups could be calculated assuming polymerization assembly. This polymerization that the extinction coefficient in both cases was assembly consisted of two glass plates (20 em x approximately the same. The previous assump­ 20 em x 0.8 em) which were separated by a strand tion was justified because it was known that of elastic fiber and clamped together with sim­ the extinction coefficient of the carbonyl group ple bureau clamps; the assembly had also been in acetates was 385-410mxlxcm-1 and in preheated to 70°C in order to prevent premature methacrylates 390-440mxlxcm-1 •6 polymerization. The thermal degradation of chloral polymer After the initiated monomer solution was filled samples was determined in nitrogen using a into the assembly, the whole assembly was im­ Perkin-Elmer TGS-1 thermobalance at a pro­ mersed immediately into an ice-water bath. grammed increase of temperature of 5°Cjmin. After I hr the assembly was taken out and al­ The purity of the monomer was monitored by lowed to dry at room temperature; this was gas chromatography with a Varian Aerograph usually accomplished overnight. The glass plates Model 920, a column packed with Poropack and Polymer J., Vol. 7, No. 2, 1975 187 P. KusisA and 0. VoGL helium as the carrier gas. ing part of the same category except that the overall rate of polymerization of chloral with RESULTS AND DISCUSSION these initiators is very similar to that of onium salts as initiators of chloral polymerization. The rate of chloral polymerization depends This group of initiators includes tertiary amines very much on the initiator used. Most initiators and particularly tertiary phosphines. fall into two general categories; one category In the case of tertiary phosphines, particularly consists of those initiators which initiate by an of triphenylphosphine, we have clearly estab­ almost quantitative initiation and the other lished previously that the initiation reaction consists of those which initiate by a very low proceeds in two steps." Firstly, an instantaneous degree of initiation. Up to now there are no reaction of triphenylphosphine occurs with one initiators known which have an initiation equi­ mole of chloral to form triphenyldichlorovinyl­ librium, where the initiating anion oxyphosphonium chloride in a fast and quanti­ added to an apprreciable amount but not com­ tative pletely to the carbonyl carbon of the chloral carbonyl group. cera I There is anthor group of initiators which re­ Ph 3P + C = 0 -> Ph3PIB-O-CH = CCh ( 3 ) acts more violently with the monomer and may I H Cl8 cause side reactions. However, when treated properly such initiators may also act as good reaction. This chloride is now the real initiator initiators; the addition of these anions to chlo­ and apparently initiates according to eq 2 and 3. ral is essentially quantitative, as in the case of The nature of the initiator determines the butyl lithium. structure of one of the end groups in the poly­ Of the convenient initiators, lithium tertiary chloral chain. Unfortunately, very little is butoxide is preferred because it reacts fast and known about the reaction terminating the growth essentially quantitative with one mole of chloral of the macromolecule. Consequently, the nature to form the corresponding alkoxide anion, with­ of the terminal end groups in chloral polymeri­ out causing any side reaction. 7 zations is not known and infrared studies do cera not give any indication of a clearly identifiable, I specific end group.
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