Break seals

After the flask was purged under high vacuum overnight, dry benzene was distilled into the flask. Then the coupling reaction was performed by breaking both break-seals... 

A discussion of this polymerization method would not be complete without mention of the development of specialized glassware utilized over the years. It has evolved from very elaborate, sophisticated, and specially designed glassware to fairly simple setups. Initially, elaborate break-seal technology was used to complete the entire polymerization process,143 similar to anionic polymerization methodology.17 Break-seal techniques were employed to fully understand many monomer structure-reactivity relationships these techniques are no longer needed. 

Reaction of K3Co(CN) with PMMA. A 1.0 g sample of PMMA and 1.0g of the cobalt compound were combined in a standard vessel and pyrolyzed for 2 hrs at 375°C. The tube was removed from the oven and the contents of the tube were observed to be solid (PMMA is liquid at this temperature). The tube was reattached to the vacuum line via the break-seal and opened. Gases were determined by pressure-volume-temperature measurements on the vacuum line and identified by infrared spectroscopy. Recovered were 0.22g of methyl methacrylate and 0.11 g of CO and C02. The tube was then removed from the vacuum line and acetone was added. Filtration gave two fractions, 1.27g of acetone insoluble material and 0.30g of acetone soluble (some soluble material is always lost in the recovery process). The acetone insoluble fraction was then slurried with water, 0.11 g of material was insoluble in water. Infrared analysis of this insoluble material show both C-H and C-0 vibrations and are classified as char based upon infrared spectroscopy. Reactions were also performed at lower temperature, even at 260°C some char is evident in the insoluble fraction. 

Only six other centers have operated similar metal bomb calorimeters, mainly modeled on the Hubbard design, and it is therefore of interest to note that Gross and co-workers have been intrepid enough to use a simple two-compartment glass apparatus separated by a break-seal for fluorine combustion (5 atm F2). Their results were in excellent agreement with those obtained in metal bombs (107). 

In order to ascertain the nature of the propagating species in the polymerisation of styrene catalysed by perchloric acid in methylene dichloride we have investigated the behaviour of this system by (a) calorimetric, (b) spectroscopic, and (c) conductimetric techniques. All experiments were carried out in high-vacuum apparatus, with highly purified and dried reagents which were mixed by breaking phials or break-seals magnetically. 

The spark punches a hole through thin glass (< ca. 0.5 mm) and can therefore not be used on breakable phials or break-seals. 

In this section only those taps and valves will be considered which are suitable for repeated use. Break-seals and seal-off points which can be considered as once-only valves are discussed in Section 3.2. Before discussing the different types of taps and valves there is a safety rule which applies to all taps which are operated by turning a key Always use two hands one hand to hold the tap barrel and the other to turn the key. 

PTFE is essentially inert, but very aggressive chemicals, such as SbFj, may cause deterioration of the sealing ability of PTFE over longer contact times. Therefore, to isolate reservoirs of aggressive chemicals for more than a few days it is better to use a glass break-seal. 

Break-seals come in many different forms and two of these are shown in Fig. 2.7. The form shown in Figure 2.7(a) is easier to make and tends to be more reliable, in that it (almost) always breaks when desired, but the type shown in Figure 2.7(6) has the advantage that, after breaking, the full diameter of the tube is open this means that liquids or gases flow through the opening more rapidly. 

In all the examples in this book of the use of break-seals, they are used as once-only devices. However, one can use them as taps for a small number of on-off operations, if the most rigorous conditions must be met. The device... 

Fig. 2.8. Break-seal ladder the different parts are not drawn to the same scale. Several portions of known volume of a liquid A are to be distilled into a container or apparatus B. Procedure One of the break-seals C is broken with the steel ball D and the required volume of A is distilled into a hanging burette (not shown) beyond E. The broken break-seal is sealed off by fusing at its seal-off point F then this first portion is distilled from the burette into B. For the next operation, D is transferred by a magnet into the next rung of the ladder. When all the break-seals have been used up, a second ladder can be attached at G and H without breaking the vacuum.

Calibrated vessels with glass break-seals 81... 

Fig. 3.3. Pressure equalisation burette in a device for potentiometric titration. For full details of operation see original publication. A break-seal ampoule containing the titrating solution B which reacts with the contents of break-seal ampoules P in reactor R, C sintered filter, Z) 10 ml burette, E 2 mm capillary, F capillary tip, Pt wires from electrodes leading to potentiometer. T, T, 7 PTFE taps, 7J three-way glass vacuum tap.

Therefore, in order to meter small quantities of materials in vacuo, the use of glass phials or calibrated break-seals, described in Sections 3.1.2 and 3.1.3. respectively, is recommended. Even smaller quantities of moderately volatile materials can be dosed by means of calibrated vapour-dosing bulbs (see Section 3.1.5). 

An alternative to the use of small spherical phials, which often has advantages in terms of the flexibility of the apparatus, is the use of calibrated vessels or ampoules with break-seals. They consist usually of a tube 3-6 mm i.d. and 20-70 mm long with a calibration mark and carrying a fragile glass membrane which can be broken when desired, so as to admit the contents of the ampoule to a mixing vessel or reaction vessel. The construction of different types of magnetic break-seals has been described in Section 2.2.4.4. 

This section would not be complete without a reference to what is probably the best-known form and the most common usage of break-seals. 

Fig. 3.9. Calibrated break-seal ampoule. If is the volume to the calibration mark C, the content as shown is = V —nr h where r is the radius of the narrow tube.

Fig. 3.15. An elaborate reactor attached to a dilatometer A which is fitted with electrodes (Pt) for conductance measurements. B mixing chamber, P phial of styrene, C bulb containing HI gas, D solution of iodine, E break-seals.

Fig. 3.19. Vacuum viscometer design. A solvent reservoir, B hanging burette, C site for freeze seal, F killing agent in break-seal phial, G stainless steel ball bearing in appendix, H coarse frit to retain glass fragments, J phial breaker, K stirrer, P initiator phial.

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