Modelling and measurements on a condensing counter-flow heat exchanger

Experimental investigations and computer modelling studies have been made on the refrigerant-water counterflow condenser section of a small air to water heat pump. The main object of the investigation was a comparative study between the computer modelling predictions and the experimental observations for a range of operating conditions but other characteristics of a counterflow heat exchanger are also discussed. The counterflow condenser consisted of 15 metres of a thermally coupled pair of copper pipes, one containing the R12 working fluid and the other water flowing in the opposite direction. This condenser was mounted horizontally and folded into 0.5 metre straight sections. Thermocouples were inserted in both pipes at one metre intervals and transducers for pressure and flow measurement were also included. Data acquisition, storage and analysis was carried out by a micro-computer suitably interfaced with the transducers and thermocouples. Many sets of readings were taken under a variety of conditions, with air temperature ranging from 18 to 26 degrees Celsius, water inlet from 13.5 to 21.7 degrees, R12 inlet temperature from 61.2 to 81.7 degrees and water mass flow rate from 6.7 to 32.9 grammes per second. A Fortran computer model of the condenser (originally prepared by Carrington[1]) has been modified to match the information available from experimental work. This program uses iterative segmental integration over the desuperheating, mixed phase and subcooled regions for the R12 working fluid, the water always being in the liquid phase. Methods of estimating the inlet and exit fluid conditions from the available experimental data have been developed for application to the model. Temperature profiles and other parameters have been predicted and compared with experimental values for the condenser for a range of evaporator conditions and have shown that the model gives a satisfactory prediction of the physical behaviour of a simple counterflow heat exchanger in both single phase and two phase regions.

Living cationic polymerization

The kinetics of the polymerization of styrene iniated by 1-chloro-1-phenyltehane/tin (IV) chloride in the presence of tetrabutylammonium chloride have been studied. Dilatometry studies at 25 °C were conducted and the orders of reaction were established. Molecular weight studies were conducted for these experiments using size exclusion chromatography. These studies indicated that transfer/termination reactions were present. The observed kinetics may be explained by a polymerization mechanism involving a single propagating species which is present in low concentrations. Reactions at 0 °C and -15 °C have shown that a "living" polymerization could be obtained at low temperatures. A method was derived to study the kinetics of a "living" polymerization by following the increase in degree of polymerization with time. Polymerizations of styrene were conducted using 1,4-bis(bromomethyl)benzene as a difunctional co-catalyst. These reactions produced polymers with broad or bimodal molecular weight distributions. These observations may be explained by the rate of initiation being slower than the rate of propagation or the presence of transfer/termination reactions. Reactions were conducted using a co-catalyst using a co-catalyst produced by the addition of 1,1-diphenylethane to 1,4-bis(bromomethyl)benzene. Size exclusion chromatography studies showed that the polymers produced had a narrower molecular weight distribution than those produced by polymerizations initiated by 1,4-bis(bromomethyl)benzene alone. However the polydispersity was still observed to increase with reaction time. This may also be explained by slow initiation compared to the rate of propagation. Polymerizations initiated by both bifunctional initiators were examined using the method of studying reaction kinetics by following the change in number average degree of polymerization. The results indicated that a straight line relationship could also be obtained with a non-living polymerization.

Studies in the anionic polymerization of methyl methacrylate

A study has been made of the anionic polymerization of methyl methacrylate using butyllithium and polystyryl lithium as initiators and using aluminium triisobutyl as a cocatalyst. The aspects of the polymerization that were examined were the effect of changing the order of addition of reagents, the temperature at which polymerization takes place and the polarity of the solvent. Trends were assessed in terms of molecular weight, molecular weight distribution and tacticity. In addition, a second monomer addition test was carried out to verify that the polymerization was truly a living one, and a kinetic study was attempted. Studies to investigate the effect of changing the order of addition of reagents showed that polymer with similar polydispersities and tacticities are produced whether the pre-mixing (mixing initiator and cocatalyst before addition of monomer) or the post-mixing (mixing monomer and cocatalyst before addition of initiator) method were used. However, polymerizations using the post-mixing mixing method demonstrated lower initiator efficiencies, possibly indicating a different initiating species. Investigations into the effect of changing the polymerization temperature show the molecular weight distribution to narrow as the temperature decreases, although a small amount of low molecular weight tailing was also observed at low temperature. A clear relationship between tacticity and temperature was observed with syndiotacticity increasing with decreasing temperature. Changes in solvent polarity were achieved by using mixtures of the standard solvent, toluene, with varying amounts of cyclohexane, tetrahydrofuran or dichloromethane. Experiments at low solvent polarity (using toluene/cyclohexane mixtures) showed problems with initiator solubility but produced polymer with lower polydispersity and higher syndiotacticity than in toluene alone. Experiments using toluene/THF mixtures yielded no polymer, thought to be owing to a side reaction between THF and aluminium triisobutyl. Increased solvent polarity, achieved using toluene/dichloromethane mixtures produced polymer with higher polydispersity and at lower yields than the conventional system, but also with higher syndiotacticity. Second monomer addition reactions demonstrated that the polymerization was 'living' since an increase in molecular weight was observed with no increase in polydispersity. Kinetic studies demonstrated the high speed of the polymerization but yielded no useful data.

Living cationic polymerization of isobutene

The polymerization of isobutene initiated by 1-chloro-1-phenylethane has been investigated, and molecular weight studies conducted using size exclusion chromatography. Polymerizations carried out in a 40/60 (v/v) mixture of dichloromethaneIcyclohexane, using titanium (IV) chloride as a catalyst in the presence of pyridine at -30 °C were found to be controlled and living. The number average molecular weights of the polymers increased linearly with monomer conversion, and the molecular weight distributions were between 1.15 and 1.20. Efficiencies of initiation were between 80 and 100%, and evidence was found to suggest that backbiting to the initiator had occurred, resulting in the formation of cyclic oligomers during the early stages of polymerization. The kinetics of polymerization can be explained in terms of active species in. equilibrium with dormant species. The effects of temperature. and dielectric constant on this equilibrium were studied and a model based upon the Fuoss equation was developed. Pyridine was found to behave as proton trap in the system, and when it was used in excess the rate of polymerization was retarded. By assuming that the catalyst and pyridine formed a one to one complex, it was possible to show that the reaction was second order with respect to the catalyst. The synthesis of low molecular weight polyisobutenes was studied. When the concentration of initiator was increased relative to that of the isobutene, such that the theoretical degree of polymerization was 20 or less, the rate of initiation was slow compared to propagation. The efficiency of initiation in these polymerizations was typically between 30 and 40 %. Optimal conditions of temperature. and.catalyst concentration were established, leading to a 60 % efficiency of initiation. A one-pot synthesis of phenol end-capped polyisobutene was attempted by adding phenol at the end of a living polymerization. Evidence to substantiate the existence of capped polymer chains in the resultant product was inconclusive. Block copolymerizations of oxetane and isobutene were conducted using 1-chloro-1phenylethane/TiCl4, but no copolymer or oxetane homopolymer could be isolated.

Living cationic polymerization of styrene monomers

This thesis describes an experimental investigation of synthesis of polystyrene under various polymerization conditions such as solvent polarity, temperature, initial concentrations of initiator, catalyst, monomer and added salts or co-catalyst, which was achieved using the living cationic polymerization technology in conjunction with gel permeation chromatography (GPC) and NMR spectroscopy. Polymerizations of styrene were conducted using 1-phenyl ethylchloride (1-PEC) as an initiator and tin tetrachloride (SnCI4) as a catalyst in the presence of tetra-n-Butylammonium chloride (nBu4NCI). Effects of solvent polarity varied by mixing dichloromethane (DCM) and less polar cyclohexane (C.hex), temperature, initial concentrations of SnC14, 1-PEC and nBu4NCI on the polymerizations were examined, and the conditions under which a living polymerization can be obtained were optimised as: [styrene]o ~ 0.75 - 2 M; [1-PEC]o ~ 0.005 - 0.05 M; [SnCI4Jo ~ 0.05 - 0.4 M; [nBu4NCIJo ~ 0.001 - 0.1 M; DCM/C.hex ~ 50/0 - 20/30 v/v; T ~ 0 to -45°C. Kinetic studies of styrene polymerization using the Omnifit sampling method showed that the number average molecular weight (Mn) of the polymers obtained increased in direct proportion to monomer conversion and agreed well with the theoretical Mn expected from the concentration ratios of monomer to initiator. The linearities of both the 1n([MJoI[M]) vs. time plot and the Mn vs. monomer conversion plot, and the narrow molecular weight distribution (MWD) measured using GPC demonstrated the livingness of the polymerizations, indicating the absence of irreversible termination and transfer within the lifetimes of the polymerizations. The proposed 'two species' propagation mechanism was found to apply for the styrene polymerization with 1-PEC/SnCI4 in the presence of nBu4NCl. The further kinetic experiments showed that living styrene polymerizations were achieved using the 1-PEC/SnCI4 initiating system in mixtures of DCM/C.hex 30/20 v/v at -15°C in the presence of various bromide salts, tetra-n-butylammonium bromide, tetra-n-pentylammonium bromide, tetra-n-heptylammonium bromide, and tetra-n-octylammonium bromide, respectively. The types of the bromide salts were found to have no significant effect on monomer conversion, Mn, polydispersity and initiation efficiency. Living polymerizations of styrene were also achieved using titanium tetrachloride (TiCI4) as a catalyst and 1-PEC as an initiator in the presence of a small amount of 2,6-di-tert-butylpyridine or pyridine instead of nBu4NCl. GPC analysis showed that the polymers obtained had narrow polydispersities (P.D. < 1.3), and the linearities of both the In([MJo/[MJ) vs. time plot and the Mn vs. monomer conversion plot demonstrated that the polymerizations are living, when the ratio of DCM and C.hex was less than 40 : 10 and the reaction temperature was not lower than -15°C. The reaction orders relative to TiCl4 and 1-PEC were estimated from the investigations into the rate of polymerization to be 2.56 and 1.0 respectively. lH and 13C NMR analysis of the resultant polystyrene would suggest the end-functionality of the product polymers is chlorine for all living polymerizations.

Synthesis and characterisation of vinyl block copolymers

A study has been made of the anionic polymerisation of methyl methacrylate using butyllithium and polystyryl lithium as initiators and the effects of lithium chloride and aluminium alkyls on the molecular weight and molecular weight distributions. Diblock copolymers of styrene-b-methyl methacrylate were synthesised at -78oC in THF in the presence of lithium chloride, and at ambient temperatures in toluene in the presence of aluminium alkyls. Studies in the presence of lithium chloride showed that the polymerisation was difficult to control; there was no conclusive evidence of a living system and the polydispersity indices were between 1.5 and 3. However, using relatively apolar solvents, in the presence of aluminium alkyls, homopolymerisation of methyl methacrylate showed characteristics of a living polymerisation. An investigation of the effects of the structures of the lithium and aluminium alkyls on the efficiency of initiation showed that a t-butyllithium/triisobutylaluminium initiating system exhibited an efficiency of 80%, compared with lower efficiencies (typically 30%) for systems based on butyllithium/triethylaluminium.The polydispersity index was found to decrease from ∼2.2 to ∼1.5 when butyllithium was replaced by t-butyllithium. The efficiency of the initiator was found to be solely dependent on the size of the alkyl group of the aluminium component, whereas the polydispersity index was found to be solely dependent on the size of the alkyl group on the lithium component. The aluminium alkyl is thought to be co-ordinated to the ester carbonyl groups of both the monomer and polymer. There is a critical degree of polymerisation, at which point the rate of polymerisation decreases, which probably relates to a change in structure of the active chain end. Characterisation of poly(styrene )-b-poly(4-vinylpyridine) and poly(styrene)-b-poly(4-vinylpyridine methyl iodide) diblock copolymers using static light scattering techniques, showed the formation of star-shaped 'reverse' micelles when placed in toluene. Temperature effects on micellization behaviour are only exhibited for the unquaternised micelles, which showed characterisically lower aggregation numbers than their quaternised counterparts. A suitable solvent was not obtained for characterisation of the styrene-b-methyl methacrylate diblock copolymers synthesized.

Block co-polymerization by transformation reactions

The aim of this study was to use the transformation of anionic to metathesis polymerization to produce block co-polymers of styrene-b-pentenylene using WC16 /PStLi and WC16/PStLi/ AlEtC12 catalyst systems. Analysis of the products using SEC and 1H and 13C NMR spectroscopy enabled mechanisms for metathesis initiation reactions to be proposed. The initial work involved preparation of the constituent homo-polymers. Solutions of polystyryllithium in cyclohexane were prepared and diluted so that the [PStLi]o<2x10-3M. The dilution produced initial rapid decay of the active species, followed by slower spontaneous decay within a period of days. This was investigated using UV / visible spectrophotometry and the wavelength of maximum absorbance of the PStLi was found to change with the decay from an initial value of 328mn. to λmax of approximately 340nm. after 4-7 days. SEC analysis of solutions of polystyrene, using RI and UV / visible (set at 254nm.) detectors; showed the UV:RI peak area was constant for a range of polystyrene samples of different moleculor weight. Samples of polypentenylene were prepared and analysed using SEC. Unexpectedly the solutions showed an absorbance at 254nm. which had to be considered when this technique was used subsequently to analyse polymer samples to determine their styrene/ pentenylene co-polymer composition. Cyclohexane was found to be a poor solvent for these ring-opening metathesis polymerizations of cyclopentene. Attempts to produce styrene-b-pentenylene block co-polymers, using a range of co-catalyst systems, were generally unsuccessful as the products were shown to be mainly homopolymers. The character of the polymers did suggest that several catalytic species are present in these systems and mechanisms have been suggested for the formation of initiating carbenes. Evidence of some low molecular weight product with co-polymer character has been obtained. Further investigation indicated that this is most likely to be ABA block copolymer, which led to a mechanism being proposed for the termination of the polymerization.

The cationic ring-opening polymerization of cyclic ethers

The kinetics and mechanisms of ring opening polymerization and copolymerizntion of different cyclic ethers were studied using mainly a cationic system of iinitiation. BF30Et2/ethanediol. The cyclic ethers reacted differently showing that ring strain and basicity are the main driving forces in cationic ring opening polymerizaion. In most cases it was found that the degree of polymerization is controlled kinetically via terminations with the counterion and the monomers, and that the contribution of each type of reaction to the overall termination differs markedly. The Gel permeation chromatography studies showed that the molecular weight distribution of the samples of polyoxetanes were bimodal. This was in accordance with previous work establishing that the cyclic tetramer is found in much higher proportions than any of the other cyclic oligomers. However the molecular weight distribution of the copolymers made from oxetane and THF or from oxetane and oxepane were shown to be unimodal. These observations could be explained by a change in the structure of the growing end involved in the cationic polymerization. In addition crown ethers like dibenzo-crown-6 and compounds such as veratrole are believed to stabilise the propagating end and promote the formation of living polymers from oxetane.

The polymerization of lactic acid anhydrosulphite by anionic initiators

This thesis is primarily concerned with the synthesis and polymerization of 5-methyl-1;3, 2-dioxathiolan-4-one-2-oxide (lactic acid anhydrosulphite (LAAS)) using anionic initiators under various conditions. Poly(lactic acid) is a biodegradable polymer which finds many uses in biomedical applications such as drug-delivery and wound-support systems. For such applications it is desirable to produce polymers having predictable molecular weight distributions and crystallinity, The use of anionic initiators offers a potential route to the creation of living polymers. The synthesis of LAAS was achieved by means of an established route though the procedure was modified to some extent and a new method of purification of the monomer using copper oxides was introduced, Chromatographic purification methods were also examined but found to be ineffective. An unusual impurity was discovered in some syntheses and this was identified by means of 1H and 13C NMR, elemental analysis and GC-MS. Since poly-α-esters having hydroxyl-bearing substituents might be expected to have high equilibrium water contents and hence low surface tension characteristics which might aid bio-compatibility, synthesis of gluconic acid anhydrosulphite was also attempted and the product characterised by 1H and 13C NMR. The kinetics of the decomposition of lactic acid anhydrosulphite by lithium tert-butoxide in nitrobenzene has been examined by means of gas evolution measurements. The kinetics of the reaction with potassium tert-butoxide (and also sec-butyl lithium) in tetrahydrofuran has been studied using calorimetric techniques. LAAS was block co-polymerized with styrene and also with 1,3-butadiene in tetrahydrofuran (in the latter case a statistical co-polymer was also produced).

The ring opening polymerization of ring strained cyclic ethers

The kinetics and mechanisms of the ring-opening polymerization of oxetane were studied using cationic and coordinated anionic catalysts. The cationic initiators used were BF30Et2!/ethanol, BF30Et2!/ethanediol and BF30Et2/propantriol. Kinetic determinations with the BF30Et2/diol system indicated that a 1: 1 BF3:0H ratio gave the maximum rate of polymerization and this ratio was employed to detenmne the overall rates of polymerization. An overall second-order dependence was obtained when the system involved ethanediol or propantriol as co-catalyst and a 3/2-order dependence with ethanol, in each case the monomer gave a first-order relationship. This suggested that two mechanisms accounted for the cationic polymerization. These mechanisms were investigated and further evidence for these was obtained from the study of the complex formation of BF30Et2 and the co-catalysts by 1H NMR. Molecular weight studies (using size-exclusion chromatography) indicated that the hydroxyl ion acted as a chain transfer reagent when the [OH] > [BF3]. A linear relationship was observed when the number average molecular weight was plotted against [oxetane] at constant [BF3:0H], and similarly a linear dependency was observed on the BF3:0H 1:1 adduct at constant oxetane concentration. Copolymerization of oxetane and THF was carried out using BF30Et2/ethanol system. The reactivity ratios were calculated as rOXT = 1.2 ± 0.30 and rTHF = 0.14 ± 0.03. These copolymers were random copolymers with no evidence of oligomer formation. The coordinated anionic catalyst, porphinato-aluminium chloride [(TPP)AICl], was used to produce a living polymerization of oxetane. An overall third-order kinetics was obtained, with a second-order with respect to the [(TPP)AICl] and a first-order with respect to the [oxetane] and a mechanism was postulated using these results. The stereochemistry of [(TPP)AlCl] catalyst was investigated using cyclohexene and cyclopentene oxide monomers, using extensive 1H NMR, 2-D COSY and decoupling NMR techniques it was concluded that [(TPP)AlCl] gave rise to stereoregular polymers.

Reversible addition–fragmentation chain transfer polymerization of 2-chloro-1,3-butadiene

Controlled polymerization of 2-chloro-1,3-butadiene using reversible addition–fragmentation chain transfer (RAFT) polymerization has been demonstrated for the first time. 2-Chloro-1,3-butadiene, more commonly known as chloroprene, has significant industrial relevance as a crosslinked rubber, with uses ranging from adhesives to integral automotive components. However, problems surrounding the inherent toxicity of the lifecycle of the thiourea-vulcanized rubber have led to the need for control over the synthesis of poly(2-chloro-1,3-butadiene). To this end, four chain transfer agents in two different solvents have been trialed and the kinetics are discussed. 2-Cyano-2-propylbenzodithioate (CPD) is shown to polymerize 2-chloro-1,3-butadiene in THF, using AIBN as an initiator, with complete control over the target molecular weight, producing polymers with low polydispersities (Mw/Mn < 1.25 in all cases).