The extruder as a polymerisation reactor for styrene based polymers van der Goot, Atze Jan

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1 Univesity of Goningen The extude as a polymeisation eacto fo styene based polymes van de Goot, Atze Jan IMPORTANT NOTE: You ae advised to consult the publishe's vesion (publishe's PDF) if you wish to cite fom it. Please chec the document vesion below. Document Vesion Publishe's PDF, also nown as Vesion of ecod Publication date: 1996 Lin to publication in Univesity of Goningen/UMCG eseach database Citation fo published vesion (APA): van de Goot, A. J. (1996). The extude as a polymeisation eacto fo styene based polymes Goningen: s.n. Copyight Othe than fo stictly pesonal use, it is not pemitted to download o to fowad/distibute the text o pat of it without the consent of the autho(s) and/o copyight holde(s), unless the wo is unde an open content license (lie Ceative Commons). Tae-down policy If you believe that this document beaches copyight please contact us poviding details, and we will emove access to the wo immediately and investigate you claim. Downloaded fom the Univesity of Goningen/UMCG eseach database (Pue): Fo technical easons the numbe of authos shown on this cove page is limited to 10 maximum. Download date:

2 7 Chapte 3 Polystyene and Styene-copolymes Applications, Synthesis and Kinetics Styene is the main component of all eactions in this thesis. This monome is chosen as it is vesatile and well-nown. Styene is polymeised in the extude by means of a adical mechanism. In most expeiments, one o two comonomes ae added to styene yielding coand tepolymes. This chapte gives an oveview of the applications of polystyene and styene copolymes. Futhemoe, the polymeisation ates of styene and styene-nbutylmethacylate ae evaluated theoetically and expeimentally. 3.1 Applications Styene and polystyene Polystyene is one of the most impotant polymes of today. Its populaity stems fom the fact that it possesses many good popeties, such as good pocessability, igidity, tanspaency, low wate absobability, and that it can be poduced at low costs. Howeve, polystyene has some disadvantages, such as sensitivity to chemicals and food mateials with high oil content, poo UV-esistance and bittleness, which limit its applications. Polystyene is mainly used in the pacaging industy (Ku 1988a, b). The monome styene is a vey vesatile monome. It can be polymeised by vaious mechanism such as fee adical-, cationic-, anionic-, and goup tansfe polymeisation. Moeove, styene has the capability to ceate adicals, which maes themal polymeisation possible. Styene can also be used in all types of poduction pocesses fo polymeisation. The most common methods ae emulsion-, suspension-, bul- and solution polymeisation. The type of pocess detemines the popeties of the polyme poduced. The most economical pocess fo the poduction of polystyene is a bul-continuous pocess, in which aound 10-20% ethylbenzene is added as a diluent to ease the flow. Fo expandable polystyene, a suspension pocess is used. In ode to adjust popeties, comonomes, chain tansfe agents o cosslin agents ae added to styene befoe it is polymeised. Chain tansfe agents ae used to egulate the molecula weight, but ae aely used in bul pocesses. T-Dodecyl mecaptane is often used as chain tansfe agent. To cosslin the styene polyme, divinylbenzene wos vey well, although othe cosslin agents can also be used. Highly cosslined polymes can only be poduced batch-wise in bul. 39

3 Chapte Blends of polystyene Since styene is a cheap and vesatile monome, many copolymes and blends ae poduced with styene as impotant component. In this section, some blends of polystyene will be mentioned. To impove the impact stength, polybutadiene is geneally added to polystyene, in the amount of 5 to 10 % (La Coste 1996). This einfoced polystyene is nown as high impact polystyene (HIPS). The ubbe component can be divided into two categoies: homopolybutadiene and polybutadiene-styene copolymes. Sometimes, satuated elastomes ae used fo einfocement instead of polybutadiene. Polybutadiene can be blended physically to polystyene, but moe often it is added to styene esulting in some gafting duing pocessing. HIPS is applied fo instance in pacaging, television- and compute housings, and piping and tubing. SBR (styene 23 %-butadiene-ubbe) is a well-nown ubbe, which is mainly applied in automobile tyes (Ku 1988b). By changing the butadiene content in the polyme, the popeties can be changed fom a ubbe to a igid polyme. Bloc copolymes of styene and butadiene ae synthesised by anionic polymeisation and theefoe quite expensive. In ode to impove the themal popeties of polystyene, poly-2,6-dimethyl-1,4-phenylene oxide (PPE) is added to the polyme. PPE is a polyme with a glass tansition tempeatue of 220 C and this polyme is miscible with polystyene. The addition of PPE to PS esults in a polyme blend with a highe glass tansition tempeatue than PS (Yee 1977) Co- and tepolymes of styene The addition of anothe monome to styene is a second method to adjust the popeties of the polyme. The most common copolymes of styene ae descibed in this section. A vey inteesting polyme is obtained afte the copolymeisation of styene with maleic anhydide. The polyme finds applications in waxes and emulsifies when it concens a low molecula weight polyme, while the high molecula weight mateials ae used as engineeing plastics, which ae mostly ubbe modified. The styene-maleic anhydide copolymes ae not only inteesting fom an industial point of view, but also fom a scientific point of view due to the fact that, afte polymeisation, an almost pefectly altenating copolyme is obtained (Dodgson 1976, Ebdon 1986, Klumpeman 1993, 1994). Anothe copolyme that is often investigated is the styene-methylmethacylate copolyme. This polyme has a high tanspaency, an excellent UV-esistance and it is vey suitable fo injection moulding. The copolyme is applied in fo example lamps, glazing and windows, but due to the high pice, the maet is limited. The polyme is poduced in suspension o in bul. On laboatoy scale, a Lewis acid such as ethyl-aluminium sesquichloide o zinchloide is added in ode to incease the polymeisation ate and the altenating egulation of the polyme fomed (Bailey 1972). Resins of the copolyme styene-acylonitil (SAN) ae used in applications such as automobile instument lenses and as an additive to highe cost esins. Besides, the esins ae used to modify and impove flow chaacteistics of ABS, PVC and othe esins. Geneally, a SAN-copolyme consists of 25 to 30% acylonitil. 40

4 Polystyene and Styene-copolymes The most well-nown tepolyme of styene is the acylonitil-butadiene-styene tepolyme (ABS), which is used in applications such as pipes and fittings. The polyme is poduced by dissolving polybutadiene into a monome mixtue of styene and acylonitil, which polymeises in an emulsion pocess. The tepolyme theefoe consists of a polybutadiene bacbone onto which styene-acylonitil copolymes ae gafted. In addition to commecial polymes, many co- and tepolymes of styene ae descibed in scientific liteatue. Examples of comonomes ae acylic acid, seveal acylates and maleates. Tepolymes descibed in liteatue ae styene-methylmethacylate-maleic anhydide and styene-methylmethacylate-maleic imide (Schmidt-Naae 1989) The copolymeisation in this thesis The copolymeisation descibed in this thesis is the copolymeisation of styene (St) with n- butylmethacylate (BMA). The latte component, which is extensively studied by Ganzeveld (1993), inceases the ate of polymeisation, and it is chosen fo easons of safety and pice. The poduct of this polymeisation is applied in speciality aeas, such as the manufactue of ecoding tapes and tones fo photocopies. In this study, the St-BMA copolymeisation is the stating point of all expeiments descibed. Changes of polyme melt viscosity o polymeisation ate ae elated to the St-BMA expeiments descibed in chapte 4. When St and BMA ae mixed in the mola atio 1:1, the polyme fomed is tanspaent and has a glass tansition tempeatue of about 30 C. Figue 3.1 shows the two monomes. Fig. 3.1 Styene (a) and n-butylmethacylate (b) 3.2 The polymeisation inetics The homopolymeisation The polymeisations of styene descibed in this thesis ae fee adical addition polymeisations. This means that afte initiation, a polyme chain gows in a athe shot time. The temination eaction excludes the polyme chain fom futhe eaction. 41

5 Chapte 3 Unde the nomal assumption that the popagation ate is independent of the polyme chain length, the inetic scheme fo polymeisation is: decomposition initiato initiation monome popagation temination d I 2 R R + M i M1 p M i + M M i+1 M tc i + M j Pi + j (by combination) td M i + M j Pi + P (by dispopotionation) j whee d, i, p, tc and td ae the coesponding ate constants in evey eaction step. When it is assumed that the adical concentation is constant (the quasi steady state appoximation) the ate of polymeisation is given by (Bamfod 1976): in which: d[ M ] f d[ I] vp = = p [ M ] (3.1) dt t = + t tc td The molecula weight of the polyme fomed may change duing polymeisation. The molecula weight of the momentaily fomed polyme is given by: M pobability of popagation [ M ][ M ] [ M ] p p mom= = = Wm (3.2) 2 pobability of temination t[ M ] f dt [ I] Equation 3.2 neglects the effect of chain tansfe to the monome and assumes a temination eaction by combination. Although equation 3.2 gives a good indication of the effect of the initiato concentation on the molecula weight of the momentaily fomed polyme, it can not be used fo pedicting the molecula weight of the polyme fomed afte complete eaction. The eason is that the atio of initiato concentation and monome concentation may change duing polymeisation. 42

6 Polystyene and Styene-copolymes When the polymeisation is finished, the numbe aveage molecula weight of the polyme can be calculated by means of: M n [ M ] 0 [ M ] = W (3.3) f m ([ I] [ I] ) 0 Equation 3.3 also assumes a temination eaction by combination. This equation states that a doubling of the initiato concentation esults in a halving of the numbe aveage molecula weight of the polyme, when the convesion is not influenced by the initiato concentation. Howeve, since a highe initiato concentation geneally esults in a highe convesion, the influence of the initiato concentation on molecula weight is educed. In bul polymeisations, deviations fom equation 3.1 occu when a cetain degee of convesion is eached. These deviations ae caused by a stong incease of the viscosity duing polymeisation, as a esult of which the polyme chain adicals can not diffuse easily though the viscous medium, leading to a deceased temination ate t, since this ate is pimaily diffusion contolled. The popagation ate p is, except fo vey high convesions, independent of the viscosity of the polyme melt, since it is chemically contolled. As a esult of these two effects, the oveall polymeisation ate inceases. This phenomenon is nown as the Tommsdoff effect o gel effect (Tommsdoff 1948) The co- and tepolymeisation In the case of a copolymeisation of two monomes A and B, the popagation step consists of at least fou diffeent eactions: 11 ~ A + A ~ AA 12 ~ A + B ~ AB 21 ~ B + A ~ BA 22 ~ B + B ~ BB whee evey eaction has its own popagation ate constant ( 11, 12 etc.) When it is assumed that the adical concentations ae constant, the well-nown copolymeisation equation can be deived fom the mass balance ove evey component. This equation is: d[ A] [ A] 1 [ A] + [ B] = (3.4) d[ B] [ B] [ B] + [ A] 2 43

7 Chapte 3 in which 1 and 2 ae the eactivity atios that ae defined as: = = (3.5) The atio d[a]/d[b] signifies the atio of incopoation of monomes A and B, which is not necessaily equal to the monome feed atio. Consequently, one of the monomes is consumed moe apidly than the othe, and the composition of the monome feed vaies, which leads to a continuous shift of the composition of the copolyme fomed. To descibe the incopoation, a feed atio and a incopoation atio ae defined as: A fa = [ ] [ A] + [ B] d A F a = [ ] d[ A] + d[ B] (3.6) (3.7) The incopoation atio F a can be calculated if the values of 1 and 2 ae nown. The values fo 1 and 2 can be oughly estimated by using the Q- and e-values accoding to Alfey and Pice (Alfey 1947, Bandup 1989): Q = (3.8) { e ( e )} 1 1 exp 1 1 e2 Q2 These Q- and e-values ae monome popeties. Fo styene, the values ae defined as Q St = 1.0 and e St = -0.8, while the values fo BMA ae detemined at Q BMA = 0.82 and e BMA = 0.28 (Bandup 1989). By using equation 3.8, the eactivity atios can be calculated at 1 = 0.51 and 2 is 0.74 (styene is monome A). A moe accuate method is the diect detemination of the eactivity atios. In liteatue, a numbe of studies is epoted, which have studied the copolymeisation of St-BMA. The measuements in these studies show that 1 anges fom 0.56 to 0.74, while 2 lies between 0.47 and 0.64 (styene is monome A). When these eactivity atios ae compaed to the atios obtained by the method of Alfey and Pice, it can be seen that the Q- and e-values give a athe good pediction fo St-BMA. Howeve, studies in liteatue ae pefomed at low tempeatues. Since the tempeatue in the extude is highe, we detemined the eactivity atios expeimentally (section 3.3.2). Besides the incopoation of the monome in the polyme, the eactivity atios can be used fo descibing the polymeisation ate. By using equation 3.5 and the popagation ate constants fo the homopolymeisation, the aveage popagation ate constant fo the copolymeisation is given by: 44

8 Polystyene and Styene-copolymes p, co = (3.9) This model fo the copolymeisation descibed by equation 3.9 is defined as the teminal model. The model needs fou popagation steps to descibe the ate of copolymeisation, implying that only the last unit in the gowing chain detemines the eactivity. Howeve, it is nown in liteatue (Fuuda 1987) that fo a styene-methylmethacylate system the expession fo the popagation ate as in equation 3.9 is not sufficient to give a good desciption of the ate of polymeisation. Not only the last unit, but also the one befoe the last unit influences the eactivity. This effect is nown as the penultimate effect. Fo a desciption of the incopoation atio of the monomes, the eactivity atios seem to be sufficient. The theoy fo homo- and copolymeisation can be extended to the polymeisation in which thee components eact (Bamfod 1976). In that case, the popagation ate consists of nine steps. Fo component A, the popagation is descibed by the following equations: 11 ~ A + A ~ AA 12 ~ A + B ~ AB 13 ~ A + C ~ AC The atio of the amount of A and B that eact at a cetain instant time is given by the following equation: in which: ( ) ( ) ( ( ) [ A] [ B] [ C] [ B] [ C] + + [ A] [ A] [ B] [ C] [ A] [ C] + + [ B] + + ) d[ A] [ A] = (3.10) d[ B] [ B] = and = etc. 21 Fo the othe components, simila equations can be deived. It should be noted that the quantity 12 means the binay eactivity atios as obtained in the copolymeisations. In othe wods 12 is the same quantity as 1 used in equation 3.5. One moe subscipt is used to distinguish the seveal subsystems. 45

9 Chapte Expeimental Up to now, most of the inetics studies concening the copolymeisation of St with n-bma ae pefomed at low tempeatues. Since the tempeatue in the extude anges fom 120 to 160 C, additional expeiments wee necessay to investigate the inetics of the eaction and the eactivity atios of the monomes Themal polymeisation of St-BMA Styene is able to polymeise themally. Though a Diels-Alde-adduct, styene poduces adicals that can ceate polyme chains (Hui 1972, Husain 1978). Figue 3.2 shows the eaction mechanism. Fig. 3.2 The fomation of styene adicals St-BMA-mixtues can also be polymeised themally. The ate of themal polymeisation at 135 C is shown in figue 3.3, and it is compaed to the data fo the themal polymeisation of styene at 140 C, as measued by Hui (1972). Fig. 3.3 The themal polymeisation of styene ( ) and St-BMA ( ) 46

10 Polystyene and Styene-copolymes It can be seen that the themal polymeisation ate of St-BMA is compaable to the themal polymeisation ate of styene. Afte one hou, a convesion of aound 25% was obtained. The polyme fomed at this tempeatue possesses a weight aveage molecula weight of 360 g/mol, with a polydispesity P of somewhat less than 2. Pue BMA showed no themal activity The eactivity atios of St-BMA The eactivity atios wee detemined by pefoming a themal polymeisation at 135 C of seveal St-BMA mixtues, in which the mole faction of styene vaied fom 0.1 to 0.9. Except fo the mixtue with a styene mole faction of 0.1, all mixtues showed themal polymeisation. When these mixtues had eached a convesion of 5 to 10%, the polymeisation was stopped by cooling the mixtue apidly. The polyme-monome mixtue was pecipitated in methanol, afte which the copolyme was caefully died in a vacuum oven. The diffeent copolymes wee analysed by elemental analysis. The detemination of the cabon and oxygen content in the copolyme gave the amount of styene in the copolyme. The esults ae shown in figue 3.4. Fig. 3.4 Influence of the faction styene in the monome feed on the faction styene in the copolyme fomed By using the method of Kelen-Tüdõs (Kelen 1975, 1980, Tüdõs 1976), the eactivity atios wee detemined at 1 = 0.40 ± 0.03, and 2 = 0.86 ± 0.03 (styene is monome A). When St and BMA ae mixed in equal mola atio, some composition dift is liely to occu duing polymeisation. Howeve, in all bulpolymeisations, a tanspaent polyme was obtained, indicating no sevee phase sepaation. 47

11 Chapte The polymeisation inetics of St-BMA The ate of copolymeisation of styene with BMA was studied by using isothemal Diffeential Scanning Caloimety (DSC). The convesion of the monome was obtained by using the following expession: H ( t) dt 0 ζ ( t) = (3.11) H ( t) dt 0 t By using equation 3.11, it was assumed that the maximum convesion was 100% and that the heat of polymeisation was linealy dependent on the convesion. Pobably, these assumptions ae not completely valid, but nevetheless the DSC-expeiments ae useful fo compaing the ates of polymeisation of the diffeent monomes and monome mixtues. Fig. 3.5 The polymeisation of St-BMA at diffeent tempeatues The esults of the DSC expeiments with a St-BMA monome mixtue ae shown in figue 3.5. The amount of initiato added was about 5 mmol/mol monome (table 3.1). The initiatos used wee Tigonox 29C50 (120 C), Tigonox C (130 C) and Tigonox 101 (140 C). As expected, a highe tempeatue esulted in a faste polymeisation. Figue 3.6 is deived fom figue 3.5. The quantity p / t, also indicated as ov, is a pseudo-fist ode polymeisation ate constant when it concens a homopolymeisation. The quantity gives an aveaged polymeisation ate in case of a copolymeisation. The expession fo ov can be deived fom equations 3.1 and 3.11, and equals: 48

12 Polystyene and Styene-copolymes p dζ 1 = ov= (3.12) dt f [I] t d The decomposition ate constant d can be calculated via: E a RT = 0 e (3.13) d d in which the tempeatue T is expessed in Kelvin. The initiato efficiency is assumed to be 1. The value fo d, which diffes fo evey initiato, can be calculated fom elations given by Azo-Nobel, the supplie of the initiatos (table 3.2). Fig. 3.6 The polymeisation ate constant vesus the convesion Figue 3.6 shows the ov vesus the convesion of the monomes fo the St-BMA copolymeisation. It can be obseved that ov is not only dependent on tempeatue, but also on the convesion. The incease in polymeisation ate at highe convesion is a esult of the gel effect. At low convesion, the ate of polymeisation is athe low, which is pobably a esult of the inhibito, which was not emoved in the DSC- and extude expeiments. Figue 3.6 shows that the effect of inhibito is moe ponounced at highe tempeatues. Figue 3.7 compaes the ates of polymeisation fo styene, BMA and the copolymeisation. It can be seen that the copolymeisation is faste than the polymeisation of styene, but significantly slowe than the homopolymeisation of BMA. Just as in figue 3.6, the fastest eaction is mostly hindeed by inhibito effects. The gel effect is most ponounced in the case of BMA. The gel effect of styene is compaable to the effect of the copolymeisation. 49

13 Chapte 3 Fig. 3.7 Compaison of styene, St-BMA and BMA Table 3.1 shows the values fo ov afte 10% convesion fo the polymeisation descibed in figues 3.6 and 3.7. Afte 10% convesion, the effect of the inhibito can be neglected and the gel-effect is not impotant yet. The data fo the ov fo St and BMA, combined with the values fo 1 and 2 can be used fo a pediction of a value fo the ov fo the copolymeisation by using the teminal model. This ov can be calculated via: 1 1 St, ov 1 1 BMA, ov ov, co= 4 St, ov BMA, ov+ (3.14) When the teminal model is used fo descibing the copolymeisation of St with BMA the aveage polymeisation ate constant is 0.39 (l / mol s) 0.5, when the monomes ae mixed in the atio 1:1. This is significantly moe than 0.24 as found by the DSC expeiments. This means that the teminal model does not descibe the ate of copolymeisation coectly at high tempeatues. To descibe the ate of polymeisation moe accuate, a penultimate model should be used (Fuuda 1987). 3.4 Conclusions The copolymeisation of St-BMA is slowe than the polymeisation of BMA, but significantly faste than the homopolymeisation of styene. The teminal model is not sufficient fo the desciption the copolymeisation when using the eactivity atios measued. The eactivity atios indicate that when St and BMA ae mixed in equal mola amounts, some composition dift is liely to occu. Howeve, the St-BMA-copolyme was completely tanspaent in all expeiments, indicating no sevee phase sepaation due to composition dift. The ate of themal polymeisation of St-BMA is compaable to that of styene itself. The BMA used showed no themal activity. 50

14 Polystyene and Styene-copolymes Table 3.1 The ate of polymeisation fo St, BMA and St-BMA-copolymeisation. T ( C) initiato [I] (mmol/mol M) ov (at 10% convesion) Styene 130 Tigonox C St-BMA 120 Tigonox 29C St-BMA 130 Tigonox C St-BMA 140 Tigonox BMA 130 Tigonox C Table 3.2 Decomposition ate constants and tempeatues belonging to the half life time t 1/2 indicated fo the initiatos used in this thesis d0 (1/s) E a (J/mol) t 1/2 = 0.1 h 1 h 10 h BPO (Lucidol) 6.94 e C 91 C 72 C Tigonox 29C e Tigonox C 2.26 e Tigonox e Tigonox T 1.17 e Nomenclatue e constant fo detemining eactivity atios - E a activation enegy J/mol f initiato efficiency f a mole faction of monome A in the feed - F a mole faction of component A in the polyme - H heat of polymeisation J/g I initiato d initiato decomposition ate constant l/mol s d0 pe-exponential facto fo the decomposition of the initiato 1/s i initiation ate constant l/mol s ov pseudo fist ode polymeisation ate constant (l/mol s) p popagation ate constant l/mol s t oveall temination ate l/mol s tc temination by combination ate constant l/mol s td temination by dispopotionation ate constant l/mol s M monome 51

15 Chapte 3 M total amount of gowing polyme chains M i gowing adical of chain length i M n numbe aveage molecula weight g/mol M w weight aveage molecula weight g/mol P M w polydispesity M n - P i polyme of chain length i Q constant fo detemining eactivity atios - 1, 2 eactivity atios - 11 eactivity atio used in the tepolymeisation inetics equations - R gas constant (8.314) J/mol K R initiato adical t time s T tempeatue K v p polymeisation ate mol/l s W m molecula weight of monome M g/mol ζ convesion - [ ] concentation mol/l [ ] o concentation befoe polymeisation mol/l Refeences Azo-Nobel, Poduct Infomation: Initiatos Alfey T., Pice C.C., J. Pol. Sci., vol. 2, p (1947) Bailey W.J., Macomolecula synthesis, vol. 4. John Wiley & Sons, New Yo, p (1972) Bamfod C.H., Tippe C.F.H., Compehensive chemical inetics, vol. 14a (Fee adical polymeisation), Elsevies publishing, Amstedam, p (1976). Bandup J., Immegut E.H., Polyme Handboo, 3d. edition, John Wiley & Sons, New Yo (1989) Dodgson K., Ebdon J.R., Eu. Pol. J. vol. 13, p (1976) Ebdon J.R., Towns C.R., Dodgson K., J. Macomol. Sci., Rev. Macomol. Chem. Phys. vol. C26, no. 4, p (1986) Fuuda T., Kubo K., Ma Y.D., Inagai H., Polyme J., vol. 19, no. 5, p (1987) Ganzeveld K.J., Janssen L.P.B.M., Can. J. of Chem. Eng., vol. 71, p (1993) Hui A.W., Hamielec A.E., J. of Appl. Pol. Sci., vol. 16, no. 3, p (1972) Husain A., Hamielec A.E., J. of Appl. Pol. Sci., vol. 22, no. 5, p (1978) Kelen T., Tüdõs F., J. of Macomolec Sci-Chem., vol. A9, p (1975) Kelen T., Tüdõs F., Tucsanyi B., Polym. Bull., vol. 2, p (1980) Klumpeman B., O'Discoll K.F., Polyme, vol. 34, no. 5, p (1993) Klumpeman B., PhD-thesis, Univesity of Eindhoven, The Nethelands (1994) Ku P.L., Advances in Polyme Technology, vol. 8, no. 2, p (1988a) 52

16 Polystyene and Styene-copolymes Ku P.L., Advances in Polyme Technology, vol. 8, no. 3, p (1988b) La Coste J. Delo F., Singh R.P., Vishwa Pasad A., Sivaam S., J. of Appl. Pol. Sci., vol. 59, p (1996) Schmidt-Naae G., Schmidt H., Litauszei B., Noiega M., Polyme Reaction Engineeing, Reichet/Geisele, VCH Belin, p (1989) Tommsdoff E., Köhle H., Lagally P., Macomol. Chem., vol. 1, p (1948) Tüdõs F., Kelen T., Földes-Beezsnich T., Tucsányi B., J. of Macomol. Sci., Chem., vol. A10, no. 8, p (1976) Yee A.F., Pol. Eng. Sci., vol. 17, no. 3, p (1977) 53