Chain Reaction Polymerization J a m e s E. McGrath Chemistry Department and Polymer Materials and Interfaces Laboratory Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Introduction The DurDOSe of this paper is to attempt to present a rea. . sonahle, concise, hut yet ul)-twdaceovervirw of chain reaction pulymerization. 'l'his is clearly a difficult task due to the enurmouslv" Iarne nature d t h e subiect. It hewmes feasiblr only because one may refer to the many outstanding papers, reviews and hooks that are now available in the literature. A fairly comprehensive, but not exhaustive list of reference books is provided a t the end of this article. The recent 2nd edition df Professor Odian's book (reference ( 2 ) ) is very thorough and highly recommended to those seeking either an introductory and/or in-depth discussion of polymerization principles. An excellent listing of appropriate scientific and nrnctical neriodicals related to nolvmer . - chemistrv and technology IS provided in Professor Rodriquez's text (reference ( 9 ) ) . Detailed exnerimental ~roceduresfor svnthesizing pol&ers are also available. hei interested reader is especialli referred to references (36-40). Many useful constants are given in the "Polymer Handbook" (Reference (16)). ~
General Considerations The transformation of a vinyl or alkene monomer into a long chain macr~~mt,lecule ria a chain reaction process isdepicted in Scheme 1. This process hus~csllyinvolves the addition of monomer to an activated or initiated form of the monomer. The reaction as indicated does involve a change in the bonding from an SD? . - hond to an son . - tvne ". bond. This Drocess of chain polymerization is usually not spontaneous but rather must be catalyzed or initiated. For most monomer systems of this type, one can discern at least three basic individual steps in the overall process of polymerization. Thus one considers an initiation step, the propagating or growth step, and the termination step. The details of these three individual events will be highly dependent on the exact mechanism of polymerization. The active intermediates that are produced may be cateeorized as heine either radicals. anions. cations. or coordinated species. For some of these mechanistic processes, most notably anionic polymerization, it is often possible to avoid the termination step. There are many monomers that can be transformed into long chain macromolecules via an addition or chain reaction process. A number of these are illustrated in Scheme 2. The common feature here, of course, is that one is converting the unsaturated carbon-carbon hond into a saturated moiety. The nature of the pendent group will help discern which reaction mechanism must be followed in order to affect this transformation. We have indicated here that in all cases. -~~~~ we are convertine a carbon-carbon double hond into a long chain macromolecule. However, it should be pointed out that chain reaction nolvrnerization is not limited to s i m ~ l v . carbon-carbon bond polymerization hut includes, in principle and in fact. manv other reactions as well. Nevertheless, i t is most useful to illustrate chain reaction polymerization via a discussion of carbon-carbon bond reactions. The types of initiators or catalysts for addition or chain reaction polymerization are briefly outlined in Scheme 3. Ordinarily, one thinks of again ei1ht.r radical, cntionic, anionic, or cwrdinatiun tslw catalyiti as the most effective means uf initiating pol$r&izntions. The radical initiators include ~eroxides.azo compounds, redox combinations, UV light, a n d in general any nrocess that will efficientlv.. nroduce an active radical species which is capable of interacting with a monomer. High energy
-
~~~~
~~~~
~
~
844
~~~
.
~~
Scheme 1. Chain Polymerization
~
~
~
Journal of Chemical Education
Relief of strain is a driving force Must be activated Basic Individual Steps 1. Initiation 2. Propagation 3. Termination
Scheme 2. Monomer
nCH,=CH, &H.-CH
Polymer
fCHrCK&
+2H,-CH+
'-1
CH, &H.-CH
"-1
--
Poly(ethylene) Poly(propylene)
I
CH, ICEJ-CHk
I
Poly(viny1chloride)
CI
CI
-
CH,
I
ncH2=7c=o II
Addition Polymers
fCH,-C+
I
Poly(methy1 methylacrylate)
c=o
-
OCH, nCF,=CF,
I
&H,
fC F . - C F k
P~ly(letrafl~~r~elhylene)
radiation, for example from cobalt 60, is also a suitable radical initiation species. The mechanism in the case of radiation can be quite complex. Ordinarily, such high energy radiation does produce radical type polymerizations. Under certain anhydrous conditions, however, it has been demonstrated that ei-
Scheme 3.
Examples of Initlators for Chain Reaction Addition Polymerizations Cationic
Radical
Anionic
1. Peroxides
1. Proton or Lewis
1. Organo Alkalis
2. AIO-corn-
Acids 2. Carbocations
2.
pounds 3. Redax systems
3. Oxonium ions
3. High energy
4. Light
4. High energy
Scheme 5.
Peroxide Initiators Polymerization Temperature Range.
Coordination
Structure
Or:
Transition Metal Complexes
Lewis Bases radiation (anhydrous)
radiation (anhydrous) 5. High energy radiation
Scheme 4.
Vinvl Chain Polvmerization
ElecIron density at lhe double bond may determine whethw a particular monomer polymerizes via anionic, cationic or free radical mechanism. CH,=C
CH,
Inductive
J
'
or Resonance
/
Inductive
R
CH,=C
CH,
Groups bonded to the proxy structure primary affectthermal stabilib andlor solubiiitv.
or
Resonance Intermediate
~ationic@, e.g. isobutene or vinyl ethers ~nionic8,acryloniWila, methyl methacrylate Free radicalo, acrylonitrile.
methyl methacrylate, vinyl acetate Some monomers (e.g styrene) can polymerize via two orperhaps aii three
mutes.
ther cationic or anionic polymerization can be initiated with high energy radiation. More traditional cationic species include the Lewis acids such as BF-.-.aluminum trichloride and the like, andlor various oxonium ions. Anionic processes have been studied in ereat detail through the use of orpanoalkali compounds such as the alkylithi'ms. Electron transfer reagents, such as sodium naphthalene complexes, are also used. One of the most important types of polymerization, especially in terms of volume of materials produced is coordination or Ziegler-Natta catalysis, which involves a variety of transition metal complexes. For example, these are usually based on titanium, vanadium, or chromium type compounds, as we will discuss later. Since we have indicated that various polymerization processes may occur via different mechanisms, one might ask the question how does one define which type of mechanism may he operative for a particular monomer? From a fairly simplistic point of view, we can get some idea of the nature of the required catalyst from the structure of the monomer. For example, in Scheme 4, it is indicated that a monomer that contains electron donating groups either via resonance or inductive type interactions will polymerize via a carbocation mechanism. Examples of this could he monomers such as isobutene or the vinvl . alkvl " ether svstems. What one finds is that monomers of this type are very responsive to cationic tvDe of initiators. such as the Lewis acids. Thus. the electron density a t the double bond can determine whether a monomer polymerizes via a cationic or anionic process. A second example given is that situation where one has electron withdrawing groups such as ester groups or nitrile groups attached to the reactive site. In this case, such a monomer is quite stable to cationic species, but can be very often rapidly polymerized by anionic initiators. On the other hand, there are a number of monomers of somewhat intermediate electron densities, such as for example vinyl acetate. These monomers
will he primarily polymerkrd by only free radical type initiators. One should also puint out that vari~msmonomers ma). contain groups that would interfere with cations or anions (e.g., vinyl halides) and therefore must only be polymerized by free radical processes. Free Radical Chain Polvmerizations Let us now turn our discussion primarily in the direction of free radical polymerizations. If one is to use a Deroxide tvoe .. initlator, i t would l~e,,fgreatinterest to untlerstnnilarmething ala,ut huw rhe yroups attached to the oxvpen hond influence the polymerizationT~nScheme 5, we have-listed several peroxide initiators and some suitable temperature ranges where they could he used. The common feature here is, of course, the relative weak oxygen-oxygen hond, which is susceptible to homolvtic cleavaee. However. the erouDs . attached to the perox; bond par~icularlyinfl"ence the stability of the resulting radicals that are formed, and this in turn defines more or less the temperature range that a particular peroxide molecule could he used to affect polymerization. For example, a tertiary hutyl group on a peroxide will produce a tertiary hutoxy radical which is relatively stable. Therefore, initiator containing these groups are useful a t relatively high temperatures. The other principal effect of structure relates to the solubility of a peroxide initiator. A high percentage of organic groups will promote organic solubility, which is important in certain tvDes of oolvmerizations such as "sus~ension"reactions. 0;ihe othkr hand, if one wishes to have H water soluble initiator, the initiator of choice mav-he ootassium oersulfate. . since the potassium salt form is quite soluble in aqueoui media. There are a wide range of initiator structures that have been prepared and many of these are commercially available. Nevertheless, there are some occasions when one wishes to generate a free radical species at, perhaps, room temperature or sliehtlv above. Two additional wavs radicals . of eeneratine .. are indicated in Scheme 6 and Scheme 7. o n e a p p r ~ ~ a stihto induce dr~wni>ositiun of n ueruxide molecule. This ran be done throughthe use of additional compounds which can transfer an electron to an oxygen-oxygen hond and facilitate .. it* drctmpmitiporrionationtype rractii~n.In his case, two macn~molerularradicals termiaaw each other, one hy abstracting a protun to \field a saturated end group and at the same time the other chain now winds up with an unsaturated end group. Both overall processes are quite rapid and involve a rather low activation energy; therefore, one may define the rate of termination as being the product of the two macroradical concentrations times a rate constant kt, where in this case kt would include hoth combinations as well any disproportionation type steps and a factor of 2 is usually included. Some typical rate constants are summarized in Scheme 19, and it is easy to see that the termination rate constant is much faster than either of the other two steps. This allows one to make a steady state assumption where one can set the rate of initiation equal to the rate of termination. An advantage of doing this is that it becomes possible to solve the equation shown in Scheme 19 in terms of the radical concentration. Upon substituting this value into the rate expression, a final rate expression is obtained which is quite useful. Basically, it states that the rate of polymerization will depend on several constants, but that it will also he proportional to the first power of the monomer concentration and to the square root of the initiator concentration. Thus, as one doubles the initiator concentration, the expected rate should increase only by a factor of about 1.4. This was confirmed by many workers and typical plots showing this effect are given in Scheme 20. Here if one plots the rate of polymerization versus initiator concentration for a variety of monomers and initiators such as methylmethacrylate and. AIBN or styrene and henzyl peroxide, for example, one ohserves slopes equal to 112. The rate expression in Scheme 20 has an additional term which we have not discussed thus far.
Scheme 21. Tromsdorf or "Gel" Effect Bulk or Concentrated Solution Polymerization
I 1
VISCOSITY CAN REDUCE TERMiNATION RATE CONSTANT
k,
GET AN AUTOACCELERATION
This is f , which is the fractional efficiency of initiation. In other words, for 100%efficiency f would have a value of 1.0. This is never achieved, although in some cases, particularly with some the azo initiators, many people have reported values perhaps as high as 0.9. More recent work has tended to indicate that this value may be rather hiah. As t he reaction proceeds other rffe& importantly influenre the reaction kinrtics. For e x a m ~ l rin , Scheme 21, the wdlknown gel effect, or ~romsdorfeffect (named after'one of the early workers in the field) is illustrated. What this shows is that after a certain percent conversion, the polymerization appears to accelerate rather rapidly. An interpretation of this phenomenon is that as the viscosity increases one can get a decrease in termination rate constant. Since the rate of polymerization is a function of the ratio of k, to kt,an accelVolume 58
Number 11 November 1981
849
Scheme 22. (A)
-
+ XY
-M,.
Chain Transfer
+ Yt
--M,X
Scheme 23.
Chain Transfer Constants of Various Agents to Slyrene at 333'K ( 8 )
Monomer Transfer Benzene
H
I R-C-CH
+
I
sheptane sec-Butyl benzene rnCresol
CH
X
X
Scheme 24.
(B)
"SolvenY Transfer
M
S.+M-SM.+SM,. k,
e.g..
--Mn.
+ CCI,
+
M,CI CCI3 (LOWERS MOLECULAR WEIGHT) +
( C ) initiator Transfer
(D) Transfer to Polymer (El Transfer to Modifier -
-
eration effect is observed. Again, recent workers in the field have suggested that this diffusion control phenomena actually occurs much earlier than has been traditionally assumed. There is a fourth process in polymerization kinetics that needs to be discussed. This is called the chain transfer reaction. Chain transfer reactions can be very important in a variety of cases as outlined in Scheme 22. Here we depict a growing chain interacting with a small molecule xy in such a way that a portion of the small molecule can terminate the active radical chain and at the same time produce a new radical y.. What this step basically does is regulate the molecular weieht. It does not necessarilv decrease the rate of ~. o l- v m e r " ization if one assumes that the new radical y.will again reinitiate more monomer and the rate of ~olvmerization can vro. ceed basically as before. There are manv. tvDes ". of chain transfer stevs. - . snch as transfer to monomer, transfer to a solvent, to an initiator, to a ~olymer.or even to a modifier. In all cases. the molecular weight will be decreased. The quantitative decrease will he dependent upon the reactivity of the growing macroradical with the small molecule species. The extent to which this happens is very dependent upon the structure of the agent in question and can be roughly related to its reactivity with a small molecule radical species. Some typical data are shown in Scheme 23. An aromatic molecule, snch as benzene would not be expected to interact particularly effectively with radicals and indeed the chain transfer constant here is rather small. On the other hand, molecules such as carbon tetrachloride or certainlv mercaDtans have enormous activitv with macroradicals and &e very kfficient chain transfer agents. One can assess the effectiveness of a chain transfer agent bv conducting polymerizations at various ratios of the cGain trksfer agent to the monomer and by plotting the reciprocal of the number average molecular weight or the number average degree of polymerization versus this ratio. Then it is possible to see how important a particular compound is as a chain transfer agent as shown .graphically in Scheme 24. Here one can see that benzene is not \.cry eW&e and butyl mercaptan i, cxtrtmely efiective. It is poisihle rodefint. the chain tranifer ~
850
Journal of Chemical Education
Effectof chain transferto various solvents on lhe degree of polymerization of Polystyrene at 333K. 1, benzene: 2, sheptane; 3, seobulyl benzene; 4,m cresol: 5, CCI.: 6, CBr.: 7,mbutyl memaptan. Taken from reference (8).
constant, C,, from the slope of such a plot. This is sometimes referred to as a Mavo dot.. aeain . . " after Dr. Mavo. " . one of the pioneers in polymer science. An overview of the kinetics of free radical ~olvmerizations which takes into account chain transfer is provided in Scheme 25. Chain transfer to the polymer chain produces a particular type of structure, namely a branched macromolecule. This can occur to some extent during free radical ~olvmerizations.I t is well known, for example, that the impo;tant polymeric material, low density polyethvlene (LDPE),which is ~roduced under high pressure with free radical initiators, does contain some long branches which are formed as a result of chain trnnskr oian active macoradical onto an already tenninntd polymer chain. This is an important phenomenon which influences the rheology and processing of 1.13PE. Another structural modification that occurs in I.DPE is shown in Schemr 26. This is rhr furmation oisomu short chain branches via basically a "back biting" mechanism which was originally suggested in 1953 by Roedel of Phillips Petroleum Co. Here, by forming a six-membered ring, it is possible to abstract a hvdroaen from the chain to Droduce a bntvl branch which can then continue the polymerization (propagation). These short chain branches are very important in disrupting the chain packing in LDPE and are principally responsible for lowering the degree of crystallinity and the melting point of polyethylene (by about 25°C). There are additional types of branching that can occur.
Scheme 25.
Overview of Free Radical Klnetlcs
Scheme 26.
Short Chain Branch Formation in LDPE
Propagation
I
etc.
-
Propagation: I-CH,CH.
-CH,-CH9.
+ CH,=CHR
I
I
I
H
-C-CH,-C.
Etc. (propagation continues)
Termination (by radical coupling,disproportionation, or chain transfer) Radical coupling: ---CH,CH
I
+ -CH,-CH
I
-
Efhyl Branches andLong Chain Branches also Occur
---CH,CHCHCH,---
I I
R R DiSprOp~rtion~tion of TWORadicals: -CH,-CH. -CH,-CH.
R R
(4)
Scheme 27.
+
Kinetic Chain Length .- (O.P.,) Proportionality Constant Depends on Mode of Termination For Termination by Combination (DP,) = 2"
k
For
R
Termination by Disproportionation inp.!
=
,,
Chain Transfer:
-CH&H.
I R
R'S'
+ CH,=CH I
+ R'SH
-
+
-CH2CH,
I R
+ R'S.
(6)
RS'CHzCH. (startof new monomer chain)
I
(7)
With the availability of spectroscopic techniques such as NMR and FTIR, it is believed that the predominant type of short chain branching in LDPE is in fact a hutyl branch. The kinetic equations discussed so far can also he used to predict the so-called kinetic chain length for free radically produced nolvmers. There will he an effect as indicated in Scheme 27 . , as tu whether the mode. (rf terminatiol~is hy comhination or b\. Bv consideration of the kinetic Da- cl~sr~ro~urtionilt~r~n. . rameters &cussed earlier, it is possible to relate the number of monomer units consummed to the number of active centers produced. Again envoking the steady state assumption, one can relate kinetic chain length to the ratio of either the propagation to initiation or the propagation to termination. By substituting the values with these parameters discussed earlier, it is possible to derive an expression for the kinetic chain length in terms of several rate constants, the first power of the monomer concentration and the square root of the initiator concentration (Scheme 28). This important equation indicates that the kinetic chain leneth will increase as a function oft hr monumt:r concentration and ~(iuallyI I ~ W ~ C S smaller as one incrt:ases the level d initlator. This is. ut n w s e . very logical if we assume again that each initiator molecule can start two chains. The distribution of molecular weights is another question that is quite important. That is addressed in a separate article in this issue. The expected distribution of molecular weights a t low conversion for the two different termination rate processes is shown in Scheme 29. Basically, one should also consider though that this is true only for rel-
Scheme 28.
Kinetic Chain Length
Number of Monomer Unifs Consumed Active Cenfer (1)
,,=R,=R,
(steady State)
R, RT k [MI (2) "=x2kr LM.1
d[Ml Since Rp = - - kp[M][M.] dt
kpZ [MI2
(3) u = - 2k, RP For Initiated PZN
1M.l (4)
fkd[l] 1'2
= (--j;;-)
" = 2(1k#kT)"2[1]"2 ko Scheme 29.
Molecular Weight Distribution
Transferor Disproponionation if P = Prob, of Chain GrowM/term.
RP p= P-
(RP + Rd 1
Combination
Volume 58 Number 11 November 1981
851
Scheme 30.
Deviations
Mostly due to chain transfer, e.g. Mx.+XY -M,X+Y. Y. + M YM- etc.
-
General Case (DP) =
Scheme 32. Type Suspen-
Comparison of Various Polymerization Processes (Heterogeneous)
Advantages Law viscosih,
sion
RATE OF GROWTH ALL RXN RATES
Simple polymer isolation Easy thermal control
Emulsion
May be of direct usable particle size Low viscosity Good thermal control
Scheme 31.
Comparison of Various Polymerization Processes (Homogeneous)
TYPe
Advantages
(batch type)
Law impurity level, casting possible. improved thermal control
Bulk
Bulk (con-
tinuous)
Solution
Improved thermal control
Disadvantages Thermal control difficult Isolation is difficult: requires devalatiliration. Requires agitation,monomer recycling, etc. Difficultto remove solvent. Solvent recovery COS~IY. Chain transfer may limit molecular weiaht.
atively low conversions. At high conversions, as in most commercial processes, one observes somewhat broader ratios of M,../M. (22). ~ h deviations k from the kinetic expressions previously discussed are most often due to the oecurance of chain transfer processes, as indicated in Scheme 30. Here one must consider the idea that the molecular weight or the chain length will he primarily a function of the rate of growth, that is the propaeation reaction divided by all of the chain breaking reactions yncluding not only termination but also the transfer steps as well, as illustrated in the expression shown in Scheme 30. Therefore, if one has a system that displays a significant chain transfer activity, it is quite possible that the kinetics will he dominated hy that event and that the end groups on macromolecules will a t least he partially derived from the structure of the chain transfer agent. Radical Polvmerization Processes T o this p ~ i n we t have not diicussed any detailed asprct of the oolvmerization nroress itself. There are essentially four impb&nt processes that should be mentioned. First of ah, one can have a bulk reaction where basically. only. the monomer and possibly the initiator are used. Alternatively, a solution rraction process may be used which hiis a monumer, initiator and a solvent present. Hulk and solution rrautlons are sometimes reterred to as homogeneous pnnesies. The other tu,u processes ($1' importance are often termed heterogc~neous processes. imd thev are known n i suspensiun and cmulsion reac,t ions. Sweral advantagw and disadvantages uf tht, various polymerization proresirs are tahuiatrd in Srhemes :I1 and ' .lL The bulk pmceis has the advantage of alloaing for a relativelv low imrmritv . " level. Furthermore, it is possible under some conditions to perform cast polymerization. For example, some of the gasoline station signs are produced from cast polymethylm~thacrylate.One can imagine though that it would he verv difficult, if not impossible, to control the temperature of sich a reaction, retailing the heat of polymeriza-
.
852
Journal of Chemical Education
Latex may be directly usable 100% conversion may be achieved.
Disadvantages Highly sensitive to agitation
rate. Particle sire difficult to control. Possible contamination by suspending agent. Washing, drying, and compaction necessary. Emulsifier, surfactants and coagulants must be removed. High residual impurity level may degrade certain polymer properties. High cost. Washing, drying, and compacting may be necessary.
High MW possible, at high
rates. Small particle size product can be obtained Operable with son or tacky
tions discussed earlier. Moreover, the viscosity becomes enormous rather auicklv and one has to somehow find a way to dissipate the heat of polymerization. Continuous processes have the advantage that in a series of reactors, for example, it may he possibleto hetter control the temperature than in one bulk hatch type process. However, one probably would still have to isolate the polymer by a relatively high vacuum devolatilization step. This can be done and is done commercially, but nevertheless it might he considered a disadvantage. In addition, a series of reactors requires one to design the engineering properly so as to he able to recycle monomer hack to the first reactor again for additional polymerization. Several of the above oroblems can he solved hv conductine a solution process with'an inert solvent. This enables one to improve the thermal control and decrease the viscosity, but it is still difficult to remove quantitatively the solvent a t the end of the process. Moreover, the solvent may serve as a chain transfer agent which will limit the molecula~weight to lower values than may be desired. The suspensT~R H2 >T~H HR ,TH i CH2 = CH2 > ~ i R E. J. Vandenberg, US. Patent 3.051.690 (Hercules) Molecular Weight Distribution More difficult to control In general broad. MJM. -2-40
.. + +
--
+
-
Solution processes, higher polymerization temperature used to narrow distribullon
Essential Steps 1. An octahedral titanium complex
Scheme 60.
has a chlorine vacancy. Alkyiation occurs-giving an alkylated Ti species which still has a chlorine vacancy. 2. Monomer is adsorbed on the vacancy and is s bonded to the Ti atom. 3. A "cis miamtion" occurs. leadino to a new Ti-carbon bond.
Some Important Examples of Ring Opening Polymerization
Scheme 58. Other Important Features a. Quantum mechanical treatments show the importance of s-bonding of the olefin to the Ti atom to lower the activation energy for the "cis migr% tion." b. TO Obtain isotactic polypropylene, the alkyl group must move back to its original position before another monomer group is incorporated. Modified m d e l s [L. A. M. Rodriguez and H. M. VanLwy. J POIF. Sci.. 4, 1971 (1966)],eliminate (b) by proposing two vacant cwrdiwtion sites, one of which is occupied by the A1 compound.
Silaxanes
Nobel prize in 1963 for their contributions. The components of a Ziegler-Natta catalyst are listed in Scheme 56. The utilization of these usually heterogeneous catalysts is well developed. However, hecause of the enormous complexities, mechanistic understanding has been relatively slow to evolve. One should also note that the catalysts may themselves also he deposited on supports such as silica or alumina. The most generally accepted mechanism (46,511 is due to Cossie and Arlman (51), as shown in Scheme 57. Although free radicals can he produced, they are not involved in these types of polymerizations. The olefin is believed to first coordinate via B bonding with vacant d orbitals in the transition metal comnlex. The availahilitv and stahilitv of these coordination sites can he influenced si&ificantly hy'the metal alkyl. Nevertheless, it is usually now considered as a monometallic mechanism. The coordinated species must undergo a cis-rearrangement to both yield the stereoregular placement and produce a new vacancy in the transition metal structure, which mav coordinate with the next monomer unit (the step). A related, hut modified, expression given 860
Journal of Chemical Education
in Scheme 58 (46,51) is more consistent with an isotactic placement during the polymerization process. Molecular weight control is also required in coordination pulym~.rizstion.i)ne important nwthid invol\,es the usthudwar diicuvert4 tn E..I. Vnndenber~ " and wns reviewed hs him 1.101. It is, of course, based on the insertion polymerization idea as outlined above. Most of the industrial interest has centered on l-alkenes, dienes. or ethvlene ~olvmerizations.Many other monomers such as meth;] methacrylate and vinyl alkil ethers have been also studied. Heteroatom-containing monomers have further sites for coordination and hence c a n yield stereospecific polymerizations under even "homogeneous'' conditions. This enormously important mechanism remains an active area of research. Ring Opening Polymerization The process of transforming a heteroatom containing ring to a linear chain is often described as ring opening polymerization. A
M (CH?), X ===- f ( C H i k X % w This process almost always involves anionic, cationic or coordination initiation (catalysis). Only in very special sit-
Scheme 61.
Heats and Entropies of Polymerizationlor Cyclic Ethers (2)
Monomer
Ring
-AH
Size
lkcal/molel
3 4 3,3-Bis(chlaromethyl)0~a~y~Iobutane 4 5
22.6 16.1 20.2 6.2
5
5.3
6 6 7
0.4 00 3.6
Ethylene oxide Oxacyclobutane 1,Mioxolane Tetrahydrofuran Tetrahydropyran m-Diaxane 1.3-Dioxeoane
-AS ( c a l I o K mole1
19.9 11.5
nations have free radical initiators been successful (52.64). Some major types of cyclic monomers that undergo ring opening reactions are shown in Scheme 60. The thermodynamic feasibility of such a process for cyclic ethers, as a function of ring size, is shown in Scheme 61. One notes that for small rings such as ethylene oxide, the reaction is highly exothermic and polymerization proceeds with either anionic or cationic initiators. Dioxane has not been polymerized. Larger cyclic ethers such as T H F will only be initiated by oxonium salts such as (CzHs)nO+(BF& (52,64). Presumably the oxonium salt initiates by coordination with the electron pair of the cyclic ether. Propagation then proceeds by nucleophilic attack of monomer a t the carbon next to the oxonium ion. Various other counterions such as (PF&, (ShCl&, etc. are also utilized. In some systems one has-essentiallv "livine" " or nonterminated oxonium ions, that are somewhat analogous to'the "living" carhanionic polymerizations discussed earlier. Many of the ring.opening polymerizations display ring-chain equilibrium, and henceone may have to seuarate cvclic monomer from high molecular weight linear c h o ~ n s:I! the end of the pol?meriznri~m.Sp:w limitst i m s preclude discussion of other systems, h111tht. interested rri~dcris d~rectedto referenct;; t29, .A /I,%', 61,and 661.
to New York. 1972. (201 ~ ~ ~I.~o,-A,, d intil,ducti,m ~ ~ k ~acn~mdecuies."Sprinper. ~ ~ ~ . (21) Kmnip. d. L.. "Chemical Micrustructwe of Polymer Chains." Wiley Interscience. New Ywk. 1980. , (221 V d l m ~ 1%. r ~"Pelymer Chemislry,"Sprinxer-Verlay. NOWY ~ r k 1971. (2:I) Mcxlre,d. A. IEdilnrl."Marn~mf,lecular Sythe6eaSrries. Wiley. New Yurk IVlllumes I - V l l nuwpublirhedl. (211 Kennedy, .I.P., m d l'wnquirt, li. I M i l c r s l , "Pdymer ChemirtrvaiSsntheLic el as^ t,,mers"lVoI, Iand Ill.Wiley, New Ysrk. 1968. Kdathmshipr i n Pidymerr." (251 Harris. F. W..and bymour,H. H.."Siructure~Ssi~~l~iIif~ Academic Pren, New York. 1977. 1 x 1 Ivin. K. .I. (Editoil, "SLructurnl Studies ofMacn,mul~euleihy Spectrmcnpic Methndr. W i l w New Ywk, 1976. (27) Roson. S. 1. "Fundamental Priwipler of Polymeric Materials.)' R:%rnesand Noble, New Ymk. 1977. (281 Hsndall..l.C.."PvlymerSpqu~nceDeteminali~m:Carhun~l:lNMRMethc~d~Academie Piesr. NPWYork, 1971. (291 Norhay. A,. and M ~ C r i l t hJ. . E.,"HIIIc~ Copolymers: Overview and Critical Survey." Academic Prera. New York. 1977. h Serier. "Polymer Pn9cesses." Vul. X X I X , (SO) Schildknecht, C. E.. IEd8loi1, H i ~ I'dymer Wiley. New Yurk. 1Y77. (SII Stevmr. M. P., "Polymer Chemistry-An Intrudurtbn: AddisunWerley, Hendinz, MA. 1975. Tsuruts. L'., and O'Drirrr,il, K. F.. "SUuctllreand Mechanism of Vinyl Pslym~ri,stim." Dekker New Ysrk, 1969. Allen. 1.' li. M., and Patrick. C. H.."Kinetirrand Mechanisms of I'slymerization Keactkmr," Wiloy. New York. 1971. ..Kmet8cs and Mechanism uf Poiymerization: Dekker. Vol. I.Vinyl Polymerization (Editor: Ham, (i.E l New York, 1967. Vol. 11. Ringopening I'ulmerization. (2 E d i l e r Keesen S. L.,and F r i r h , K. C., 1969. Yo1 Ill. SLOP.GIUWLI Pdymerilati~m. Edit,,,: Sdoman. D.H. 1972. H i r h Polvmer Series. "Couoivmerizstiun." VsI. XVIII. E d i t o r : Ham. Geurgel, W e y . NEW Y k k , 1964. (:16) Cl,llinr. E. A , Baies..l..and Rillmeyer, F. W.,Jr., "Experiments i n Poiymer Science." Wiioy. New Yurt. 1973.
..
~
~
B~aun.D.Cherdrun.H..Dem,W.,"TschniquesufP~lymerSynLh~rirandCharaclerizatien." Wiley, New Ymk. I971 &,rpnwn, W. R..andCampbell,T. W.,"P~eparatiue Methudrol'Pdymar Chemirfry." 2nd Fd.. Wiiey. New York. 1968.
Sandler. S. K.. and Kan,. W., "Polymer Synthesis." Academic Press, New Yurk. Vui. 1. 1974,vsl. 2. 1977,vul. P. 1980. M ~ C a f b r y .E. L.. "l.aborat,~rg Preparatiun fvr Macrumol~cular Chemistry: McCrsw-Hill. New Ycrk, 1970. van~ 11. ~w , " l h p e~ r t i e r of ~ Pulymeri."~2nd F.d. lElsevier. ~ New Yurk.~1976. Sawad*. H.."Tharmodynamicr of Polymerizsti~,n."Dekker. New Yurk. 1977. Smart, M., "Carhaniuor. Elerlmn Tranafer Pmcerses and Living Pslymerr," Wiley. New Yurk. 1968. (441 Morton. M., " A l k n i c Pulym~riration:Principles and Practice." Acalemic Press, New
Concluding Remarks
This article has attempted to discuss the salient features and importance of chain reaction polymerization. It is hoped that some of the readers will be stimulated sufficiently to "dig deeper" into the cited references. There it will be possible to find much greater details on the subjects that were covered in this review. References 111 Flury, P. d.:'Principlesc,fPolymcr Chemistry,"ComellUniverrity Presr.Ithaca, NY. 1953. (21 Orlian.l:.. "Principles uiPslymerizalion," 2nd Ed.. Wiley. New Yerk. 1981. (:I1 Lam. K. W.. "Oreanic Chemistry of Synthetic I'i,lymera." Wiley, New York. 1967. Idi Ailr.,rk , ., . . ~~~, ~ H.~I .and ~ ILomoe. F. W.."CimLemoorarv . . Pslymer Chemistry." Prmfice-Hall, l i n r ~ e w w acliits, ~YRI. 161 Riiimeyer, F.,"'~eith~,ukufPulymerScienre:2nd Ed. Wiiay. New Ywk. 1971. 161 Saunderr, K..J,"Organic I'olymer Chemistry."Chapman andHall. 1973 171 Sevm
