Generation of Poly(viny1 Alcohol) and Arrangement of Structural Units
Charles E. Carraher, Jr. Wright State University Dayton. Ohio 45435
We have found that many students have difficulty in adequately defining, interrelating, and distinguishing between terms such as alcoholysis, acetylation, hydrolysis, saponification, solvolysis, etc. The following exercise allows for a discussion of these terms as well as ~ e r m i t t i n ethe e m ~ h a s i s of preferential dryradation reaction; such as ;if vicinaidiols, analvtiral dt:terminatiun of head-to-rail orirntation in \,ins1 polymers and the relationship between viscosity and molecular weight. It can he utilized in organic chemistry lahnratories with emphasis on synthesis and in physical and analytical chemistry laboratories with emphasis on physical characterization and it is an ideal exercise to point out the natural flow between synthesis and characterization. Nature utilizes small differences to produce large changes. Small temperature changes bring about exponential differences in reaction rates; small changes in structure of reactants can alter the product obtained via changes in the steric and/or electronic nature. In fact two critical parameters in determining reaction rates, what products will he formed, etc. are the steric and electronic nature of the reactants. Steric geometry and electronic effects are also important in ~ r I t is well established that the oolv. o l.v m e chemistrv. . " merization of vinyl monomers leads to a polymer in which the structural units occur in a reeular fashion. For instance the addition of vinyl monomer to ;free radical can occur in either of two ways
In (I) the unpaired electron can participate in resonance with thesubstituent X thus favoring i t over formation of (11). Steric factors also favor formation of (I) leading to a regular "hand-to-tail" configuration where the suhstituent occurs on alternate carbon atoms.
H
H
H
H
the ratio usually changes toward unity as temperature increases indicating (for those cases) that a difference in activation energy is a t least partly responsible for the difference in relative rate of monomer inclusion into growing chains. Conversly polypropylene also occurs in predominately a head-to-tail arrangement and the amount of resonance stabilization is small here. In fact, to date all vinyl polymers produced via ionic and free radical processes contain a major proportion of head-to-tail linkages. Since vinyl alcohol is unstable with respect to isomerization to acetaldehyde, its polymer is prepared by indirect methods-mainly by the deacetylation (hydrolysis,deesterification, alcoholysis are terms which can alsu be associated with this process) of poly(viny1 acetate). Methanol or ethanol are commonly utilized along with acid or base as a catalyst to effect alcoholysis. Alkaline deesterification is more rapid and will he utilized here. The poly(viny1 alcohol) is insoluble in methanol solution and precipitates from the solution. The terms alcoholysis, acetylation, deacetylation, hydrolysis, esterification, deesterification, andlor saponification have been utilized to describe similar reactions. This is proper for some reactions hut improper for others. Hydrolysis reactions are a subgrouping of solvolysis reactions where the reaction occurs in aqueous solution involving elements of the water molecule, namely H+ and OH-. For instance the reaction between an organic acid chloride and water is a hydrolysis reaction-the other terms cited above not being correctly applicahle.
A reaction involvina an alcohol as a reactant is referred to as an alcohulysii reaction. It mav alsu he a subgrouping of snlvolvsis reactions if the alcohol also is the ~ r i m a r vsolvent. React&s leading to the formation of esters-are called esterification reactions. Thus the reaction between a diol and a diacid chloride yielding a polyester is both an alcoholysis and esterification reaction 0 0
II
CI-C-R-C-CI
II
+
HO-R'-OH
+
n
ii
+O-R'-0-C-R-Ct Polyester The relative importance of steric and electronic nature on the regular polymer structure is still in question and possibly varies from monomer to monomer. From studying the relative rates of inclusion of two monomers into a single growing polymer chain relative "reactivity ratios" are fonnd. The effect of temperature on the monomer reactivity ratio is small. In cases where the reactivity ratios were examined with accuracy,
-
n
II
+
2HC1
Reactions occurring in the opposite direction, i.e. involving the formation of an alcohol and acid or acid chloride are termed deesterification andlor dealcoholysis. In special cases where (normally with natural fats) the alkaline hydrolysis of an ester occurs (yielding a soap and alcohol) the reaction is termed a saponification reaction. The below reaction can then be termed a hydrolysis, saponification and/or deesterification reaction. Volume 55, Number 7, July 1978 1 473
related t o the fraction of head-to-tail orientations compared to non-head-to-tail orientations.
1 I
H&-O-C-(CHs)n-CH, NaOH
I
+
- II
II
HC-O-C-(CH,),--CH, 0
I
Exercise
CH,OH
0
+
CHOH
II
CH,OH Glycerol
H2C-O-C-(CHJn-CH,
Acetylation reactions involve inclusion of the acetyl,
0
II
CH3-C-, moiety into compounds. The reverse reaction, i.e. t h e splitting o u t of the acetyl unit, is then referred t o a s a deacetylation reaction. Thus the following reaction can he termed as a n acetylation, alcoholysis, and esterification reaction.
n . R-OH
+
CH,-C-CI
II
-
n
ii R-O-C-CH,
+
HCI
T h e reverse reaction can he termed a deesterification or deacetylation reaction. T h e present reaction occurring in alkaline solution to form a n alcohol can then he termed a deacetylation deesterification, or hydrolysis reaction, emphasizing t h a t reaction occurred in a n aqueous media, or alcoholysis emphasizing t h a t reaction occurred in the alcohol media utilizing the elements of alcohol.
T h e major uses for poly(viny1 alcohol), PVOH, fall into three erouos. One makes use of its water soluhilitv where t h e PVOH serves a s a thickening agent for emulsi& and suspension systems and as a packaging film where water solubility is important. A second category uses PVOH which is chemically modified t o give it water insolubility. This is often effected by wet spinning PVOH from water into a concentrated solution of sodium sulfate containing formaldehyde and sulfuric acid resulting in the formation of some formal groups. Some (interchain) crosslinking is desirable t o reduce shrinkage of the fiber. This modified PVOH has a higher water absorption than most other fibers permitting it t o replace such materials a s cotton in clothing. A third use for PVOH is a s the intermediate in the synthesis of poly(viny1 acetals), by far t h e most important being poly(vinyl hutyral) used as the plastic interlayer for aircraft and auto "safety glass." Safetv-elass laminates are made hv nlacine a drv sheet of the pol;(Ginyl hutyral) between two-pieces Gf glass. This is subjected to mild pressure to seal and then further autoclaving a t higher pressure and temperature t o complete the lamina~ o l.v ( v i -n vhutwal) l tion wocess. Tvpical thicknesses for the . layeiare 0.02-0.b5 cm. T h e following exercise describes the deacetylation of poly(viny1 acetate). The resulting poly(viny1 alcohol) will then he exposed to periodic acid which will preferentially degrade vicinal alcohol groupings which occur whenever the headto-tail sequence did not occur, causing a lowering in polymer molecular weight. Molecular weight charge is then directly 474 / Journal of Chemical Education
Deacetylation of Poly(viny1Acetate)
Commercial paly(viny1 acetate) (PVA) (3 g) and anhydrous methanol (75 ml) are added to a 250-ml round bottomed flask fitted with a reflux condenser leading to a CaC12drying tube. The methanol-PVA mixture is refluxed until the PVA dissolves (-20 mi") whereupon a solution of anhydrous methanol (25ml) containing KOH (2 g) is added. The solution is refluaed for 1 hr. As the polyvinyl alcohol (PVOH) is formed, it precipitates from the solution as a fine powder. PVOH is collected usingaBuchner funneland washed with four 25-mI portions of anhydrous methanol. PVOH is partially dried by passing a current of air over the product for -30 mi". The partially dry PVOH is added to a 3W-ml beaker containing distilled water (100 ml; dissolution is hastened by heating the water to about 80°C) and dissolved. The solution is then added droowise to a 1-1beaker eonwining tarrlmr 12kl I ~ which I i,stirrco wgon,usly u i t h a me
